Powers states that his observations by no means confirm Cope's view that these differences are in the main referable to variation in the completeness of metamorphosis, and on the contrary, he regards metamorphosis as on the whole a levelling process, tending to obliterate diversity. The enormous differences in size and proportions which he describes can only be appreciated by reference to his figures. They affect almost all features of bodily organisation. These striking differences he looks upon as brought about by differences in nutrition, "diversities in habitual locomotion," and diversity in the age at which metamorphosis occurs, and to sexual difference. Apart from sexual difference he regards the chief distinctions, in brief, as "acquired variations of the larva."
As an example he gives the great elongation of some of the forms as "due first to slow growth, second to the free-swimming habit, third to the prolongation of larval life, and finally to the assumption of sexual maturity as males," either in the branchiate or non-branchiate condition. He describes the rapid growth of some and the slow growth of others. A larva of intermediate type may grow about a centimeter a month, but a rapidly growing specimen may grow more than four times as much. The slower rate of growth may, he says, be induced by winter feeding, and other treatment.[20]
When, however, he goes on to describe the influences which he regards as exerted by the habit of freely swimming, I am led to wonder whether after all in most of these illustrations, the primary distinctions are not in reality genetic. "Specimens raised in the same aquarium or in similar aquaria, side by side with all conditions as uniform as it is possible to make them, seldom fail to furnish striking examples of broad-headed, short-bodied, and short-tailed types which are habitually found at the bottom, while others, slender and elongated, are free swimmers, and maintain themselves in almost as continual suspension and motion as does a gold fish." Later, again, he writes, "Yet despite the uniformity of these favourable conditions, the larvae soon began to split up into two noticeably distinct groups, the one of unusually compact proportions, the other of uniform intermediate build, such is most commonly met with." It is to my mind scarcely possible to resist the inference that, though there may be definite responses to certain conditions, yet the chief distinctions are genetic, and that it is these distinctions which confer the power to respond. The parts respectively played by cause and effect are always difficult to assign; but when it is stated that "a weak-limbed, long-bodied and long-tailed animal becomes well nigh perforce an undulatory swimmer, while the strong-limbed, short-tailed, heavy-bodied specimen, when these characteristics are rapidly forced upon it, is, under certain circumstances, just as forcibly induced to become a crawler," we feel how erroneous any estimates of causation are likely to be.
One of the most remarkable and interesting sections of Powers' paper is that in which he describes the differences in bodily structure and habits which he attributes to cannibalism, and the whole account of the phenomena should be read in the original. It appears that there are two extremely distinct types of larvae, those with narrow heads and slender bodies which live for the most part on small Crustacea such as Daphnias, and those with huge mouths and very wide heads, which disregard such small animals altogether and live on amphibian larvae, whether of their own or other species. As the illustrations show, the differences between these two types are very great, and the differences in instinct and behaviour are no less. The cannibals take no heed of the pelagic crustacea, lying sluggishly at the bottom, rousing themselves immediately to a violent attack on the larger living things which approach them. Nothing but the most incontrovertible evidence based on abundant control experiments should convince us that such differences are not primarily genetic, and in the present state of knowledge I incline to think that the families really consist of individuals which are ready to assume the cannibal habit if opportunity offers, and others which are congenitally incapable of it. It may readily be that if all chance of cannibal diet be excluded, the full development of the wide head and mouth, or the other peculiarities, would never become pronounced, but I doubt whether such change could be induced in any individual taken at random.
CHAPTER XI.
Sterility of Hybrids. Concluding Remarks.
When we consider the bearing of recent discoveries on those comprehensive schemes of evolution with which we were formerly satisfied, we find that certain details of the process are more easy to imagine. We readily now understand how varieties once formed, can persist, but at the same time difficulties hitherto faced with complacency become formidable in the light of the new knowledge. So generally is this admitted by those familiar with modern genetic research that most are rightly inclined to postpone the discussion. The premisses, indeed, on which such a discussion must be based are almost wholly wanting.
The difficulties to which I chiefly refer are not those created by the phenomena of adaptation, though they are serious enough. In treating of that subject I have felt obliged to express scepticism as to the validity of nearly all the new evidence for the transmission of acquired characters. At the present time the utmost we are bound to accept is the proof that (1) in some parthenogenetic forms variations, or perhaps we may say malformations, produced in response to special conditions, recur in one or perhaps two generations asexually produced after removal to other conditions. (2) That violent maltreatment may in rare instances so affect the germ-cells contained in the parents as to cause the individuals resulting from the fertilisation of those cells to exhibit an arrest of development similar to that which their parents underwent.