Chap. VII.] THEORY OP NATURAL SELECTION. 265 



from each other in single characters, but in many parts; 

 and he asks, how it always comes that many parts of 

 the organisation 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 trans- 

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

 had been simultaneously developed. The best answer, 

 however, to the above objection is afforded by those 

 domestic races which have been modified, chiefiy 

 through man's power of selection, for some special pur- 

 pose. 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 transfor- 

 mation, — 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 at- 

 tributed partly to the principle of correlated growth, and 

 partly to so-called spontaneous variation. 



A much more serious objection has been urged by 

 Bronn,and recently by Broca, namely, that many charac- 

 ters appear to be of no service whatever to their pos- 



