Chap. IV . 
NATURAL SELECTION. 
91 
to keep tlie best dogs without any thought of modifying 
the breed. 
Even without any change in the proportional numbers 
of the animals on which our wolf preyed, a cub might 
be born with an innate tendency to pursue certain kinds 
of prey. Nor can this be thought very improbable; for 
we often observe great differences in the natural ten¬ 
dencies of our domestic animals; one cat, for instance, 
taking to catch rats, another mice ; one cat, according to 
Mr. St. John, bringing home winged game, another hares 
or rabbits, and another hunting on marshy ground and 
almost nightly catching woodcocks or snipes. The ten¬ 
dency to catch rats rather than mice is known to be in¬ 
herited. 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 sup- 
]dant 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 preserva¬ 
tion 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 pur¬ 
sues deer, and the other more bulky, with shorter legs, 
which more frequently attacks the shepherd’s flocks. 
Let us now take a more complex case. Certain plants 
excrete a sweet juice, apparently for the sake of elimi¬ 
nating something injurious from their sap: this is 
