Art. XVIII.—On the Origin of a New Species by Isolation.
[Read before the Philosophical Institute of Canterbury, 1st November, 1916; received by Editors, 30th December, 1916, issued separately, 16th August, 1917.]
In April, 1907, Captain J. Bollons, of the Government s.s. “Hinemoa,” brought back from the Great King Island a number of large snails, some containing the live animal, which I described under the name of Placostylus bollonsi (12, vol. 40, 1908, p. 340, pl. xxv; 11, p. 763, pl. 30, figs. 11, a, b). I pointed out that this species offered a splendid example of the origin of a new species by isolation, and I now propose to go more fully into this interesting subject.
The genus Placostylus occurs with us in the northern part of the North Island only, which points to its immigration from the north, and, geologically speaking, most likely in comparatively recent times. Placostylus is found in New Zealand, Lord Howe Island, New Caledonia, Solomon Islands, Fiji, and a stray species in New Guinea. Hedley (4, p. 335) considers this Placostylus area, New Guinea excluded, as a zoological province, a unity explicable only on the theory that these islands form portions of a scattered continent and are connected by shallow banks formerly dry land. This continental area he calls the “Melanesian Plateau” (later named “Antarctica” by Forbes). New Caledonia is, with about thirty-five species of Placostylus, the metropolis of the genus, and some of its species are closely allied with those of Lord Howe Island and New Zealand. Subfossil, one species is known from Koutoumo Island, New Caledonia (P. sessilis Gassies), and two from the Loyalty Group; post-Tertiary, one from Lord Howe Island; and P. hongii ambagiosus Sut. is found in large numbers subfossil on Cape Maria van Diemen.
According to Hedley (6, p. 98), Heurteau was the first to recognize the geological connection between New Caledonia and New Zealand, and the subject was reviewed by Crosse (1, p. 443). I give here a translation of some of his remarks: “Considered from a geological standpoint, New Caledonia is a direct continuation of New Zealand, despite the enormous expanse of sea now separating the two countries. We find the same Triassic, Jurassic, and Cretaceous formations, disposed in similar order, containing analagous fossils, and there is an abundance of eruptive rocks.” There also exists a close relationship between the animals and plants of New Caledonia and New Zealand. In the Recent fauna there is the same absence of all mammals, except bats and rats, and the same poverty of reptiles. Snakes are absent from both, and the Amphibia consist of one in New Zealand and none in New Caledonia. There is a close correspondence between Melanopsis, Placostylus, Rhytida, Athoracophoridae, the Charopa group of Endodonta, and some of the Phenacohelicidae, Rhytidopsis being nearly allied to our Phenacohelix, and Monomphalus to our Allodiscus. The land Mollusca of Lord Howe Island have a close affinity to those of New Caledonia (7, p. 403).
Speaking of the Great Glacier Epoch of New Zealand, Hutton (8, pp. 176, 182) comes to the conclusion that an elevation of 3,000 ft. to 4,000 ft. would be sufficient to produce the phenomena of the period of glaciation. In the older Pliocene came the last great upheaval, all the islands were joined together, and the land stretched away to the south, east, and north, reaching, in a north-westerly direction, Lord Howe Island. A bank submerged to a depth of about 1,000 fathoms extends from New Zealand to Lord Howe Island, and represents the former Pliocene land extension.
These short geological and faunistic considerations clearly indicate where we have to look for the home of the ancestors of our Placostyli, which is beyond a doubt Lord Howe Island, and indirectly New Caledonia. In 1854 Gaskoin described Placostylus bivaricosus (13, p. 25, pl. 12, figs. 1–4) from Lord Howe Island, a species which is closely allied to the New Zealand P. hongii, which had the name of the Maori chief Hongi Ika (born 1787; died 1828) given to it by Lesson. The resemblance of the two species is so great that Hedley was fully justified in writing, “This species (P. bivaricosus) speaks eloquently of a recent land connection extending on the one side to New Caledonia, on the other to New Zealand.” (3, p. 140). Etheridge holds that the nearest allies in New Caledonia are P. caledonicus Petit and P. porpliyrostomus Pfr. (2, p. 132). As already mentioned. Lord Howe Island has a post-Tertiary Placostylus, which is found embedded in coral-sand rock of aerial, not sedimentary, origin, and which was called P. bivaricosus solidus by Etheridge (10, p. 27; 2, p. 131). This variety is distinguished by the peculiarity of the peristome; the outer and inner lip broaden, exposing repeated laminae of growth; the callosity on the body-whorl thickens greatly, supporting strong tubercles and emarginations. We must, of course, conclude that this variety is the forerunner of P. bivaricosus.
