Darwin's Artificial and Natural Selection 103 



striking differences between species in nature is their infer- 

 tility, and the infertility of their offspring when intercrossed. 

 This is a very general rule, so far as we know. In regard to 

 the different races of domesticated forms, the most significant 

 fact is that, no matter how different they may be, they are 

 perfectly fertile inter se. In this respect, as well as in others, 

 there are important differences between domesticated races 

 and wild species. The further difference, that has been 

 pointed out by a number of writers, should also not pass 

 unnoticed, namely, that the domestic forms differ from each 

 other in the extreme development of some one character, and 

 not in a large number of less conspicuous characters, as is the 

 case in wild species. 



These considerations show that, interesting and suggestive 

 as are the facts of artificial selection, they fail to demon- 

 strate the main point for which they are used by Darwin. 

 With the most rigorous attention to the process of artificial 

 selection, new species comparable in all respects to wild ones 

 have not been formed, even in those cases in which the 

 variation has been carried farthest (where the history of the 

 forms is most completely known). 



There is another point on which emphasis should be laid. 

 If by selecting the most extreme forms in each generation 

 and breeding from them the standard can be raised, it might 

 appear that we could go on indefinitely in the same direction, 

 and produce, for instance, pigeons with legs five metres long, 

 and with necks of corresponding length. But experience has 

 shown that this cannot be done. As Darwin frequently re- 

 marks, the breeder is entirely helpless until the desired varia- 

 tion appears. It seems possible, by selecting the more extreme 

 of the fluctuating variations in each generation, that a higher 

 plane of variation is established, and even that more extreme 

 forms are likely to arise for a few generations ; but, even if 

 this is the case, a limit is soon reached beyond which it is 

 impossible to go. 



