ve 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 sexual 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 constitute 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 _sexual_
colouration: it could apply only to colouration as affected by
physiological functions common to both sexes. Yet it is in order to
furnish a "preferable substitute" for Mr. Darwin's theory of _sexual_
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.