Chap. IV. NATURAL SELECTION. 91 



to keep the 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 bom with an innate tendency to pursue certain kinds 

 of prey. Nor can tins 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- 

 plant 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 



