366 ME. a. J. EOMAISTES OK PHYSIOLOGICAL SELECTION". 



antecedently probable. I will, therefore, next proceed to state 

 such evidence as I have been able to collect, tending to show that 

 the facts of organic nature are such as we should expect they 

 ought to be, if it is true that physiological selection has played a 

 considerable part in their causation. And to do this I will begin 

 by taking the three cardinal objections to the theory of natural 

 selection with which I set out, namely sterility, intercrossing, 

 and inutility. For, as we shall see — and this in itself is a sugges- 

 tive consideration — all the facts which here present formidable 

 obstacles to the theory of natural selection are not only explained 

 by the theory of physiological selection, but furnish to that 

 theory some of the best evidence which I have been able to find. 



Argument feom Sterility between Species. 



As now repeatedly observed, the theory of natural selection is 

 not, properly speaking, a theory of the origin of species : it is a 

 theory of the development of adaptive structures. Only if species 

 always differed from one another in respect o£ adaptive structures 

 would natural selection be a theory of the origin of species. 

 But, as we have already seen, species do not always, or even 

 generally, thus differ from one another. In what, then, do they 

 differ ? They differ, first, chiefly and most generally, in respect 

 of their reproductive systems ; this, therefore, I will call the 

 primary difference. Next, they differ in an endless variety of 

 more or less minute details of structure, which are sometimes of 

 an adaptive character, and sometimes not. These, therefore, I 

 will call secondary differences. Now, these secondary differences, 

 or differences of minute detail, are never numerous as between 

 any two allied species ; in almost all cases they admit of being 

 represented by units. Yet, if it were possible to enumerate all 

 the specific differences throughout both the vegetable and animal 

 kingdoms, there would be required a row of figures expressive of 

 many millions. Or, otherwise stated, the secondary features 

 which serve to distinguish species from species are minute differ- 

 ences of structure, sometimes useful and sometimes not, which 

 may occur in any parts of organisms, but which never occur in 

 many parts of the same organism. Thus we perceive that, if we 

 have regard to the whole range of species, what I call the secon- 

 dary differences are in the highest imaginable degree variable or 

 inconstant. The only distinction which is at all constant or 

 general is the one which I call primary, or the one which belongs 



