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 zzzer 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. 
