Page:The Zoologist, 4th series, vol 4 (1900).djvu/362

332 instances cited by Mr. Distant are quite inappropriate as illustrations of his fundamental suggestions, though possibly they might be brought forward in support of a theory that assimilative colouration has been sporadically developed from recent geological times down to the present day.

On glancing through the quotations given, one is at once struck by the practically complete absence of anything in the way of experimental evidence as to the direct action of the environment, and this is the more strange seeing that such evidence is actually available. It will be sufficient to refer to the splendid series of exhaustive experiments made by Prof. Poulton upon the influence of both food and surrounding colours on the larvæ and pupæ of various Lepidoptera. These experiments have been carried out with the most scrupulous care and exactitude, and the conclusions which can be drawn from them are worth scores of hasty observations and occasionally inaccurate generalizations made by travellers and sportsmen. It would be beside the point to enter into details as to the results obtained by Prof. Poulton, since we are only discussing the possibility of the survival of primæval assimilation; but it may be mentioned that these experiments definitely prove that certain of these organisms possess, in varying degrees, the capacity of being modified in colour by the direct action of different light-waves; and, further, I consider it is sufficiently demonstrated that this capacity can only be satisfactorily explained by the theory of natural selection, and not by any theory of purely internal or external causes.

But to resume: with reference to the attempt to explain the crimson colour in the wings of the African Touracos (Turacus and Gallirex) as being due to the occurrence of copper in the districts which they frequent, I quite agree with the objections raised by Dr. Bowdler Sharpe. In my experience of these beautiful birds, both in Natal and Mashonaland, I can find no connection between their distribution and the occurrence of coppers; and besides, the fact, noted by Mr. Distant, that they have been known to moult several times in captivity in England without impairing the brilliancy of their feathers, appears to entirely negative any such direct connection. Moreover, no attempt is made to explain why it is only these birds which are affected in this curious way; or why, in them, the effects are confined to a limited number of feathers.

It is noticeable that wherever phenomena are explained upon the theory of natural selection a detailed explanation of the "why and wherefore" is always expected, whereas exponents of theories of purely environmental causes are so frequently content to confine themselves to the widest of generalities.