distinct from the corresponding European species. So again many North
American forms which until lately were named as distinct species, are
now considered to be local races.) Variations may be divided into two
classes; those which appear to our ignorance to arise spontaneously,
and those which are directly related to the surrounding conditions, so
that all or nearly all the individuals of the same species are
similarly modified. Cases of the latter kind have recently been
observed with care by Mr. J.A. Allen (34. Mammals and Birds of East
Florida, also an Ornithological Reconnaissance of Kansas, etc.
Notwithstanding the influence of climate on the colours of birds, it is
difficult to account for the dull or dark tints of almost all the
species inhabiting certain countries, for instance, the Galapagos
Islands under the equator, the wide temperate plains of Patagonia, and,
as it appears, Egypt (see Mr. Hartshorne in the American Naturalist,
1873, p. 747). These countries are open, and afford little shelter to
birds; but it seems doubtful whether the absence of brightly coloured
species can be explained on the principle of protection, for on the
Pampas, which are equally open, though covered by green grass, and
where the birds would be equally exposed to danger, many brilliant and
conspicuously coloured species are common. I have sometimes speculated
whether the prevailing dull tints of the scenery in the above named
countries may not have affected the appreciation of bright colours by
the birds inhabiting them.), who shews that in the United States many
species of birds gradually become more strongly coloured in proceeding
southward, and more lightly coloured in proceeding westward to the arid
plains of the interior. Both sexes seem generally to be affected in a
like manner, but sometimes one sex more than the other. This result is
not incompatible with the belief that the colours of birds are mainly
due to the accumulation of successive variations through sexual
selection; for even after the sexes have been greatly differentiated,
climate might produce an equal effect on both sexes, or a greater
effect on one sex than on the other, owing to some constitutional
difference.
Individual differences between the members of the same species are
admitted by every one to occur under a state of nature. Sudden and
strongly marked variations are rare; it is also doubtful whether if
beneficial they would often be preserved through selection and
transmitted to succeeding generations. (35. Origin of Species fifth
edit. 1869, p.104. I had always perceived, that rare and
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