logical mechanism whereby
these adaptive changes of colour are produced differs in different
animals; but it is needless for our purposes to go into this part of the
subject. Again, there are yet other cases where protective colouring
which is admirably suited to conceal an animal through one part of the
year, would become highly conspicuous during another part of it--namely,
when the ground is covered with snow. Accordingly, in these cases the
animals change their colour in the winter months to a snowy white:
witness stoats, mountain hares, ptarmigan, &c. (Fig. 108.)

[Illustration: FIG. 108.--Seasonal changes of colour in Ptarmigan
(_Lagopus mutus_). Drawn from stuffed specimens in the British
Museum, 1/6 nat. size, with appropriate surroundings supplied.]

Now, it is sufficiently obvious that in all these classes of cases the
concealment from enemies or prey which is thus secured is of advantage
to the animals concerned; and, therefore, that in the theory of natural
selection we have a satisfactory theory whereby to explain it. And this
cannot be said of any other theory of adaptive mechanisms in nature that
has ever been propounded. The so-called Lamarckian theory, for instance,
cannot be brought to bear upon the facts at all; and on the theory of
special creation it is unintelligible why the phenomena of protective
colouring should be of such general occurrence. For, in as far as
protective colouring is of advantage to the species which present it, it
is of corresponding disadvantage to those other species against the
predatory nature of which it acts as a defence. And, of course, the
same applies to yet other species, if they serve as prey. Moreover,
the more minutely this subject is investigated in all its details,
the more exactly is it found to harmonise with the naturalistic
interpretation[38].

[38] Were it not that some of Darwin's critics have overlooked the
very point wherein the great value of protective colouring as
evidence of natural selection consists, it would be needless to
observe that it does so in the _minuteness_ of the protective
resemblance which in so many cases is presented. Of course where the
resemblance is only very general, the phenomena might be ascribed to
mere coincidence, of which the instincts of the animal have taken
advantage. But in the measure that the resemblance becomes minutely
detailed, the supposition of mere coincidence is