Hudson Lecture for 1958
Darwinism in New Zealand: Some Examples, Influences and Developments
New Zealand Geological Survey
When I was honoured by the invitation of the Council of the Wellington Branch to present the Hudson Lecture for 1958, I was asked to speak on a subject which would serve also to commemorate the hundredth anniversary of the announcement of the theory of the Origin of Species by Natural Selection jointly by Charles Darwin and Alfred Russell Wallace. No subject could be more appropriate for a Hudson Lecture, as the late G. V. Hudson was an enthusiastic Darwinian, and the titles of his published entomological papers include such topics as “the Darwinian principle of sexual selection”, “explanation of variation in cryptic Lepidoptera”, “melanism and wet climates”, and “semi-apterous females in certain Lepidoptera, with an attempted explanation”. The celebrated announcement we commemorate was made by Sir Charles Lyell and Joseph Dalton Hooker on behalf of the authors at a meeting of the Linnean Society on July 1, 1858, just over 100 years ago, and “had the effect”, on T. H. Huxley1 and many of his contemporaries, “of a flash of light which to a man who has lost his way in a dark night, suddenly reveals a road which, whether it takes him straight home or not, certainly goes his way”. My chief qualification to present a lecture on this topic is that I am a thoroughgoing and unashamed Darwinian in my beliefs and in my approach to Natural History. Having said this, I can almost retire from the scene, except as a mouthpiece for Darwin and his contemporaries, for my intention is to present for your consideration an address composed very largely of quotations from Darwin's letters and books, and New Zealand illustrations of the principles he did so much to establish, some of them from work with which I have been associated.
Darwin and Hooker
Darwin's only direct contact with New Zealand was in December, 1835, when H.M.S. “Beagle” called at the Bay of Islands. Both Darwin and New Zealand were not at their best at that place and time; he was tired and homesick, New Zealand at the worst phase of pre-Waitangi chaos. No wonder the country and its inhabitants, Maori, whaler, trader, weeds, and rats, made such a sad contrast with the tropical paradise of Tahiti which the Beagle had just left. In the Journal of the Beagle, New Zealand can claim credit for none of the stimulating ideas and observations that were to ferment in Darwin's mind for the next 25 years. Already, however, he began to influence New Zealand naturalists. William Colenso spent Christmas Day with Darwin, and we have Hooker's word for it that Darwin remembered this in later life, after he had read with interest Colenso's papers in the early Transactions of the New Zealand Institute.2 But in 1839, four years after his return to England, Darwin met Joseph Dalton Hooker, a student hurrying to complete what we should now call a “pressure cooker” degree in order to join Sir James Clark Ross on his Antarctic Expedition.3 Hooker devoured the “Beagle” Journal in proof before he set out to visit in succession the islands and continents of the Southern Ocean that share so many of their plants with New Zealand. On his return Hooker began (in 1843) a correspondence with Darwin that tells how New Zealand topics played their part in the development of Darwin's theory.
[Footnote] 1 Darwin Francis (ed.), 1887. Life and Letters of Charles Darwin (London: Murray) II: 197. (Hereafter abbreviated to Life and Letters.)
[Footnote] 2 Hooker, J. D., 1883. Letter to von Haast, cited in The Life and Times of Sir Julius von Haast (see ref. 84).
[Footnote] 3 Life and Letters, II: 19.
New Zealand's Flora and the Southern Continent
In 1844 Darwin began a long discussion with Hooker on the relationships of Southern Hemisphere floras. Darwin's modest herbarium was now added to Hooker's own collections. To explain the well-known affinities between the floras of New Zealand, South America, and the Subantarctic islands of all three southern oceans, Hooker believed a former Antarctic continental extension was necessary. The persistent demand for continental extension across the oceans to explain the present distribution of life seems in retrospect unnecessary for pre-Darwinian biologists and perhaps shows how ready was the soil of contemporary thought for the seed of “descent with modification”. The paleontologist Edward Forbes (one of the first to record New Zealand fossils) started the vogue with an “Atlantis” extending across the Atlantic Ocean, and Hooker embraced the idea for the South American-New Zealand relationships. But Darwin opposed such speculation with a faith in the ability of plants to be dispersed across the sea, and later claimed that he “had never let down or upheaved our mother earth's surface for the sake of explaining any one phenomenon”.4 It shocked his philosophy to create land without some other and independent evidence. Nevertheless, he was enthusiastic over Hooker's introduction to the New Zealand flora when he read it in 1853, “How few generalizers there are among systematists”, he complained5, and the echo of his complaint can still be heard. But his reservation about continental extensions remained. “To imagine such enormous geological changes within the period of now-living beings on no other ground but to account for their distribution, seems to me, in our present state of ignorance on the means of transportal, an almost retrograde step in science”.6 So he began a long series of experiments on the viability of seeds after immersion in salt water, carefully planting out the treated seeds and rejoicing in each successful germination as “a triumph over Hooker”. (Hooker, of course, provided the seeds from Kew for these experiments.) Darwin's children, we are told, were tremendously eager that he “should beat Dr. Hooker”7
A second dominant influence and correspondent was Charles Lyell, father of modern geology, whose “Principles of Geology” embodies a consistent interpretation of the rocks in terms of processes we can now observe. The extension of uniformitarianism to the field of biology was Darwin's life work and he called Lyell his “Lord High Chancellor in Natural Science”.
He wrote to his Lord High Chancellor in 1856, irate at the geological strides Lyell's disciples were taking8 “Here poor Forbes made a continent to North America and another (or the same) to the Gulf weed; Hooker makes one from New Zealand to South America and round the world to Kerguelen Land.… And all this within the existence of recent species! If you do not stop this, if there be a lower region for the punishment of geologists I believe, my great master, you will go there”. To Hooker, one of the offenders, he wrote9. “…a pretty nice little extension of land they make altogether. I am fairly rabid on the question, and therefore, if not wrong already, am pretty sure to become so”. What Darwin chiefly objected to was the assumption that such vast changes had occurred in the comparatively short lifetime of existing species, since his studies of organic change led him to the conclusion that the members of each species of plant and animal had been in biological continuity at no great geological interval from now. These arguments he put forth to Lyell in a long letter (June, 1856)10 Lyell, however,
[Footnote] 4 Life and Letters, II: 38, 59.
[Footnote] 5 Life and Letters, II: 39.
[Footnote] 6 Life and Letters, II: 60 (footnote); Gardeners' Chronicle, May 26, 1855.
[Footnote] 7. Life and Letters, II: 55.
[Footnote] 8. Life and Letters, II: 72.
[Footnote] 9: Life and Letters, II: 73.
[Footnote] 10. Life and Letters, II: 74.
was not yet a convert to Darwin's ideas on organic evolution, and an event in New Zealand during the previous year had impressed upon him how rapidly earth movements could take place. Walter Mantell, a founder of the New Zealand Society, precursor of the Wellington Branch of the Royal Society of New Zealand11, Sir Frederick Weld, and Captain E. Roberts of the Royal Engineers, had all supplied Lyell with accounts of the Wellington earthquake12. “Nine feet”, he replied to Darwin13 “did the Rimutaka chain of New Zealand gain in height in January 1855, and a great earthquake has occurred in New Zealand every seven years for half a century nearly”. Modern historians and prophets of seismicity may not substantiate these figures, but they played their part in the battle of the land-bridges. In respect to New Zealand, Darwin later14 relaxed his objections to continental connection: “It is only against the former union with the oceanic volcanic islands that I am vehement. What a perplexing case New Zealand does seem: is not the absence [he meant scarcity] of legummosae, etc, fully as much opposed to continental connexion as to any other theory? … The presence of a frog in New Zealand seems to me a strongish fact for continental connexion, for I assume that sea-water would kill spawn, but shall try”. At this early date, of course, the primitive relationships, presumptively ancient origin, and peculiar life-history of the New Zealand frog Leiopelma were unknown15.
