Crosses between the two species of
zebra, or even between the zebra and the quagga, or the quagga and the
ass, might have led to a very different result. Again, in pre-Darwinian
times it was so universally the practice to argue in a circle, and
declare that the fertility of the offspring of a cross proved the
identity of species of the parents, that experiments in hybridity were
usually made between very remote species and even between species of
different genera, to avoid the possibility of the reply: "They are both
really the same species;" and the sterility of the hybrid offspring of
such remote crosses of course served to strengthen the popular belief.
Now that we have arrived at a different standpoint, and look upon a
species, not as a distinct entity due to special creation, but as an
assemblage of individuals which have become somewhat modified in
structure, form, and constitution so as to adapt them to slightly
different conditions of life; which can be differentiated from other
allied assemblages; which reproduce their like, and which usually breed
together--we require a fresh set of experiments calculated to determine
the matter of fact,--whether such species crossed with their near allies
do always produce offspring which are more or less sterile _inter se_.
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