It cannot be doubted, I think, that Mr. Darwin has satisfactorily proved
that what he terms selection, or selective modification, must occur,
and does occur, in nature; and he has also proved to superfluity that
such selection is competent to produce forms as distinct, structurally,
as some genera even are. If the animated world presented us with none
but structural differences, I should have no hesitation in saying that
Mr. Darwin had demonstrated the existence of a true physical cause,
amply competent to account for the origin of living species, and of man
among the rest.
But, in addition to their structural distinctions, the species of
animals and plants, or at least a great number of them, exhibit
physiological characters--what are known as distinct species,
structurally, being for the most part either altogether incompetent to
breed one with another; or if they breed, the resulting mule, or
hybrid, is unable to perpetuate its race with another hybrid of the
same kind.
A true physical cause is, however, admitted to be such only on one
condition--that it shall account for all the phenomena which come
within the range of its operation.
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