Now, we have in New Zealand a shell which, though somewhat larger, corresponds very well with the subfossil Lord Howe variety—P. hongii ambagiosus, which I described and figured in 1906 (11, p. 768). This subspecies is found subfossil, but also still living, and this no doubt represents the form which reached New Zealand from Lord Howe Island when the former stretched far out to the north-west. On Lord Howe Island also occurs a variety, P. bivaricosus etheridgei Brazier, which is a large, thin, elongated form, with a simple, very little thickened peristome, which is not notched (10, pl. v, figs. 1, 2, 7, 8). New Zealand possesses a similar
example, the P. hongii ambagiosus gradually losing its incrassate notched peristome, the shell getting somewhat thinner, forming the species known as P. hongii; and the farther these snails have gone from the north southwards the more simple the outer lip has grown. However, we must not forget that the species of Placostylus are polymorphic, and variations in the characters of the shell are not rare. Why heavy, massive snails should go on slowly evolving lighter and more simply constructed shells I cannot explain; the fact only remains.
I beg leave to bring forward here a hypothesis regarding a similar development in marine shells. The genus Pugnellus Conrad has nearly the whole of its shell covered by a thick calcareous deposit, so that only a small part of it can be seen. Species of this genus are found in the Cretaceous of Austria, southern India, North America, North Africa, Borneo, Chile, Patagonia, and in the older Tertiary of New Zealand (Wangaloa). We have in New Zealand the genus Conchothyra Hutton, which is nearly allied to Pugnellus, but has a tremendous calcareous covering, hiding nearly the whole shell. This I consider as an extreme form of Pugnellus, and it is peculiar to the Cretaceous and older Tertiary of New Zealand. Pugnellus is followed by Tylospira, which has part of the body-whorl and spire covered with a thick enamel. There is one Recent species in Australia; it also occurs in the Miocene of Australia and New Zealand. At last we come to the most simple genus of the series, Struthiolaria, which has only a thin broad inner lip, and a thickened sinuous outer lip. It is found in the Cretaceous, Eocene, and Oligocene of Patagonia; Miocene of Australia; Miocene, Pliocene, and Recent in New Zealand; &c. I am well aware that Cossmann includes Pugnellus in the family Strombidae, and Tylospira with Struthiolaria in the family Struthiolariidae. Zittel, however, unites them all in the former family. My opinion is that we have to class all these genera either under Strombidae or under Struthiolariidae, and I am more in favour of the latter, as I consider Pugnellus to be the ancestor of Tylospira and Struthiolaria.
To return to our subject. Lesson and Martinet, in their work Les Polynésiens, say that Bulimus hongii was plentiful amongst flax-bushes near the North Cape, and that Bulimus vibratus was abundant on the Three Kings. The latter species (Placostylus fibratus Martyn) is a somewhat related New Caledonian species, and our P. hongii was sometimes, though wrongly, going under that name. However, the use of two specific names shows that at one time it was known that the Three Kings harboured a species distinct from that on the mainland, but the curious thing is that not a single collection in New Zealand contained an example of the Great King Placostylus. If I had not made a note of Lesson and Martinet's statement, had not asked Captain Bollons to look for Placostylus on the Great King, and if he had not been successful in finding it, this interesting shell would still be unknown.
The characteristic features of Placostylus bollonsi are its rather thin large shell, its simple aperture, its network-like sculpture, the large obtuse broadly rounded apex or protcconch with its peculiar ornamentation, the total absence of a spermatheca in the animal, and the large calcareous egg, which involves a large embryonic shell. The thimble-shaped large protoconch with its fine oblique axial riblets distinguish P. bollonsi from all the other species of the genus.