Darwin and the Kowhai
Darwin's experiments on seed viability after long immersion in sea water continued. One of New Zealand's few Leguminosae, Edwardsia, the kowhai, was also one of Hooker's Antarctic genera, and since most leguminous seeds seemed easily killed by salt water, he asked Hooker for kowhai seeds. Apparently Hooker demurred, for Darwin was led to complain16: “I believe you are afraid to send me a ripe Edwardsia pod, for fear I should float it from New Zealand to Chile”. Eight years later he had not forgotten Edwardsia17. “I have read with uncommon interest” (he wrote to Hooker) “Travers' short paper on the Chatham Islands. I remember your pitching into me with terrible ferocity because I said I thought the seed of Edwardsia might have been floated from Chile to New Zealand: now what do you say, my young man, to the three young trees of the same size on one spot alone of the island and with the cast-up pod on the shore? If it were not for those unlucky wingless birds, I could believe that the group had been colonised by accidental means.” Travers' observation of the only three kowhai trees he found at the Chatham Islands18 is now almost forgotten, but kowhai pods still wash up on our beaches, and I vividly recollect an able young naturalist telling me a few years ago how he succeeded in germinating seeds from such jetsam. Indeed, Guppy19 found that kowhai seeds germinated after four months floating in sea water, so in this respect Darwin's point was well taken.
[Footnote] 11 Bastings L., 1953. History of the New Zealand Society, 1851–1868. Trans. Roy. Soc. N. Z., 80: 359–66.
[Footnote] 12 Ongley, M., 1943. Trans. Roy. Soc. N. Z., 73: 84–9.
[Footnote] 13 Darwin, Francis, and Seward, A. C. (eds.), 1903. More Letters of Charles Darwin. London, Murray, 1: 92 (hereafter abbreviated to More Letters).
[Footnote] 14 More Letters, 1: 418.
[Footnote] 15 Turbott, E. G., 1942. Trans. Roy. Soc. N. Z., 71: 247–53; 1949. Rec. Auck. Inst. Mus., 3: 373–6 (and references).
[Footnote] 16 Life and Letters, II: 180.
[Footnote] 17 More Letters, 1: 475.
[Footnote] 18 Travers, H. H., 1867. J. Linn. Soc. Bot.,, 9: 142–3.
[Footnote] 19 Guppy, H. B., 1906. Observations of a Naturalist in the Pacific…, II: 580.
In 1858 the New Zealand frog was only a rumour, and Hooker's arguments for continental status had not won Darwin's support. In the “Origin” New Zealand was treated as an oceanic island which had gained its biota by “the many and curious means of occasional transport, a subject which has hardly ever been properly experimented on”20. Darwin's own experiments and arguments on the drifting of seeds, on the period of their survival after immersion in seawater, and the speed of currents, have seldom been followed up by later workers, but a related topic, transport of the planktonic larvae of shallow water marine invertebrates, has been of interest to New Zealand biologists, since numbers of mollusca, crustaceans, and echinoderms are common to the shores of Eastern Australia and New Zealand. Successful transport of a planktonic larva depends on the duration of its planktonic life in relation to the speed of the current and the stream-line distance to be covered. When I first described this relationship to a meeting of New Zealand ecologists21, I remarked that none of these factors was known for the Tasman Sea. Oceanographic studies have since brought us some knowledge of stream-line course, and are likely to determine current speeds, and Professor H. B. Fell has initiated studies of the viability of marine invertebrate larvae to supply data for the third factor in the equation. These are truly Darwinian investigations.
New Caledonia and New Zealand
Hooker's insistence, the New Zealand frog, and Haast's (unfounded) report of a New Zealand mammal finally won the day for New Zealand's former continental status. “About New Zealand”, he wrote22, “at last I am coming round, and admit it must have been connected with some terra firma, but I will die rather than admit Australia. How I wish the mountains of New Caledonia were well worked”. Only in the last decade have we learned something23 of the beeches (Nothofagus) and conifers (e.g. Libocedrus) of upland New Caledonia and New Guinea, which support a former link with New Zealand along a geologically plausible route. Miss L. B. Moore has recently confirmed another link by her comparisons and descriptions of the lily genus Xeronema shared by New Zealand and New Caledonia24. But Hooker's Antarctic connection remains a favourite speculation for biologists. (“Without speculation, there is no good and original observation”, Darwin wrote to Wallace, December, 185725.) Darwin was well aware of the limited dispersal ability of beech forest. Years later, discontinuous beech distribution in Asia led him to remark26: “In a geological sense, we must, I suppose, admit that every yard of land has been successively covered with a beech forest between the Caucasus and Japan”. Similar thoughts about New Zealand vegetation were later expressed by Cockayne27: “It is not a movement of individuals, except in the association, but a movement of the whole association that has to be considered".
New Zealand Flora
Not all Darwin's interests in the New Zealand flora were biogeographic. His studies of variation in nature, and its relation to species formation, led him to the hypothesis that in large genera (i.e., those with many species), the individual species
[Footnote] 20 Darwin, C., 1859. On the Origin of Species by Means of Natural Selection… (page references to 1950 reprint of First Edition. London: Watts & Co.: 315 (hereafter abbreviated to “Origin”).
[Footnote] 21 Fleming, C. A., 1952. N. Z. Sci. Rev., 5: 60–1; sec also Fig. 2 in ref. 67.
[Footnote] 22 More Letters, 1: 474.
[Footnote] 23 Van Steenis, 1953. Results of the Archbold Expedit. J. Arnold Arboretum, 34 (4): 300–74.
[Footnote] 24 Moore, L. B., 1957. Pacific Science, 11: 355–62.
[Footnote] 25 Life and Letters, II: 108; but see also p. 84. and More Letters, II: 133.
[Footnote] 26 More Letters, II: 8–9.
[Footnote] 27 Cockayne, L., 1919. New Zealand Plants and Their Story, Wellington: 196.
are themselves more variable than those in smaller, more conservative genera, a view cautiously expressed in the “Origin”. He used Hooker's New Zealand flora as a test, calculating the percentage of species with named varieties in genera with more or less than three species. The New Zealand flora was also analysed in relation to Darwin's ideas on the value of the dioecious habit in trees28. With Hooker, he discussed “the favoured development of trees and bushes in New Zealand and the consequent… development of separation of sexes to prevent too much intermarriage”. He was also fascinated with the demonstrations of “survival of the fittest” (Herbert Spencer's term came later) when European weeds displaced native ones, and questioned Hooker (1857) on the relative abundance of introduced and native thistle (Sonchus) in New Zealand29.
Glaciation in New Zealand
When the Austrian frigate Novara was setting out on the expedition that brought Hochstetter to our shores, Darwin's advice was sought30. “In New Zealand,” he replied, “urge them to look out for erratic boulders and marks of old glaciers”.
