MB. G. J, EOMANES OIN" PHYSIOLOGICAL SELECTIOIT. 341 



In the first place, differences of type in nature are by naturalists 

 classified as differences of species, principally because they are 

 found to be mutually sterile. Thus it is but circular reasoning 

 to argue that all natural species are shown by nature herself to 

 differ from artificial varieties in presenting this peculiarity of 

 mutual sterility ; for it is mainly in virtue of presenting this 

 peculiarity that they have been classified as species. The real 

 question, therefore, that stands to be considered is simply this : 

 Why should the modifications of organic types supposed to have 

 been produced by natural selection have so frequently and 

 generally led to mutual sterility, when even greater modifications 

 of such types known to have been produced by artificial selection 

 continue to be mutually fertile ? 



In the next place, the distinction in question is not absolute. 

 On the one hand, some few domesticated varieties, when 

 crossed with one another, exhibit a more or less marked degree 

 of sterility ; and, on the other hand, a large number of wild 

 species, when crossed with one another, exhibit fertility, and 

 this in all degrees. So that the distinction between natural 

 species and artificial varieties in respect of fertility is, as a matter 

 of fact, not absolute, but breaks down in both its parts. 



Nevertheless, although this distinction is not absolute, it is 

 undoubtedly, and as a general rule, valid. That is to say, it is 

 unusual or exceptional to find complete fertility between natural 

 species, and it is still more so to find even partial sterility between 

 artificial varieties. Therefore, notwithstanding his success in 

 showing that there is no absolute distinction between species 

 and varieties in this respect, Mr. Darwin plainly peceived that 

 there still remained a relative distinction of a most general and 

 important kind. In order to mitigate the severity of this dis- 

 tinction, he furnished elaborate proof of the following facts. 



1st. That with natural species the cause of sterility lies exclu- 

 sively in differences of the sexual system. 



2nd. That the conditions of life which occur under domesti- 

 cation tend to enhance fertility, and this to such an extent as to 

 render the domesticated descendants of mutually sterile species 

 mutually fertile, as in the case of our domesticated dogs. 



Now, these two facts undoubtedly help to explain why the 

 great changes of organic types produced by artificial selection 

 have not resulted in superinducing mutual sterility ; but they do 

 not appear to throw any light at all on the question, why it is 



