Mules and other hybrids are freely
produced between very distinct species, but are themselves infertile or
quite sterile; 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.
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