Hochstetter's companion, the redoubtable Julius Haast, stayed on to investigate the Southern Alps and sent his reports back to Hooker31. Darwin was enthusiastic about Haast's observations on glaciers: he had foreseen that many problems of discontinuous distribution could be solved on the assumption that life zones had followed climate zones in large-scale shifts during the glacial period and was eager to learn of evidence that the Ice Age had affected both hemispheres.
The Short-tailed Bat
Lyell once asked why no bats had become flightless after colonising islands, since many birds had done so. Darwin replied32: “With respect to bats at New Zealand… not having given rise to a group of non-volant bats, it is, now that you have put the case, surprising; more especially as the genus of bats in New Zealand is very peculiar, and therefore has probably been long introduced”. Years later, Hutton and Drummond33, supported by the observations of the late Edgar F. Stead, considered that the New Zealand bat Mystacina, instead of hawking food on the wing, feeds in the branches, climbing with its forelimbs folded to serve once more as legs.
A browse through the first edition of the “Origin” (1859) uncovers a host of topics relevant to problems of New Zealand biology, past and present. Darwin's first chapter, “Variation Under Domestication” includes his observations on the contrasting proportions of wings and legs in wild and domestic ducks. With domestication, greater body weight, and less need for flight, a domestic duck's legs have become heavier, its wings lighter than those of a wild duck. Darwin in several places referred to the somewhat mysterious laws of “correlation of growth” that have been much clarified by workers on “functional morphology” such as S. Brody34 on domestic animals, and D. M. S. Watson35 on fossil lineages. The application of principles of functional morphology—of the correlations in growth rates, and of quantitative dependence of the growth of one part on the growth of
[Footnote] 28 More Letters, II: 251.
[Footnote] 29 More Letters, 1: 102.
[Footnote] 30 Life and Letters, II: 93–4.
[Footnote] 31 More Letters, II: 154.
[Footnote] 32 Life and Letters, II: 336.
[Footnote] 33 Hutton, F. W., and Drummond, J., 1923. The Animals of New Zealand (ed. 4): 39.
[Footnote] 34 Brodie, S., 1945. Bioenergetics and Growth, New York.
[Footnote] 35 Watson, D. M. S., 1949. Pp. 45–63 in Genetics, Paleontology and Evolution, Princeton.
another, have been put to good use for the benefit of man in the development of domestic meat-producing animals by research workers such as John Hammond in the United Kingdom and C. P. McMeekan in New Zealand, but the same principles have seldom been applied to the interpretation of differences between wild animals. Darwin's domestic duck (he referred in particular to the Aylesbury36), heavier in body and stouter in leg than its wild progenitor, can be compared with our New Zealand Takahe, which shows similar contrasts with the Pukeko, its closest relative, presumably closely similar to the stock from which the Takahe sprang37. The Pukeko is an adaptable moderate-sized waterfowl with adequate powers of flight, widely distributed in the Southern Hemisphere. Notornis differs chiefly in its large size, stout legs and neck, reduced wings, and oversized beak. Increase in size has been a common tendency among herbivorous vertebrates, and has a simple physiological explanation. The food requirements of an animal can be broken up into a “maintenance ration”, required to maintain life, and a “production ration”, which provides the energy for all activity, locomotion, reproduction,
[Footnote] 36 Origin: 9, 117.
[Footnote] 37 Fleming, C. A., 1951. Some general reflections on Notornis. Notornis,, 4 (5): 103–6.
growth, etc. The maintenance ration is related to the weight, but is not proportional to it, increasing roughly as the animal's surface area—i.e., as the ⅔ power of the weight. But the production ration is directly proportional to the energy used, and thus to the weight. Thus food consumption consists of a maintenance ration varying as the square of a linear dimension plus a production ration varying with its cube. For the same amount of activity, a larger animal has a greater thermodynamic efficiency than a small one. Darwin's Aylesbury duck and New Zealand Notornis have this physiological advantage over wild ducks and pukekos, provided that other factors allow their survival.
But flight is more difficult in large birds than in small ones, since they must fly faster to avoid crashing. Efficiency of a wing is a function of its area (proportional to the square of a linear dimension) but the load to be carried is proportional to the cube of a linear dimension, so that a bird twice the linear size of another would have to support eight times the weight of its smaller relative with only four times the wing area. A swamp-hen about twice the size of a pukeko, as Notornis is, would have much less efficient wings unless wings and muscles changed in compensation. This was Haldane's argument that a conventional “angel” would need a 4ft keel on its sternum to support the large muscles necessary for flight. But energy used in flight is a debit against the production ration of a bird, and is a more expensive item in a large bird than in a small one. This is the chief reason why large birds have become flightless when flight was no longer necessary for survival38. The importance of flight has generally kept birds' average size low, except in Darwin's domestic ducks, in the protection of the farm-yard, and New Zealand flightless birds, surviving in the specially protected conditions of primitive New Zealand, freed from the need to escape from carnivorous predators.
Large size also accounts for the robust legs of farm ducks and Notornis, for the strength of a supporting column is related to its cross-section (square of linear dimension) and must increase disproportionately to keep up with a cubic increase in weight. Similarly, the capacity of a column to support weight varies inversely as the square of its length (Euler's principle) so that leg bones of heavy animals tend to be shorter than those of smaller related species.
I have made this long digression on the excuse of Darwin's farm duck to emphasize that intensive directed research on domestic animals can still provide many analogies for the interpretation of wild ones, as Darwin well knew.
Some of the changes that have occurred under domestication, by the “letting up” of the pressure of selection, can be paralleled in the faunas of oceanic islands. Thus in New Zealand, where a generally sparse fauna developed in isolation without the restraints of severe competition and in the absence of predatory mammals, “permissive evolution” has allowed the survival of melanistic forms of birds, similar in origin to the “sports” in cage-birds, dogs and cattle. In the stilt, a world-wide bird generally of pied plumage, a black form developed in New Zealand alone, but has largely given way to the more normal pied form since European colonisation changed what Darwin liked to call the “polity of Nature”. Any bird-watcher who has seen how completely a mob of pied stilts can merge into a background of river boulders will endorse the claim for adaptive value in their disruptive colour pattern. To account for the occurrence of “black” forms in so many New Zealand birds, it seems necessary to postulate some adaptive advantages linked with melanic pigmentation which are generally less important for survival than the “normal”
[Footnote] 38 Romer, A. S., 1945. Vertebrate Paleontology, Chicago: Univ. Press.
Fig. 2.—Melanism. In the stilt, a world-wide bird generally of pied plumage, a black form developed in New Zealand alone, but has largely given way to the more normal pied form since European colonisation changed what Darwin liked to call the “polity of Nature”.
colour pattern, but which were able to be effective in the special ecological conditions of primitive New Zealand. In Lepidoptera, to be sure, E. B. Ford39 has found melanic forms to be hardier than the normal form which nevertheless dominates in wild populations, except where man's alteration of the environment in industrial areas has swung the balance in favour of the melanic form.
“Variation Under Nature”
Darwin's second chapter, “Variation Under Nature”, begins with a discussion of species definitions in relation to natural variability which, he claimed, bridged differences between monstrosities, varieties, sub-species, and species. It includes the prophetic if rhetorical question “who can say that the dwarfed condition of shells in the brackish waters of the Baltic, or dwarfed plants on alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited…?”40 There are several New Zealand ectotypes, that cause the systematist to doubt whether the distinctive characters are controlled by genetics or solely by environmental influences, but we have a number of studies overseas and locally on New Zealand plants to show that in such ecological races are generally genetically controlled and not the result of purely environmental (phenotypic) effects.
