This theory is, also, strengthened by some few other facts in regard
to instincts; as by that common case of closely allied, but certainly
distinct, species, when inhabiting distant parts of the world and
living under considerably different conditions of life, yet often
retaining nearly the same instincts. For instance, we can understand
on the principle of inheritance, how it is that the thrush of South
America lines its nest with mud, in the same peculiar manner as does
our British thrush: how it is that the male wrens (Troglodytes) of
North America, build "cock-nests," to roost in, like the males of our
distinct Kitty-wrens,--a habit wholly unlike that of any other known
bird. Finally, it may not be a logical deduction, but to my
imagination it is far more satisfactory to look at such instincts as
the young cuckoo ejecting its foster-brothers,--ants making
slaves,--the larvae of ichneumonidae feeding within the live bodies of
caterpillars,--not as specially endowed or created instincts, but as
small consequences of one general law, leading to the advancement of
all organic beings, namely, multiply, vary, let the strongest live and
the weakest die.
CHAPTER 8. HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication.
Laws governing the sterility of hybrids.
Sterility not a special endowment, but incidental on other
differences.
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