THEORY OF NATURAL SELECTION 221 



of natural selection, can a variety live side by side with the 

 parent species? If both have become fitted for slightly dif- 

 ferent habits of life or conditions, they might live together; 

 and if we lay on one side polymoi-phic species, in which the 

 variability seems to be of a peculiar nature, and all mere 

 temporary variations, such as size, albinism, &c., the more 

 permanent varieties are generally found, as far as I can 

 discover, inhabiting distinct stations, — such as high land or 

 low land, dry or moist districts. Moreover, in the case of 

 animals which wander much alx)Ut and cross freely, their 

 varieties seem to be generally confined to distinct regions. 



Bronn also insists that distinct species never differ from 

 each other in single characters, but in many parts ; and he 

 asks, how it always comes that many parts of the organisa- 

 tion should have been modified at the same time through 

 variation and natural selection? But there is no necessity 

 for supposing that all the parts of any being have been 

 simultaneously modified. The most striking modifications, 

 excellently adapted for some purpose, might, as was formerly 

 remarked, be acquired by successive variations, if slight, 

 first in one part and then in another; and as they would be 

 transmitted all together, they would appear to us as if they 

 had been simultaneously developed. The best answer, how- 

 ever, to the above objection is afforded by those domestic 

 races which have been modified, chiefly through man's power 

 of selection, for some special purpose. Look at the race 

 and dray horse, or at the grey-hound and mastiff. Their 

 whole frames and even their mental characteristics have been 

 modified; but if we could trace each step in the history of 

 their transformation,— and the latter steps can be traced,— 

 we should not see great and simultaneous changes, but first 

 one part and then another slightly modified and improved. 

 Even when selection has been applied by man to some one 

 character alone, — of which our cultivated plants offer the 

 best instances, — it will invariably be found that although 

 this one part, whether it be the flower, fruit, or leaves, has 

 been greatly changed, almost all the other parts have been 

 slightly modified. This may be attributed partly to the prin- 

 ciple of correlated growth, and partly to so-called spon- 

 taneous variation. 