Darwin claimed that variations affected all parts of an organism and scoffed at the circular argument of zoologists who claimed that “important organs never vary; for these same authors practically rank that character as important… which does not vary”41. Students of gastropod Mollusca have recently had some severe shocks to their faith in the reliability of two characters formerly considered stable, and thus reliable guides to affinity. The gastropod protoconch, the record of the larval life, has been found to vary on occasion in response to temperature conditions during development, and the radula, or ribbon-like teeth, long used as a final argument for specific distinctness, has been proved by Dr. R. C. Dell to show unsuspected variation in different individuals of a common New Zealand whelk42.
We have in New Zealand a good number of what Darwin called “polymorphic genera” in which “species present an inordinate amount of variation”43 such as
[Footnote] 39 Ford, E. B., 1938. Evolution, Clarendon Press: 53.
[Footnote] 40 Origin: 38.
[Footnote] 41 Origin: 39.
[Footnote] 42 Dell, R. K., 1956. Dom. Mus. Bull.,, 18: 105.
[Footnote] 43 Origin: 40.
he had observed in the genus Rubus and in several general of Brachiopoda (lamp shells) where “hardly two naturalists can agree which forms to rank as species and which as varieties”. Darwin suspected that such variations “are of no service or disservice to the species and …have not been seized on and rendered definite by natural selection”. Systematists are still plagued by such polymorphic swarms, and New Zealand paleontologists have still to rationalize the nature of such variability in certain Tertiary Brachiopoda; in one case the late Dr. J. A. Thomson named 10 species of one genus in a single thin bed, but admitted in a paper44 read before this Society in 1919 that “there are so many intermediates that it is obvious that evolution either had taken place only a short time previously or was still in progress”. Modern workers would not be so prodigal of specific names, I suspect, but would adopt Darwin's explanation that selection, in such cases, was failing to keep such populations confined rigidly to the narrow range of variation we are used to.
“Struggle for Existence”
The third chapter in the “Origin” is devoted to the “Struggle for Existence”, the general principles of which, as set forth by Darwin and Wallace in 1858, need no explanation. Darwin explained it in a nutshell as “one general law, leading to the advancement of all organic beings—namely, multiply, vary, let the strongest live and the weakest die”45. Such epigrammatic brevity, out of context, has led to misunderstanding, and, indeed, to the unjustified restriction of the term “Darwinian selection” to differential survival or mortality, which is only one aspect of selection.
So-called Darwinian selection, which operates in the life-time of the individual, has recently been neatly demonstrated by analysis of the dimensions of the molar teeth in a fossil population of the extinct cave bear of Europe46. This bear hibernated in caves, and suffered a mortality peak near the end of winter, so that the fossil samples from a cave near Odessa (Ukraine) show an age grouping with modes at annual intervals. Frequency plots of “paracone index” of the second molar in successive ages show a continuous reduction in mean values with increasing age, indicating a strong linear selection in favour of lower paracone index, due to the death of young bears suffering from quite slight malocclusion between the molars. This example graphically demonstrates how effectively, in a natural population, selection can operate on minor differences; but as the differences were not genetically controlled no evolution took place.
The classic case of Natural Selection operating on wild populations is the spread of “industrial melanism” during the past century among moths living near the industrial cities of Great Britain47. Smoke and smog killed the lichens of tree trunks and replaced them with a black sooty covering. Change of background colour reversed the normal selection pressure in favour of “normal” coloration, so that whole populations changed colour in the course of about fifty years.
But this crude form of selection involving the death of the unfit was far less important in Darwin's mind than many of his critics have claimed. It is important to emphasize that Darwin himself used the term Struggle for Existence “in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny”48 (my Italics). During the past two decades the use of mathe-
[Footnote] 44 Thomson, J. A., 1920. Trans. Roy. Soc. N. Z., 52: 370.
[Footnote] 45 Origin: 209.
[Footnote] 46 Kurten, B., 1957. Evolution, 11 (4): 412–416.
[Footnote] 47 Ford, E. B., Polymorphism and Taxonomy, in The New Systematics, Clarendon Press: 508.
[Footnote] 48 Origin: 54.
matical methods has given a firm basis for selection theory, and Fisher49 has shown that even genes with small selective advantage will eventually spread over entire populations
Malthus on Populations
Darwin claimed his theory to be “the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms”.50 The main points in Malthus's doctrine are that organisms tend to multiply exponentially whereas the food supplies on which they depend increase arithmetically if at all. The same relationship applies to the exponential increase of New Zealand's population and its demands, on the one hand, and the arithmetical increase in electricity supplies by the addition of power stations and (on a more restricted time scale) of water in the storage lakes.
Some change in the economy of the North Atlantic and South-West Pacific, not at all understood, has recently allowed us to study the exponential growth of two populations of a sea bird. The first study51, by Fisher and Vevers in 1943–4, showed the increase of the North Atlantic gannet, presumably after depredations by the human predator of the eighteenth and early nineteeth centuries. Nearer home, the Cape Kidnappers gannetry increased in numbers from about 50 pairs in 1879 to 2,337 pairs in 1946–47, but when the figures are shown graphically52, they show a general tendency to a flattening of the curve, doubtless because the limitations of the environment exert their Malthusian effect.
Darwin noted the fluctuations in animal numbers, and recorded that epidemics often ensue after a sudden rise as “a limiting check independent of the struggle for life. But even some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through facility of diffusion among the crowded animals, been disproportionately favoured; and here comes in a sort of struggle between the parasite and its prey”53 Mr. P. C. Bull54 has recently confirmed this relationship in the cyclic changes in New Zealand rabbit populations and their parasites (nematodes and sporocytes). After the rabbit population in Hawke's Bay expanded in response to a favourable change in conditions, “crowding followed and gave rise to high mortality rates through parasites and disease”. The peak in the density of parasites was attained shortly after the peak in rabbit numbers, the population fell, and the density of parasites decreased thereafter.
Chapter IV of the “Origin” deals with Natural Selection—the question of “how will the struggle for existence … act in regard to variation?”55 Darwin foresaw many criticisms that this “blind” force could not mould the complicated organs of the myriad forms of organic life through the differential survival of random variation. Recent work has given some substance to generalization about selection. Muller has estimated56 that the number of mutations, of observed average magnitude of effect, necessary to convert a one-celled organism into a horse, by chance alone, without selection, would be about 1,0001,000,000. This impossible figure shows the importance of selection in collecting favourable mutations and minimizing waste of variation. But the apparent improbability of an organism reaching the form we now see by selection of random variations can perhaps be resolved in our
[Footnote] 49 Fisher, R. A., 1930. The Genetical Theory of Natural Selection, Clarendon Press.
[Footnote] 50 Origin: 55.
[Footnote] 51 Fisher, J., and Vevers, H. G., 1943–4. J. Anim. Ecol., 12: 173–213; 12: 49.
[Footnote] 52 Fleming, C. A., and Wodzicki, K. A., 1952. Notornis, 5 (2): 71, Fig. 35.
[Footnote] 53 Origin: 60.
[Footnote] 54 Bull., P. C., 1956. N.Z. Sci. Rev., 14 (5): 51–7, Fig. 2.