When Placostylus reached New Zealand from the north-west the Three Kings were united with what is now the North Island, and must have been populated by the same species we still find in the north, P. hongii ambagiosus,
which so closely resembles P. bivaricosus solidus. It is generally admitted by geologists that the separation of the Three Kings took place at about the time when Cook Strait came into existence. The depth of the sea between the Three Kings is estimated at about 40 to 50 fathoms (12, vol. 23, p. 420). Mr. P. G. Morgan, Director of the New Zealand Geological Survey, kindly informed me that “probably the late Pliocene was a period of elevation in New Zealand, and during early Pleistocene subsidence began. By the middle of the Pleistocene much of New Zealand was some hundreds of feet below its present level. This is shown by raised beaches on many parts of the coast-line.”
No trace of a shell resembling the Placostylus of the mainland has been found on the Great King, and it is evident that P. bollonsi is the result of a slow and gradual evolution from a form resembling P. hongii ambagiosus which has been going on ever since the isolation of the Three Kings.
Wherever Placostylus occurs it is gregarious, and our P. hongii is known to lay a considerable number of calcareous eggs—twenty to thirty, or even more. P. bollonsi was found by Captain Bollons in one place only, under dead leaves of a grove of karaka-trees, and the decomposed leaves of that tree form its chief food. The specimens I kept alive for some time never touched a fresh, green karaka-leaf. There is no doubt that the supply of food was limited, and this will assist us to solve the question how the new species was evolved. We only know the kind of snail that was living on the Great King when separation from the mainland took place, and the result of the evolution—P. bollonsi. What went on during the thousands of years we do not know, but I may be allowed to propose a hypothesis. There is no doubt that if the snails went on laying the comparatively large number of eggs in the restricted area of their food-supply there would be some day overcrowding, and the struggle for existence would begin. Those who kept on producing a large number of offsprings might see them starving, but those who began to reduce the number of eggs would be better off. The laying of fewer eggs may have been accompanied by an increase in their size, and so this process may have been going on and on till the large-sized shell of P. bollonsi was evolved. This snail lays, no doubt, only one egg, measuring 13 mm. by 18 mm., whereas the eggs of P. hongii measure only about 5 mm. by 6 mm. That the laying of only one egg has been going on for a very long period is proved by the fact that the spermatheca or receptaculum seminis of the animal was completely lost, not even a rudiment of it could I find in the five specimens I dissected. It had become useless, there being only one egg to fertilize. P. hongii, on the other hand, has a very well developed spermatheca.
To give an idea of the long time that must have passed since the isolation of the Three Kings, I quote here the estimate based on the radioactivity of the earth's crust—one million years since the Quaternary Period, which, of course, is considerably younger than the Pleistocene. I do not belong to those who think this to be correct, as I very much doubt whether it will ever be possible to calculate the age of the earth in a manner approaching accuracy. I merely mention it to show what a long space of time we have to reckon with, and if we take it to be only one hundred thousand years, that allows ample time to comprehend the evolution of this species.
I mentioned the fact that the species of Placostylus are living together in considerable numbers, and yet Captain Bollons found only about a dozen live snails. He has been in the locality several times since to look for more
specimens, but found only dead shells, and he considers the species to be now extinct. That a species laying only one egg would not increase in number is evident, yet Lesson and Martinet say that in the last century these snails were abundant on the Great King. I cannot find any record that there are rats on the island. These, of course, would dispose of some snails. Then we have to remember that Maoris were at one time living on the Great King, and they may have eaten a good number of these snails, but I am unable to say whether Maoris ever used Placostylus as an article of food. Remembering the terrible wreck of the “Elingamite” at 10 a.m. on the 9th November, 1902, and that a party of those saved had to live without food for several days on this island, I think it is possible that some person or persons may have discovered these snails and eaten all those they found. It would gladden my heart if I knew this had been the case.
The most interesting question, however, is, How did the snails know that they produced too many eggs? How did they perceive that laying fewer eggs, increasing most likely their size, and finally sticking to one egg only, would be their salvation? We are only too ready to underestimate the mental faculties of the lower animals, and there is no likelihood that we shall ever get an approximately correct knowledge of what is going on in their small brains. We cannot deny that they possess mind, which means will and intelligence, and may be described as accumulated consciousness and accumulated faculty. And a still more interesting fact is that thousands and thousands of years back the Placostylus on the Great King practised Malthus's “principle of population.” There is nothing new under the sun.
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