378 MB. Q. J. ROMANES ON PHYSIOLOGICAL SELECTION. 



the evolution of species. The cases to which I allude are those 

 where specific types have been modified by the agencies of what 

 Mr. Spencer calls " direct equilibration." Of these agencies the 

 most important that happen to be known to us are use and disuse. 

 A little thought will show that the moulding power of these 

 agencies on specific types must be quite as independent of physio- 

 logical selection as it is of natural selection. But a little more 

 thought will show that this moulding influence must always be 

 in some one line of morphological change : it cannot proceed in 

 many diverging ways at once ; but must slowly transmute a whole 

 specific type into some other specific type. Now, i£ this change 

 should happen to go on in a portion of a species living in one part 

 of the world, when that portion becomes transmuted into a different 

 specific type, there is no reason why the now modified descendants 

 should prove barren when crossed with their unchanged, or dif- 

 ferently changed, parent-form, which may be still living in any 

 other part of the world. 



In view of all these considerations, I should regard it as a 

 serious objection to my theory if it could be shown that sterility 

 between allied species is invariably absolute, or even if it could be 

 shown that there are no cases of fertility unimpaired. What my 

 theory would expect to find is exactly what we do find, namely, 

 a considerable majority of instances where sterility occurs in all 

 degrees, with comparatively exceptional instances where secondary 

 distinctions have been able to develop without being associated 

 with the primary distinction. 



On the whole, therefore, I cannot but candidly consider that all 

 the facts relating to the sterility of natural species are just what 

 they ought to be, if they have been in chief part due to the principle 

 which I am advocating. Mr. Darwin appears to have clearly 

 perceived that there must be some one principle serving to explain 

 all these facts, so curiously related and yet so curiously diverse ; 

 for he says, and he says most truly, " We have conclusive evidence 

 that the sterility of species must be due to some principle quite 

 independent of natural selection." And I trust enough has now 

 been said to show that, in all probability, this hitherto unnoticed 

 principle is the principle of physiological selection. 



Aegument i-egm the Peevention oe Inteeoeossing. 

 This argument is the same from whatever cause the prevention 

 of intercrossing may arise. Where intercrossing is prevented by 