[Footnote] 55 Origin: 69.
[Footnote] 56 Muller, H. J., cited by de Beer (ref. 58).
minds if we compare it with other seeming improbabilities. Fisher has pointed out57 that improbability has a different aspect, depending on whether we consider it from the time before or the time after an event. The chance of a bisexual organism leaving at least one offspring of his own sex is about ⅝. What is the probability then that any man of the time of King Solomon, 100 generations ago, should be the ancestor of a continuous direct male line until today? Fisher considers the odds against this contingency would need 44 figures to express: he assessed the chance of winning first prize in a million-ticket lottery as almost inconceivably more probable. And yet every man living is the living proof that this contingency, highly improbable 100 generations ago, has occurred.
Many critics have argued that complex structures could not have been favoured by natural selection in their early stages of evolution. What use, for instance, were the electric organs of certain fish, now capable of powerful discharges for catching prey and for defence, in their early stages of development? Darwin pointed out that “it would be extremely bold to maintain that no serviceable transitions are possible”. In the case cited, it has since been discovered58 that weak electric discharges by certain fish function like radar equipment and are fully adaptive.
Darwin discussed at length the geographic conditions favouring the operation of natural selection; he concluded59 “that for terrestrial productions a large continental area, which will probably undergo many oscillations of level, and which consequently will exist for long periods in a broken condition, will be the most favourable for the production of many new forms of life”, an idea coming rather close to Sewell Wright's concept of rapid evolution in a widespread species whose “adaptive field” is diversified into numerous “adaptive peaks”. On the other hand, organisms living in a confined area, and exposed to less severe competition, may evolve slowly to become “living fossils”, of which Darwin cited the platypus and lungfish as examples. He illustrated such variations in the speed of evolution by the only diagram in the first edition of the “Origin”, a representation of “divergent branching lineages”.
This figure, referred to throughout the book, may be taken as the prototype of the many phylogenetic trees constructed by Darwin's successors, who proceeded (m J. S. Huxley's words60) “to plant wildernesses of family trees over the beautyspots of biology”. Darwin's description of the phylogenetic tree61 is no desert plant: “As buds give rise by growth to fresh buds, and these if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications”.
Darwin and Lamarckism
Darwin was handicapped by ignorance of what we now know of the principles of Mendelian genetics. He fell back, reluctantly, on the Lamarckian principle of change with “use and disuse”, yet many cases of the failure of this principle inclined him “to lay very little weight on the direct action of the conditions of life” in producing variation62. He reverted to the reduced wings of the Aylesbury duck and flightless birds in nature. “As the larger ground-feeding birds seldom take flight except to escape danger, I believe that the nearly wingless condition of several birds which now inhabit or have lately inhabited several oceanic islands tenanted by no beast of prey, has been caused by disuse”, a frankly Lamarckian
[Footnote] 57 Fisher, R. A., 1954. In Evolution as a Process London: Allen & Unwin: 91.
[Footnote] 58 de Beer, G., 1958. Endeavour, 17 (66): 68.
[Footnote] 59 Origin: 94.
[Footnote] 60 Huxley, J. S., 1942. Evolution, the Modern Synthesis, London: Allen & Unwin: 22.
[Footnote] 61 Origin: 113.
[Footnote] 62 Origin: 116–7.
explanation. Yet he also had a selective interpretation63 “If under changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up a useless structure” Later, in his chapter on Instinct, Darwin quite definitely rejected Lamarck's doctrine of “use and disuse”.64 “It will be thought that I have an overweening confidence in the principle of natural selection when I do not admit that such wonderful and well-established facts [on behaviour of colonial insects] at once annihilate my theory”. But his analysis of “such wonderful facts” about West African driver ants led him to “believe that natural selection, by acting on fertile parents, could form a species which should regularly produce neuters, either all of large size with one form of jaw, or all of small size with jaws having a widely different structure; or lastly, and this is our climax of difficulty, one set of workers of one size and structure, and simultaneously another set of workers of a different size and structure …” “No amount of exercise, or habit, or volition, in the utterly sterile members of a community could possibly have affected the structure or instincts of the fertile members, which alone leave descendants. I am surprised that no one has advanced this demonstrative case of neuter insects against the well-known doctrine of Lamarck”.
“The Noble Science of Geology”
One of the most masterly chapters in the “Origin” is entitled “On the Imperfection of the Geological Record”. Here Darwin's training and the strong influence of his “Lord High Chancellor” gave him confidence His two main points were that geological time was vast (this was before Kelvin pulled a red herring across the geologists' trail), that the known formations represented only fractions of each period, and that the failure of fossils to demonstrate conclusively the changes demanded by his theory was due to the imperfection of their record of the history of life. “The noble science of geology loses glory from the extreme imperfection of the record The crust of the earth must not be looked at as a well-filled museum, but as a poor collection made at hazard and at rare intervals”.65 In a prophetic discussion he hit on the idea that the observed stratigraphic range of a fossil is generally only part of its true duration, a “teilzone”, or part of its true duration or “biozone” (these terms were invented much later by paleontologists) and he concluded that some of the abrupt appearances and disappearances of fossils in the stratigraphic record of any one country are due to “a large amount of migration during faunal and other changes”.66 Since his day, when only European geology was at all well known thick continuous rock sections have provided many instances of gradations in fossils and intermediate links between successive species, but the factor of migration often causes abrupt change.
Gradual changes in geological time, interruption of the record by immigration and emigration, and the “teilzones” resulting from this history are well shown by scallops in the New Zealand Pleistocene, in the Wanganui-Rangitikei district. Here two related lineages alternate in the geological section, each lineage being represented by a slightly different population at each level.67 In one lineage (Pecten benedictus Lamarck) a succession of fairly constant populations, P. b. marwicki (Finlay), is succeeded high in the section by a very distinct form P. b. tepungai
[Footnote] 63. Origin: 128.
[Footnote] 64. Origin 205, 207, 208.
[Footnote] 65 Origin: 412.
[Footnote] 66 Origin: 250.
[Footnote] 67 Fleming, C. A., 1957. N. Z Geol Suru Pal Bull, 26: 54, Fig. 12.
Fig.. 3.—Differences in mean characters between population samples of two consecutive chrono-subspecies of scallop (Pecten) in the Castlecliffian Stage of Wanganui Basin. The population intermediate in time is intermediate in the three characters measured: A, height/length ratio B, percentage of sample with concentric lamellae on flanks. C, right valve inflation (%). (Open oblong indicates observed range; arrow heads are 2σ each side of mean.)
Fleming. But a population intermediate in time between these two forms is also intermediate in several of its characters that can be measured. “On the theory of descent,” Darwin wrote, “the full meaning of the fact of fossil remains from closely consecutive formations, though ranked as distinct species, being closely related, is obvious”.68 The consecutive Pecten populations are now ranked as subspecies, not species as they would have been classed in Darwin's time, later in life he “speculated on what nomenclature would come to and concluded it would be trinomial”69.
Many gradual transitions in fossils like the ones illustrated can now be cited to answer what Darwin thought “difficulties of the gravest nature”.
Acclimatization in New Zealand
Paleontology can only tell us of the course of evolution, not of its mechanism and causes. It is virtually impossible for the human observer to judge why many changes in animals were favoured by selection—i. e., to assess the selective valency
[Footnote] 68 Origin: 285.
