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. I saw, also, that
the preservation in a state of nature of any occasional deviation of
structure, such as a monstrosity, would be a rare event; and that, if at
first preserved, it would generally be lost by subsequent intercrossing
with ordinary individuals. Nevertheless, until reading an able and
valuable article in the "North British Review" (1867), I did not
appreciate how rarely single variations, whether slight or strongly
marked, could be perpetuated. The author takes the case of a pair of
animals, producing during their lifetime two hundred offspring, of
which, from various causes of destruction, only two on an average
survive to pro-create their kind. This is rather an extreme estimate
for most of the higher animals, but by no means so for many of the lower
organisms. He then shows that if a single individual were born, which
varied in some manner, giving it twice as good a chance of life as that
of the other individuals, yet the chances would be strongly against its
survival. Supposing it to survive and to breed, and that half its young
inherited the favourable variation; still, as the Reviewer goes onto
show, the young would have only a slightly better chance of surviving
and breeding; and this chance would go on decreasing in the succeeding
generations. The justic