MR. G. J. ROMANES ON PHYSIOLOGICAL SELECTION. 407 



as I have myself argued (pp. 361, 399-400), upon the general 

 theory of evolution it must be accepted as a fact that, so soon as 

 varieties have diverged from their parental type sufficiently far to 

 take rank as species, some such change in the reproductive system 

 as that of sterility vfith allied forms has usually been found to 

 have occurred. Now, it is perfectly true that this is the well- 

 known fact, and, moreover, as I have previously endeavoured to 

 insist, that it is the fact which more than any other stands to be 

 explained by any theory of the origin of species. But, obviously, 

 the theory of physiological selection is something more than a 

 mere re-statement of this fact : it is an explanation of the fact in 

 terms of evolutionary pbilosophy. 



First, let it be observed that the supposed objection is not con- 

 cerned with, any question touching the validity of the evidence 

 adduced to sbow that the particular kind of variation on which 

 my theory depends does actually take place ; nor is the objection 

 concerned with any doubt as to the extent in which this varia- 

 tion may have operated in the origination of species. On the 

 contrary, the objection goes upon the ground of accepting all 

 the evidence which I have adduced upon tbese points, and then 

 representing that, granting it all, it merely amounts to are-state- 

 ment of fact. Well, let the evidence be granted, and, therefore, 

 let it be assumed that the majority of natural species are so 

 many records of a particular kind of variation having taken place 

 in the reproductive systems of ancestors. The issue then re- 

 solves itself into the question whether this is a mere re-state- 

 ment of fact, or whether it serves to throw any new light in the 

 way of explanation. 



By an explanation I understand the pointing out of effects as 

 due to the operation of causes. In the present instance, the effect 

 which has to be explained is the differentiation of specific types. 

 This I have sougbt to do by invoking the agency of a well-known 

 event — viz., that of variation — and showing that whenever this 

 cause affects the reproductive system in a particular way, a new 

 species must arise as an effect. JSTow, I believe that this mode of 

 viewing the problem as to the origin of species is not only new, 

 but, if true, serves to solve th.e problem, or to explain the facts. 

 The facts, indeed, were there before, as must always be the case 

 before an explanation can be suggested ; but an explanation 

 consists in placing the facts m a certain relation to one another 

 — i. e. in a relation of proved causality. In the present instance 



