The tendency to catch rats rather than mice is known
to be inherited. Now, if any slight innate change of habit or of structure
benefited an individual wolf, it would have the best chance of surviving
and of leaving offspring. Some of its young would probably inherit the
same habits or structure, and by the repetition of this process, a new
variety might be formed which would either supplant or coexist with the
parent-form of wolf. Or, again, the wolves inhabiting a mountainous
district, and those frequenting the lowlands, would naturally be forced to
hunt different prey; and from the continued preservation of the individuals
best fitted for the two sites, two varieties might slowly be formed. These
varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that, according to
Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill
Mountains in the United States, one with a light greyhound-like form, which
pursues deer, and the other more bulky, with shorter legs, which more
frequently attacks the shepherd's flocks.
It should be observed that in the above illustration, I speak of the
slimmest individual wolves, and not of any single strongly marked variation
having been preserved. In former editions of this work I sometimes spoke
as if this latter alternative had frequently occurred. I saw the great
importance of individual differences, and this led me fully to discuss the
results of unconscious selection by man, which depends on the preservation
of all the more or less valuable individuals, and on the destruction of the
worst.
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