THE FACTORS OF ORGANIC EVOLUTION. 109 



there is little to choose between them. Mr. Spencer's 

 words show that he attributes, if not half, still not far 

 off half the modification that has actually been pro- 

 duced, to use and disuse. Erasmus Darwin does not 

 say whether he considers use and disuse to have 

 brought about more than half or less than half ; he 

 only says that animal and vegetable modification is 

 " in part produced " by the exertions of the animals 

 and vegetables themselves ; the impression I have 

 derived is, that just as Mr. Spencer considers rather 

 less than half to be due to use and disuse, so Erasmus 

 Darwin considers decidedly more than half — so much 

 more, in fact, than half as to make function unques- 

 tionably the factor most proper to be insisted on if 

 only one can be given. Further than this he did not 

 go. I will quote enough of Dr. Erasmus Darwin's 

 own words to put his position beyond doubt. He 

 writes : — 



" Thirdly, when we enumerate the great changes 

 produced in the species of animals before their 

 nativity, as, for example, when the offspring repro- 

 duces the effects produced upon the parent by acci- 

 dent or culture, or the changes produced by the mix- 

 ture of species, as in mules ; or the changes produced 

 probably by exuberance of nourishment supplied to 

 the foetus, as in monstrous births with additional 

 limbs ; many of these enormities are propagated and 

 continued as a variety at least, if not as a new species 

 of animal. I have seen a breed of cats with an addi- 

 tional claw on every foot; of poultry also with an 

 additional claw and with wings to their feet ; and of 



