360 ME. G. J. TtOMANES 01^ PHYSIOLOGICAL SELECTIOK. 



yellow and white varieties, or between blue and red varieties of 

 tbe same species, while eacb continues fertile within its own 

 limits. And similarly in all the other cases. 



Now, in these facts one may only see evidence of changes in 

 the organism reacting on the reproductive system in such a way 

 as to produce this particular effect. I shall have more to say on 

 this subject later on ; here it is enough to remark that it matters 

 little to my theory whether the changes be thus due to reac- 

 tion on the reproductive system, or have arisen in the reproduc- 

 tive system, as it were, independently ; for, as above observed, 

 whether the causes of the change be supposed intrinsic or extrinsic, , 

 the change itself is really all that we are now concerned with. 

 This change, however produced, is a change in the direction of what 

 I call racial incompatibility, and therefore, if it had taken place in 

 any wild species, must necessarily have constituted a physiological 

 barrier to intercrossing between the two varieties, which, according 

 to my theory, is the primary condition required for the develop- 

 ment of varieties into species. And that such a state of matters 

 is at least as likely to occur in a wild species as in a domesticated 

 descendant is obvious. For domestication, as a rule, increases 

 fertility, and therefore is, as a rule, inimical to sterility, some- 

 times even breaking down the physiological barriers between 

 natural species. Therefore, if at other times even under domes- 

 tication the reproductive system may vary so as to erect these 

 barriers between artificial varieties, much more are such barriers 

 likely to be erected between varieties when these arise in a 

 state of nature. Indeed, the difficulty is to find such cases in a 

 state of domestication, the great difference between mongrels and 

 hybrids consisting in this very fact of the former being so 

 usually fertile, and the latter so usually sterile. But I trust that 

 enough has now been said to show that even among our domestic 

 productions we may find evidence of racial incompatibility, or of 

 that particular variation in the reproductive apparatus which is 

 required by the theory of physiological selection. 



As regards varieties in a state of nature, it must be noticed, 

 first of all, that racial incompatibility is not likely to be observed. 

 For, on the one hand, if such incompatibility is in any degree 

 pronounced, for this very reason the two forms would be ranked 

 by naturalists as distinct species ; while, on the other hand, if not 

 so pronounced, the fact of incompatibility could only be revealed 

 by careful observation. For these reasons the evidence which I 