[Footnote] 69. More Letters, 1: 474.
of differences. “From the extraordinary manner in which European productions have recently spread over New Zealand,” Darwin writes70 “and have seized on places which must have been previously occupied, we may believe, if all the animals of Great Britain were set free in New Zealand, that in the course of time a multitude of British forms would become thoroughly naturalized there and would exterminate many of the natives … Yet the most skilful naturalist, from an examination of the species of the two countries, could not have foreseen the result.” Few of the British mammals now abundant in New Zealand (hedgehog, rabbit, stoat, ferret, for example) had been introduced when Darwin wrote, and the succeeding century of acclimatization has been a rather sad and expensive fulfilment of his prediction.
In his chapters on geographic distribution Darwin summarised a mass of data, accumulated since “Beagle” days, to show that the bond of inheritance explains the distribution of related animals “Broken or interrupted range” (which implies former continuity) “may often be accounted for by the extinction of the species in the intermediate regions,”71 often, as Darwin pointed out72 due to geographical and climatic changes in recent geological time. The New Zealand scallops mentioned earlier are a case in point five living subspecies of Pecten benedictus are now known between the Red Sea and Hawaii, separated from each other by wide areas where no relatives are known73 to testify to their former continuity, a still wider distribution in the Pleistocene is shown by extinct populations in the Mediterranean and New Zealand Similarly (if we may change phyla abruptly) the Gannet (Sula serrator Linn.) is now distributed in three discrete populations, differentiated into well-marked subspecies, in the North Atlantic, in South African seas, and in the Tasman, but in this case there is no fossil evidence to elucidate the history of the separated populations.
Alpine Plants and Pleistocene Glaciation
The demonstration by Louis Agassiz and others that central Europe and North America had suffered under an Arctic climate at quite a recent geological date enabled Darwin to explain the discontinuous distribution of many alpine plants and
[Footnote] 70, Origin: 286.
[Footnote] 71 Origin: 391.
[Footnote] 72, Origin: 391.
[Footnote] 73 Fleming, op. cit.: Fig. 4.
animals, on distant “mountain summits separated from each other by hundreds of miles of lowlands”. At the onset of a glacial period, he explained, temperate inhabitants retreated and were supplanted by Arctic ones, until a uniform Arctic fauna and flora covered the central parts of Europe. As the snow and ice retreated, Arctic forms followed the melting snow up the mountains “When the warmth had fully returned, the same Arctic species which had lately lived in a body together on the lowlands …… would be left isolated in distant mountain-summits (having been exterminated on all lesser heights) and in the Arctic regions.”74 Although the physical signs of glaciation in New Zealand have long suggested a similar history for our alpine plants it is only recently that concrete evidence of the former lowland distribution of mountain plants has been available. New Zealand paleobotanists, in the first instance Mr. D. R. McQueen, have, for example, found leaves and seeds of the alpine toatoa Phyllocladus alpinus and, apparently, a variety of Astelia linearis (now restricted to high altitudes) almost at sea level near Wellington, in deposits studied by J. W. Brodie75—deposits which Cotton and Te Punga had already diagnosed, on quite independent evidence, as the result of frozen-soil processes during a periglacial climate.76 Thanks to nuclear science, we now know that these alpine plants were living on the motorway near Porirua Harbour in a period of glacial advance about 20,800 years ago.
Darwin knew of the “direct evidence of former glacial action in New Zealand”, and believed that “the same plants found on widely separated mountains in this island, tell the same story” of former lowland distribution during the Glacial Period.77.
[Footnote] 74 Origin: 312.
[Footnote] 75 Brodie, J. W., 1957. N. Z. J. Sci, Tech., B38:· 623–43. Owing to Mr. McQueen's departure from New Zealand, the paleobotanical evidence has not yet been adequately documented. Miss R. Mason (Botany Division, D.S.I.R) has recently confirmed that seeds of Astelia linearis Hook. f. var. linearis are present in sample N 160/502, Tawa Flat (N.Z. Geol. Surv. B542).
[Footnote] 76 Cotton, C. A., and Te Punga, M. T., Trans. Roy Soc. N.Z., 82: 1001–31.
[Footnote] 77 Origin: 316
Fig. 6.—Lowland distribution of alpine plants during a glacial phase of the Late Pleistocene (about 20,000 years ago). Crosses show localities for fossil remains of two plants (Astelia aff linearis and Phyllocladus alpinus) of which the living representatives now live in the upper cold temperate and sub-polar belts (stippled and black) of the southern Tararua Range where P. alpinus is recorded only from Mt. Omega (O) and Mt. Renata (R).
Darwin and Hutton
A whole lecture could be devoted to the influence of Darwin's writings on a century of New Zealand biologists, and I make no attempt to cover the field. One of the first to accept his views was a young army officer, Lieutenant F. W. Hutton, 24 years of age, freshly returned from service in the Crimea and Indian Mutiny, who wrote a review of the “Origin”78 four years before he sold out of the army to migrate to New Zealand. Hutton's review was published at a time when many reactions to the “Origin” were similar to that of Bishop Wilberforce, who asked “Is it credible that all favourable varieties of turnips are tending to become men?”79 Hutton later described80 his conversion to Darwinism after a geological excursion to the Isle of Wight with Ramsay, Director of the Geological Survey of Great Britain, which led to his review On its appearance, Darwin wrote to Hutton:81 “You have done the subject a real service by the highly original, striking, and condensed manner in which you have put the case”, and to Hooker82 “on Lieutenant Hutton's Review (who he is I know not); it struck me as very original. He is one of the very few who see that the change of species cannot be directly proved, and that the doctrine must sink or swim according as it groups and explains phenomena. It is really curious how few judge it this way …” (Most took G. K. Chesterton's attitude that change of one species into another was unlikely because “nobody, so to speak, ever caught them at it”). Several months later he wrote to Lyell:83. “I am pleased that you approve of Hutton's review. It seemed to me to take a more philosophical view of the manner of judging the question than any other review”. Hutton continued to think on Darwinian lines
[Footnote] 78 Hutton, F. W., 1861. The Geologist, 4: 132–6, 183–8.
[Footnote] 79 Quart Rev., July, 1860.
[Footnote] 80 Hutton, F. W., 1899. Darwinism and Lamarckism, London: 32–3.
[Footnote] 81 More Letters, 1: 183.
[Footnote] 82 Life and Letters, II: 362
[Footnote] 83 Life and Letters, 1: 193.
for the rest of his fruitful life, and wrote a small volume, “Darwinism and Lamarckism” in 1899, embodying the most significant advances since Darwin's death.
New Zealand Pioneers
Von Haast, in Canterbury, was also an early convert who supplied Darwin with data on New Zealand glaciation and summarized the “Origin” in his inaugural address of the Canterbury Philosophical Institute.84 Darwin urged the Institute “to record the rate and manner of spreading of European weeds and insects, and especially to observe what native plants most fail”85 He also wondered if Haast was author of a “really well done dialogue on Natural Selection in a New Zealand paper”. Poor von Haast's wit was not up to that of the real author of the piece Samuel Butler “discovered the leading idea of his life in New Zealand”. That idea was evolution, and he found it in the “Origin”86 for which (he wrote to Darwin with a gift of Erewhon) he could “never be sufficiently grateful”87 but later in life he was to attack the “Origin” and its author with self-righteous bitterness. To Butler's anti-Darwinism (and Lamarckism) we must ascribe George Bernard Shaw's similar prejudices, and, according to Bertrand Russell88 certain tendencies in Soviet biology during the past decade (although I suspect they may also be attributed to ecclesiastical criticism by the Orthodox Church in Tsarist times89).
