ilO CONCLUDING EEMABKS. Chap. XXVHI. 



modifications of structure ; and various general reasons could 

 be assigned against such a belief. 



On the other hand, we have abundant evidence of the 

 constant occurrence under nature of slight individual differ- 

 ences of the most diversified kinds ; and we are thus led to 

 conclude that species have generally originated by the natural 

 selection of extremely slight differences. This process may 

 be strictly compared with the slow and gradual improvement 

 of the racehorse, greyhound, and gamecock. As every detail 

 of structure in each species has to be closely adapted to its 

 habits of life, it will rarely happen that one part alone will 

 be modified ; but, as was formerly shown, the co-adapted 

 modifications need not be absolutely simultaneous. Many 

 variations, however, are from the first connected by the law of 

 correlation. Hence it follows that even closely-allied species 

 rarely or never differ from one another by one character 

 alone ; and the same remark is to a certain extent applicable 

 to domestic races ; for these, if they differ much, generally 

 differ in many respects. 



Some naturalists boldly insist x that species are absolutely 

 distinct productions, never passing by intermediate links into 

 one another ; whilst they maintain that domestic varieties 

 can always be connected either with one another or with their 

 parent-forms. But if we. could always find the links between 

 the several breads of the dog, horse, cattle, sheep, pigs, &c, 

 there would not have been such incessant doubts whether 

 they were descended from one or several species. The grey- 

 hound genus, if such a term may be used, cannot be closely 

 connected with any other breed, unless, perhaps, we go back to 

 the ancient Egyptian monuments. Our English bulldog also 

 forms a very distinct breed. In all these cases crossed breeds 

 must of course be excluded, for distinct natural species can 

 thus be likewise connected. By what links can the Cochin 

 fowl be closely united with others ? By searching for breeds 

 still preserved in distant lands, and by going back to 

 historical records, tumbler-pigeons, carriers, and barbs can 

 be closely connected with the parent rock-pigeon ; but wa 



1 Godron, ' De l'Espece,' 1859, torn. ii. p. 44, &c. 



