Chap. YI. AFFECTED BY NATUKAL SELECTION. 193 



ent countries often have to struggle for their own subsistence, 

 and are exposed to a certain extent to natural selection, and 

 individuals witli sliglitly-dillerent constitutions would succeed 

 best under dilTerent climates. A good observer states tliat in 

 cattle susceptibility to the attacks of flies is correlated with 

 color, as is the liability to be poisoned by certain plants ; so 

 that even color would be thus subjected to the action of nat- 

 ural selection. Other observers are convinced that a damp 

 climate affects the growth of the hair, and tliat with the hair 

 the horns are correlated. Mountain-breeds alwaj-s differ from 

 lowland-breeds ; and a mountainous country would probably 

 affect the hind-limbs fmm exercising them more, and possibly 

 even the form of the jielvis ; and then, by the law of homolo- 

 gous variation, the fiont-liml)s and the head would probably be 

 affected. Tlie shape, also, of the pelvis might affect by press- 

 ure the shape of certain parts of the young in the womb. 

 The laborious breathing necessary in high regions would, we 

 have reason to believe, increase the size of the chest ; and 

 again- correlation would come into play. The effects on the 

 whole organization of lessened exercise, together with abun- 

 dant food, is probably still more important ; and this, as H. von 

 Nathusius has lately shown in his excellent Treatise, is appar- 

 ently one chief cause of the great modilication which the 

 breeds of swine have undergone. But Ave are far too ignorant 

 to speculate on the relative importance of the several known 

 and unknown causes of variation ; and I have made these 

 remarks only to show that, if Ave are unable to account for the 

 characteristic differences of our domestic breeds, which never- 

 theless are generally admitted to have arisen through ordinary 

 generation from one or a few parent-stocks, we ought not to 

 lay too much stress on our ignorance of the precise cause of 

 the sliglit analogous differences between species. I might 

 have adduced for this same purpose the dilferences between 

 the races of man, which are so strongly marked ; I may add 

 that some light can apparently be thrown on these differences, 

 through sexual selection of a particular kind, but without 

 entering on full details my reasoning Avould appear frivolous. 



Uillliarian Doctrine howfac true: JJcauti/ /tow acquired. 



The foregoing remarks lead me to say a few Avords on the 

 protest, lately made by some naturalists, against th«> utilitarian 

 doctrine that CA'cry detail of structure has been produced for 

 9 



