Page:The Zoologist, 4th series, vol 4 (1900).djvu/360

330 an ancient geological record, it is also in that phase of animal existence that the beginnings of colouration must have developed. It therefore seems possible that assimilative colouration may have been a first and a very general consequent in animal development; and that the subsequent protective resemblance acquired by numerous living creatures through the process of natural selection, when life had advanced to the competitive stage, is far too frequently used as an explanation for whole series of uniform phenomena in colouration, which have probably survived unaltered from remote antiquity." (Pp. 383, 384.) And again: "As adaptation implies a previous state of variation, which again predicates a more or less stable condition from which variation arose, we come to the conclusion that the pre-variable condition was a unicolorous one, and from the data—scanty indeed—at our disposal, are inclined to suggest that the unicolorous hue was originally due to assimilative colouration." (P. 471.) In other words, it is suggested that the present unicolorous hues of such organisms as green birds and caterpillars, isabelline desert animals and flat fishes, &c, are preferably to be explained on the ground that they are survivals of an assimilative colouring which was acquired in early geological times, its persistence being due not to the direct action of natural selection, but to the fact that this colouring happened, quite by chance, to be of vital importance to the animals.

Now, apart from other objections, the acceptance of such an hypothesis appears to me to land us at once upon the horns of a dilemma. Either we have to believe that these unicolorous animals have existed as we now see them since the "early stages of animal life," or we have to assume that these organisms, with their numerous ancestry, right back to the low generalised form from which they sprung in "remote antiquity," must have existed through countless ages of time and innumerable geological and climatic changes in an unchanging environment to which their primæval assimilative colouring chanced to be so well adapted, that natural selection has been quite unable to affect them in that respect throughout the entire period; although, be it noted, their structure has undoubtedly undergone, in most cases, very considerable modification.

This conclusion is to me almost as untenable as the previous one; and, as it is difficult to perceive in what other way the present phenomena of colour can be explained upon the suggestion of the survival of ancestral assimilative colouring, it seems to me that this hypothesis must fall to the ground. The fundamental error of the suggestion appears to lie in the fact that the development of colour has been regarded in a purely abstract light, and not in connection with the development of any particular animal or group of animals, as must be done in order to arrive at any reliable results.