M. Q, J. EO MANES OK PHYSIOLOGICAL SELECTION'. 361 



have to give of incompatibility in a state of nature is derived 

 chiefly from species, as I will now explain. 



3. Species. — According to the general theory of evolution, 

 which in this paper is taken for granted, the distinction between 

 varieties and species is only a distinction of degree ; and the dis- 

 tinction is mainly, as well as most generally, that of mutual 

 sterility, whether absolute or partial. Therefore I am here sup- 

 plied with an incalculable number of instances, all tending to 

 support my theory ; for in so many instances as variation has led 

 to any degree of sterility between parent and varietal forms, or 

 between the varying descendants of the same form, in so many 

 instances it is a simple statement of fact to say that physiological 

 selection must have taken place. There remains, however, the 

 question whether the particular change in the reproductive system, 

 which led to all these cases of mutual sterility, was anterior or 

 posterior to changes in other parts of the organisms. For, if it 

 was anterior, these other changes — even though they be adaptive 

 changes — were presumably due to the sexual change having 

 interposed its barrier to crossing with parent forms ; while, if the 

 sexual change were posterior to the others, the presumption 

 would be that it was those other changes which, by their reaction 

 on the reproductive system, induced the sexual change. I shall 

 have to consider this alternative later on. Meanwhile, therefore, 

 it is enough to point out that under either possibility the prin- 

 ciples of physiological selection are present ; only these principles 

 are accredited with so much the more causal influence in the pro- 

 duction of species in the proportion that we find reason to 

 suppose the sexual change to have been, as a rule, the prior 

 change. Hence, under either alternative, and on the datum that 

 species are extreme varieties, we have presented many millions of 

 instances of fertility within the varietal form, with sterility 

 towards allied forms. Why, then, should we feel any difiiculty in 

 supposing that the same thing happens in a lesser degree ? Nay, 

 rather, would it not be a most extraordinary fact if it did never 

 happen in lesser degrees ? Tet, if it does ever happen in lesser 

 degrees, we have a variation of the kind required by physiological 

 selection, although not yet of a degree sufficient to constitute the 

 variety a new species — seeing that species is practically a name 

 reserved by naturalists to designate this particular kind of 

 variation, when it has arrived at a certain observable degree of 

 departure from the parent form. 



