Darwin, and After Darwin Volume I Part 20

Wallace argues, the immense tufts of golden plumage in the bird of paradise owe their origin to the fact that they are attached just above the point where the arteries and nerves for the supply of the pectoral muscles leave the interior of the body--and the physiological rationale is not altogether obvious,--are there no other birds in which similar arteries and nerves are found in a similar position? Why have these no similar tufts? And why, in the birds of paradise themselves, does it require four years ere these nervous and arterial influences take effect upon the plumage?

Finally, one would inquire how the colour is determined and held constant in each species. The difficulty of the Tylor-Wallace view, even as a matter of origin, is especially great in those numerous cases in which the colour is determined by delicate lines, thin plates, or thin films of air or fluid. Mr. Poulton, who takes a similar line of argument in his _Colours of Animals_ (p. 326), lays special stress on the production of _white_ (pp. 201-202).

As regards the latter point, it may be noticed that not in any part of his writings, so far as I can find, does Mr. Wallace allude to the highly important fact of colours in animals being so largely due to these purely physical causes. Everywhere he argues as if colours were universally due to pigments; and in my opinion this unaccountable oversight is the gravest defect in Mr. Wallace's treatment both of the facts and the philosophy of colouration in the animal kingdom. For instance, as regards the particular case of s.e.xual colouration, the oversight has prevented him from perceiving that his theory of "brilliancy" as due to "a surplus of vital energy," is not so much as logically possible in what must const.i.tute at least one good half of the facts to which he applies it--unless he shows that there is some connection between vital energy and the development of striations, imprisonment of air-bubbles, &c. But any such connection--so essentially important for his theory--he does not even attempt to show.

Lastly, and quite apart from these remarkable oversights, even if Mr.

Tylor's hypothesis were as reasonable and well-sustained as it is fanciful and inadequate, still it could not apply to _s.e.xual_ colouration: it could apply only to colouration as affected by physiological functions common to both s.e.xes. Yet it is in order to furnish a "preferable subst.i.tute" for Mr. Darwin's theory of _s.e.xual_ colouration, that Mr. Wallace adduces the hypothesis in question as one of "great weight"! In this matter, therefore, I entirely agree with Poulton and Lloyd Morgan.