THEOR T OF NA TUBAL SELECTION, 20 i 



with th© parent species? If both have become fitted for 

 slightly different habits of life or conditions, tbuy might 

 live together; and if we lay on one side pol^morpliic 

 species, in which the variability seems to be of a peculiar 

 nature, and all mere temporary variations, such as size, 

 albinism, etc., the more permanent varieties are generally 

 found, as far as lean discover, inhabiting distinct stations, 

 such as high land or low laud, dry or moist districts. 

 Moreover, in the case of animals which wander much 

 about 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 

 organization 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 vari- 

 ations, if slight, first in one part and then in another; and 

 as they would be transmitted all together, they would ap- 

 pear 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, chiefly 

 through man^s power of selection, for some special purpose. 

 Look at the race and dray-horse, or at the greyhound and 

 mastiff. Their whole frames, and even their mental char- 

 acteristics, have been modified; but if we could trace eauli 

 step in the history of their transformation — and the hitter 

 steps can be traced — we should not see great and simulta- 

 neous changes, but first one part and then another slightly 

 modified and improved. Even when selection has been ap- 

 plied 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 tlie 

 flower, fruit, or leaves, has been greatly changed, almost 

 all the other parts have been slightly modified. This may 

 be attributed 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 ciiaracters 



