Art. III—On Hybridization, with reference to variation in Plants.
[Read before the Wellington Philosophical Society, July 28, 1863,].
Amongst the plants indigenous to the Middle Island of New Zealand, there are none which range more widely, both in altitude and in latitude, nor which present a greater amount of variation, than the veronicas Indeed, as Dr. Hooker remarks, in the notes to the Conspectus of this genus, published in his “Handbook of the New Zealand Flora,” so numerous are the intermediate forms between very distinct looking species, as to render the species excessively difficult of discrimination, and to compel the adoption, for the purposes of an interim classification, of purely arbitrary characters founded on “prominent prevalent differences only.” Dr. Hooker, in a communication to myself, in reference to a large and varied collection of specimens which I forwarded to him in the early part of 1864, whilst he was engaged in compiling the Handbook, remarked on the possibility that the variation referred to might be due to natural hybridization, and asked me whether I thought this was the case. In reply, I expressed an opinion against the supposition, and the following paper contains the substance of the grounds urged by me against it.
Before, however, entering upon the special question under discussion, I will venture to call attention to the two principal theories now prevalent respecting the origin of the various species of organic life, found within particular areas. The first is, that the surface of the globe, at an early period, became divided into a number of great areas of population, each of which contained a distinct fauna and flora, distinguishable by characteristics proper to that particular area only; and that the various species now found within it, have, from time to time, been since created in order to supply the place of representative species which have died out. The second is, that every group of organisms has a purely derivitive origin, and that each existing species is but the modified descendant, preserved by means of natural selection, of some other species: whilst, probably, in most cases, so great a divergence has taken place from the original type, as to transgress the conventional circle which we draw round generic type, and induce us to refer it to some other genus than that to which it would, originally, have been assigned. It has been well observed, that if the first of these theories be true, all attempts to trace the origin of present and past faunas and floras must necessarily be futile; for their origin would be sufficiently elucidated in the dogma that “they were created on the spot,” and that such a theory would render palæontology a useless study, and reduce it. to a mere leviathan catalogue of fossils. Notwithstanding, therefore, the “weighty difficulties which surround the theory of natural selection” (as observed by the great expounder of that theory), I have found it necessary, in order to account for the facts which have passed under my own comparatively limited observation, to assume, as a postulate, the derivative origin of species; and to accept, with little reservation, the explanations afforded by the theory of “descent with modification by means of natural selection.” Now, at the time I wrote to Dr. Hooker, I had, partly as the result of reading, and partly from observation, arrived at certain opinions on the subject of plant variation, which may be stated in the following propositions.
1st, That certain classes of plants exhibit a greater tendency than others to acquire modifications, as the result of changes in the conditions of life.
2nd, That variation resulting from this cause may be sudden, or may result slowly from the operation of this cause acting continually and regularly, upon the same species, in the same locality.
3rd, That the acquired modifications will be transmitted to posterity, whether acquired suddenly or slowly.
4th, That, under domestication, variation exhibits itself the more readily, because the plant is usually subjected to a more rapid succession of changes in the conditions of life, many of which are specially applied analogically, in order to produce some particularly desired result.
In arriving at these general conclusions, I assumed that the fertilized ovule is essentially a bud, and that it embodies to the fullest extent, and is a perfectly natural expression of all the incidents, if I may use the term, for the time being acquired by the parent plant. The actual condition of the fertilized ovule and bud, has recently been the subject of a provisional hypothesis by Mr. Darwin, termed Pangenesis, in his great work on the subject of Variation under Domestication, in which he
supposes that the whole organization, in the sense of every separate atom or unit, reproduces itself—that each ovule or bud, as the case may be, is an absolute epitome of the whole parent organism, or, to use his own words, “includes and consists of a multitude of germs thrown off from each atom or unit of the organism.” The assumption which I ventured to make in my communication to Dr. Hooker, had reference, however, to the offspring of a single species only.
In the production of hybrids, or the offspring of two different, though allied, species, on the other hand I conceive that a violence is done. That an attempt is made to fuse together two organisms, each of which has, for the time being, acquired all the modifications necessary to enable it to continue the struggle for existence, under the particular conditions of life to which it is exposed, even though ultimately destined to disappear before some more powerful type. The result of such an union, therefore, would be, that the fertilized ovule would produce a plant par-taking of the character of both parents, accompanied by a tendency to eleminate the characters belonging to the one or the other, in order to resume a normal homogeneous structure, whilst this tendency would, as it appears to me, be most readily and successfully ministered to by fertilizing the ovule of the hydrid, with pollen from one of the parents. I conceive that, unless so fertilized, the chances are infinitely great against the production of fertile ovules at all. In cases where fertile ovules happen to be produced by the self fertilization of hybrid plants, I assume that the pollen engaged in the act of fertilization is either normal, or so nearly so, in structure, as to effect fertilization almost as perfectly as pollen from one of the parent plants would do, and that such normal, or nearly normal, pollen, as well as a sufficiently perfect stigmatic surface, are produced as the result of the tendency before alluded to to revert to the parental type.