An unexpected correspondent of Darwin was Canon James Stack, of Kaiapoi, whose help was asked90 in getting data from the Maoris for The Expression of Emotions in Man and Animals (1872). We must also remember Darwin's influence on Cheeseman, whose studies on fertilisation of Pterostylis91 were stimulated by Darwin's book on orchids92 and were cited in the second edition.
Then we have Buller, with his rather uninspired Illustrations of Darwinism, presented to the Wellington Branch as a delayed Presidential Address93 His successor as president was an army officer, Major-General Schaw, who was not a Darwinian fellow-traveller, and who took as his text94 a sentence from G. V. Hudson's Manual of New Zealand Entomology that our New Zealand water-beetle is “only what a ground-beetle might naturally become if forced to lead an aquatic existence”. Schaw stated that his imperfect experience led him “to believe that any ground-beetle now forced to lead an aquatic existence … will not become a swimming-beetle, but, if it cannot get out, will inevitably become a dead beetle”; enthusiastic Darwinians have frequently stimulated similar reactions to ellipses in their logic.
No student of barnacles, fossil or living, in New Zealand or elsewhere, can avoid indebtedness to Charles Darwin's painstaking monographs95 which would have kept his name immortal in zoology even if he had never written about evolution. Several New Zealand fossil and living barnacles bear the names Darwin gave them.
[Footnote] 84 von Haast, H. F., 1948. Life and Times of Sir Julius on Haast, Wellington: 227.
[Footnote] 85 von Haast, op. cit.: 293.
[Footnote] 86 Irvine, W., 1955. Apes, Angels, and Victorians, Weidenfeld and Nicholson: 221.
[Footnote] 87 Jones, H. F., 1919. Samuel Butler, Author of Erewhon: A Memoir, Macmillan, 1: 157.
[Footnote] 88 Russell, B., 1956. Portraits from Memory, London: Allen & Unwin, 73–4.
[Footnote] 89 Kline, G. L., 1955. Darwinism and the Russian Orthodox Church Continuity and Change in Russian and Soviet Thought, Harvard University Press.
[Footnote] 90 von Haast, op. cit.: 515
[Footnote] 91 Cheeseman, T. F., 1873. Trans. N. Z. Inst., 5: 352–7.
[Footnote] 92 Darwin, C., 1863. The Various Contrivances by which Orchids are Fertilized by Insects London Murray.
[Footnote] 93 Buller, W., 1895. Trans. N. Z. Inst., 27: 75–104.
[Footnote] 94 Schaw, 1894. Proc. N. Z. Inst., 26: 640.
[Footnote] 95 Darwin, C., 1851–54. A Monograph on the Subclass Cirripedia, Ray Society.
The Science of Genetics
Since Darwin's death, in my opinion there have been two major developments in evolutionary theory: in genetics, and in understanding of speciation. Despite his immense study of breeding, inheritance, and evolution by artificial selection, Darwin knew little of the laws of inheritance. As late as 1899, Hutton, in New Zealand, wrote96 “the mystery… still confronting us as inscrutable as ever… is now called transmission of characters”. The rediscovery of Gregoi Mendel's work at the turn of the century was followed by a brilliant integration of Mendel's laws with the advancing science of cytology until it was established “as firmly as Newton's laws of motion or the atomic theory, that hereditary resemblances are determined by discrete particles, the genes, situated in the chromosomes of the cells… which conform to distribution patterns known as Mendelian inheritance”97. At first selectionists rejected mutations as the raw material of evolution, as the first mutations discovered were often deleterious and showed discontinuous steps instead of gradual variation. Likewise geneticists rejected selection because they knew their mutations were the source of hereditable variation, and that they arose suddenly apparently without the need for selection. But as studies progressed, most mutations were found to exert only slight effect and the interaction of gene mutations and recombinations in the gene complex, itself the result of long selection, provides just the supply of variation required for Darwinian evolution. I am no geneticist, and will leave the subject at this point, except to quote once more an eminent zoologist in relation to the proposition sometimes put forward that speed of evolution rates could be affected by increasing mutation rates through the effects of nuclear radiation. The mutation rate is slow: a given gene mutates in one in about half a million individuals (in organisms as different as bacteria, maize, fruit flies, and man). “It is also clear that this rate is itself the result of selection, and that although seemingly slow, it has been adequate to provide the requisite basic hereditable variation… on which selection has worked to produce whatever evolution has taken place. In other words, mutation not only need not but must not be more rapid than a slow rate.”98
Geographic Isolation and Speciation
The other major advance has been the gradual realization that in the general case (with the exception of the origin of new species in some plants by polyploidy), speciation, that is to say, the splitting of one species into two separate species, has always been preceded by some form of spatial. generally geographic, isolation, such as is entailed in the formation of geographic races. Some of Darwin's earliest thoughts on evolution arose from observation that “the several islands of the Galapagos Archipelago are tenanted… in a quite marvellous manner, by very closely related species; so that the inhabitants of each separate island, though mostly distinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world”99. As early as 1844 he concluded100 “that those areas in which species are most numerous have oftenest been divided and isolated from other areas, united and again divided…” and “that isolation is the chief concomitant or cause of the appearance of new forms”. Yet in the “Origin” he allows the possibility of what we would now call “sympatric speciation”, and declares101 “I believe that many perfectly defined species have been formed on
[Footnote] 96 Hutton, op. cit.: 60.
[Footnote] 97 de Beer, G., 1958. Endeavour, 17: 66.
[Footnote] 98 de Beer, op. cit.: 87.
[Footnote] 99 Origin: 339.
[Footnote] 100 Life and Letters, II: 28.
[Footnote] 101 Origin: 150.
strictly continuous areas” so that he was left with the major difficulty why organisms do not all intergrade—the problem J. S. Huxley102 has termed “how discontinuity of groups is introduced into the biological continuum”. The subject was later touched on by A. R. Wallace103, who asked Darwin, “How do species ever arise except when a variety is isolated?” but the question was rhetorical. The same year Moritz Wagner, in Germany, insisted on isolation by geographic barriers, and in 1876 Darwin admitted to Wagner, “I do not believe that one species will give birth to two or more species so long as they are mingled together in the same district104”. Two years later105 he coined the word “specification” (“speciation” of modern terminology) for the process contrasted with “phyletic evolution”. Hutton, in New Zealand, clearly saw that this emphasis on isolation was the foundation of the new Darwinism, so that he wrote106: “In addition to selection, isolation is shown to be necessary for organic evolution; and this, in my opinion, is the only real advance since Darwin's death”.
The flowering of genetics at the beginning of the century led to long neglect of the factor of geographic isolation until it was revived by Rensch, and more strongly, by Ernest Mayr in 1940, in a paper107 which I personally rank as a milestone in my thinking, expanded in book form in 1942108. In the same year J. S. Huxley produced his “Evolution, the Modern Synthesis”, in which he still failed to give geographic isolation its due place in speciation; the triumph to Mayr's ideas came in 1943 when Huxley accepted and incorporated them in the American edition of his text book.
