ile; and it is this infertility or sterility of the hybrids
that is the characteristic--and was once thought to be the criterion--of
species, not the sterility of their first crosses. Hence we should not
expect to find any constant infertility in the first crosses between the
distinct strains or varieties that formed the starting-point of new
species, but only a slight amount of infertility in their mongrel
offspring. It follows, that Mr. Romanes' theory of _Physiological
Selection_--which assumes sterility or infertility between first crosses
as the fundamental fact in the origin of species--does not accord with
the general phenomena of hybridism in nature.]

[Footnote 64: The exact number is 1219.51, but the fractions are omitted
for clearness.]

CHAPTER VIII

THE ORIGIN AND USES OF COLOUR IN ANIMALS

The Darwinian theory threw new light on organic colour--The
problem to be solved--The constancy of animal colour indicates
utility--Colour and environment--Arctic animals
white--Exceptions prove the rule--Desert, forest, nocturnal, and
oceanic animals--General theories of animal colour--Variable
protective colouring--Mr. Poulton's experiments--Special or
local colour adaptations--Imitation of particular objects--How
they have been produced--Special protective colouring of
butterflies--Protective resemblance among marine
animals--Protection by terrifying enemies--Alluring
coloration--The coloration of birds' eggs--Colour as a means of
recognition--Summary of the preceding exposition--Influence of
locality or of climate on colour--Concluding remarks.

Among the numerous applications of the Darwinian theory in the
interpretation of the complex phenomena presented by the organic world,
none have been more successful, or are more interesting, than those
which deal with the colours of animals and plants. To the older school
of naturalists colour was a trivial character, eminently unstable and
untrustworthy in the determination of species; and it appeared to have,
in most cases, no use or meaning to the objects which displayed it. The
bright and often gorgeous coloration of insect, bird, or flower, was
either looked upon as having been created for the enjoyment of mankind,
or as due to unknown and perhaps undiscoverable laws of nature.

But the researches of Mr. Darwin totally changed our point of view in
this matter. He showed, clearly, that some of the col