In fact, I take it to be a law of nature, that modifications acquired either suddenly or slowly as the results of the operation of external causes, become essentially and naturally incorporated into the organism, and will, the conditions of life remaining the same, be transmitted to the offspring, whilst it is equally a law of nature that the offspring arising from the union of organisms generically allied, but specifically distinct, tends to throw off the peculiar characteristics derived from the one parent or the other. I conceive also that unless this tendency ot reversion be aided or ministered to by impregnating the ovule with perfectly normal pollen from one of the parents, the hybrid would rarely produce ovules capable of fertilization.
Now, in applying the above views to the case of the New Zealand veronicas, I take into consideration the perculiar physical characters of the country.
The Middle Island contains a mountain district some hundreds of miles long, by fifty or sixty broad, washed by the sea on three sides, and yet attaining, on a base of twenty or thirty miles, a summit level in many parts above the limits of perpetual snow. This mountain district is composed of a great variety of rocks, and is broken up in a manner probably without precedent on the surface of the globe, at any similar elevation above sea level. For example, the Waiau river, flowing in a valley, separated by a chain of mountains only five or six miles through,
from the nearly parallel valley of the river Clarence, has, for about forty miles of its course, an uniformly lower level of nearly twelve hundred feet. Besides this difference in altitude, the western side of the Waiau valley is bounded by the Spencer mountains, fully nine thousand feet high, sheltering it from the north-west winds, which break, with terrific violence, over the lower mountain ridge, dividing it, on its eastern side, from the valley of the Clarence. The climates of these two valleys are consequently perfectly dissimilar. The dividing ridge, however, is covered on both sides with alpine forms of senecio, dracophyllum, veronica, gaultheria, pimelia, etc., amongst the shrubs, and of celmisia, grasses, sedges, etc., amongst the herbaceous plants. This is, certainly, a somewhat extreme case, and I only quote it in order to show within what narrow limits, large differences may exist in the climatal and other conditions to which the same plants may be exposed. Besides being so much broken, these mountain ranges are usually very steep, and their flanks are furrowed by innumerable streams. Land slips are frequent, and indeed every facility exists for the transport, not only of seeds, but also of growing plants from higher to lower levels.
It may easily be understood that these peculiarities in the mountain chains referred to, give rise, all over the region, to great variations of soil and climate, and (putting out of question the extreme case above quoted), the differences, in these respects, which are ordinarily observed between the northern and southern slopes rising from valleys having an east and west course, and the eastern and western slopes of valleys running north and south, are very great indeed. The Hurunui valley, for example, runs nearly east and west, and whilst on the one side there is a comparatively rich vegetation, including in the wooded gullies up to two thousand five hundred feet, such ferns as lomaria vulcamia, asplenium Hookerianum, and hymenophyllum scabrum, on the other side, at the distance of only a mile or so, far below this altitude, we find nothing but stunted alpine growth.
As I have before observed, the veronicas especially have a very wide range, both lateral and ascending, sundry forms of this plant being found all over the Middle Island, from sea level up to great altitudes. I find that the purely alpine forms, at alpine elevations, vary very little indeed preserving everywhere an almost perfect similarity in their prominent prevalent characters. I find, moreover, that those species which affect low altitudes vary also but little, and that in fact it is only in the intermediate zone, amongst the innumerable gullies and valleys which occur in the mountain districts, that any great number of varieties are found.
In such valleys and gullies we see clumps of veronicas sometimes upwards of an acre in extent, at times composed of but one species, and at others consisting of several species, all the plants of each species, however, presenting a perfect identity in general appearance and structure, or, at all events, only exhibiting such small anomalies as constantly occur in the separate plants of any dominant species, without the necessity of supposing them to result from hybridization.
Moreover, each of the several species thus found in society, will also be found growing separately, in widely distant localities which happen to possess similar climatal and other physical conditions, and always presenting the same general appearance.
I therefore believe that even should a complete suite of all the veronicas found in this country, exhibit a gradual and almost imperceptible passage from one extreme to the other, this may properly be referred to the fact that we have, condensed within the narrow area of the Middle Island, a variety of geological and of resulting climatal and other physical conditions only to be found in an immensely wider area elsewhere, and that hence all the observed varieties may in great measure be assigned to the modifying influences of varying external causes.
The above observations will apply equally to pimelias, veronicas, celmisias, and epilobiums, at least. In none of these have I seen that tendency to “sport” which results from hybridization under domestication. In all, where the external conditions are the same, I observed a nearly perfect identity in the more prominent specific characters of the plants, however distant the localities in which they may be found, and whether associated with other species or not, and for these reasons I ventured to express my opinion, that the varieties observed did not result from hybridization.
The foregoing was the general argument used by me in addressing Dr. Hooker on the subject under discussion, and I may now add that although we cannot, consistently with observed facts, and with the laws fairly deducible from those facts, reject hybridization as one of the agents concerned in the production of new forms in a state of nature, we are not warranted in assuming that it is an active agent. That, on the contrary, we are rather justified in believing that except under domestication, hybridization plays but a very small part in producing permanent modifications of structure.