The process of geographic speciation can be seen in progress wherever local populations, more or less isolated, have diverged in response to local differences in selective pressure. Geographic races, now often designated by trinomial names as Darwin predicted, occur in every group of organism and differ in physiology and ecology as well as morphology109. A simple case of speciation occurs when the
[Footnote] 102 Huxley, J. S. 1940. The New Systematics, Clarendon Press: 2
[Footnote] 103 More Letters, 1: 294.
[Footnote] 104 Life and Letters, III: 159.
[Footnote] 105 Life and Letters, III: 160.
[Footnote] 106 Hutton, op. cit.: 5, 9.
[Footnote] 107 Mayr, E., 1940. Amer. Naturalist, 74: 249–78.
[Footnote] 108 Mayr, E., 1942. Systematics and the Origin of Species, Columbia Univ. Press.
[Footnote] 109 Mayr, E., 1947. Evolution, 1 (4): 263–88.
same stock has invaded an isolated area on two or more occasions; divergence of the early invaders has led to the development of some form of ecological, behavioural, physiological, or seasonal isolation so that the new arrivals do not blend. Two notably cases at our own back door are the two species of white-eye formerly on Noifolk Island (a third final invasion having arrived about 50 years ago) and at Lord Howe Island, and the two parakeets of New Zealand derivation occupying Antipodes Island110. A further much publicised example is the formation of a ring of subspecies in the large gulls of the northern circum-polar shores—close relatives of our black-backed gull. Each of the forms grades into its neighbour, from Europe eastward round the Arctic Ocean to North America Western Europe has been occupied by races from opposite ends of this chain, which there behave as almost totally different species, the herring and lesser blackbacked gulls These two, though connected by intergrading forms (or subspecies) are themselves isolated, not only in plumage, but in voice, breeding season and migration habits, and very rarely hybridise. Since “there is no geographic speciation that is not at the same time ecological and genetic speciation”111 owing to the differences of selection pressure in different environments, there are good chances that two geographic races will diverge during their separation to an extent that when they meet through “double invasion” or “back invasion” or breakdown of barriers, they will prove better adapted to separate ecological niches in their common habitat112. When two daughter stocks meet again after geographic isolation they commonly show such ecological differences, which are subsequently accentuated by selection operating on the principle that no two species can survive if they occupy an identical ecological niche (Gause's Law). So we find the two white-eyes of Lord Howe and the two parakeets of Antipodes Island differ in size and, I suspect, in food requuements, the two British gulls in habits and the 14 species of Darwin's finches of the Galapagos, the result of multiple repetition of this process, have diverged to occupy a great variety of ecological positions in the Galapagos Archipelago113. Such “adaptive radiation” is the process whereby, by analogy, Hutton supposed the moas became diversified after an early archipelagic stage in the history of New Zealand, so that they are now classed by Oliver in 7 genera and 27 species114.
[Footnote] 110 Fleming, C. A., 1952. N.Z. Sci. Rev., 10: 86.
[Footnote] 111 Mayr, op. cit., 1947: 285.
[Footnote] 112 Lack, D., 1947. Darwin's Finches, Cambridge: Univ. Press: 137–40.
[Footnote] 113 Lack, op. cit.: 102.
[Footnote] 114 Oliver, W. R. B., 1949 Dom. Mus. Bull., 15.
Fossil Evidence of Allopatric Speciation
Seldom can we find fossil evidence to illustrate the beginnings of the speciation process, but the history of the cockle genus Bassina in the New Zealand Tertiary perhaps illustrates a species (B. speighti) which occupied most of New Zealand in the Oligocene and Miocene splitting to form two forms (B. parva and B. yatei) which were allopatric geographic races in the Pliocene, eventually becoming sympatric in about the Early Pleistocene before the extinction of B. parva. A marked size differentiation suggests the presence of ecological differences necessary for sympatric existence In view of the imperfection of the geological record, with Bassina as with other fossils, I must add, quoting Darwin115 that it is “no more likely that I should be right in nearly all points, than that I should toss up a penny and get heads twenty times running”.
Fig. 9.—Fossil evidence of geographic speciation in bivalve Mollusca. Bassina speighti (A, Mid-Tertiary) was succeeded in the Pliocene by two forms, C and B, at first strictly allopatric in the north and south of New Zealand, but later overlapping as sympatric species (B. parsa and B. yatei) in central districts.
Nature and the Philosopher
Darwin once advised a novice writer, “with a book, as with a fine day, one likes to end with a glorious sunset”116. This thought may have prompted the final paragraph of the Origin “It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to
[Footnote] 115 Life and Letters, II: 240.
[Footnote] 116 More Letters, 1: 238.
reflect that these elaborately constructed forms. have all been produced by laws acting around us…. There is a grandeur in this view of Life, with its several powers, having been originally breathed into a few forms or into one; and that, while this planet has gone cycling on according to the fixed laws of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.”117
This frank, but somewhat self-conscious philosophy was penned for the critical Victorian reader who was soon to read it.
I have preferred to end with two quotations that may help to show more adequately than I could hope to express in my own words the role that a belief in organic unity and organic change has played in the thoughts and feelings of naturalist philosophers like Darwin, Wallace, and G. V. Hudson, whose memories we honour tonight, since long before Darwin's time.
The first extracts, by the poet-philosopher Goethe (who died while Darwin was on the Beagle), were translated by T. H. Huxley in the first number (1869) of the periodical “Nature”, but were written about 1778.
“Nature! We are surrounded and embraced by her: powerless to separate ourselves from her, and powerless to penetrate beyond her.
“She is ever shaping new forms what is, has never yet been: what has been, comes not again. Everything is new, and yet nought but the old.
“The one thing she seems to aim at is Individuality; yet she cares nothing for individuals. She is always building up and destroying.
“She is the only artist; working up the most uniform material into utter opposites.
“Each of her works has an essence of its own; and yet their diversity is in unity.
“She changes for ever and ever, and rests not a moment. Her steps are measured, her exceptions rare, her laws unchangeable.
“She has divided herself that she may be her own delight. She causes an endless succession of new capacities for enjoyment to spring up, that her insatiable sympathy may be assuaged.
“The spectacle of Nature is always new, for she is always renewing the spectators Life is her most exquisite invention; and death is her expert contrivance to get plenty of life.
“She separates all existences, and all tend to intermingle. She has isolated all things in order that all may approach one another.
“Everyone sees her in his own fashion. She has brought me here and will also lead me away. I trust her.”
Thus the pre-Darwinian philosopher. Huxley predicted that the vision of the poet would remain as a truthful and efficient symbol of the wonder and mystery of Nature long after the theories of the philosophers were obsolete. Now poetry expresses the reality of feelings, whereas the philosophy of science endeavours to establish the reality of verifiable general laws. Darwin was too good a scientist to let his feelings obtrude much into his publications, but he did so in his personal notebook of 1837 (22 years before the “Origin”) when he began to feel the meaning of his early conclusions on the relationship of animals and man, and scribbled for his own use a note118 that will be my final quotation because it expresses simply, almost naively, a naturalist's love and sympathy for the life he studies.
“If we choose to let conjecture run wild,” he wrote, “then animals, our fellow brethren in pain, disease, death, suffering, and famine—our slaves in the most laborious works, our companions in our amusements—they may partake our origin on one common ancestor—we may be all melted together.”
[Footnote] 117 Origin 414–5
[Footnote] 118 Life and Letters, II 6.