408 ME. G. J. ROMANES ON PHYSIOLOGICAL SELECTION. 



this, so far as I am aware, has not been previously done. Tlie 

 facts of variation have been known, and the facts of specific 

 sterility have been known ; but hitherto it has not beeiv sug- 

 gested that the former may stand to the latter in the relation of 

 cause to eifect, or that when a particular kind of variation occurs 

 in the reproductive system a new species must necessarily ensue. 

 The very general association between mutual sterility and specific 

 differences of other kinds has, indeed, forced itself upon the 

 attention of naturalists ; but naturalists have attempted to ex- 

 plain the association by this, that, and the other collateral cause, 

 such as divine interposition, uniform conditions of life, and so 

 forth. The present theory, on the other hand, seeks to explain 

 this association as itself an association of cause and effect ; the 

 theory regards a species as nothing more than a variety, where 

 the variation happens to have affected the reproductive system in 

 a particular way — thus leading to physiological separation, and 

 so eventually to other morphological changes, as previously 

 argued. Now, whatever may be thought as to the probability of 

 this explanation, to me it appears evident that it is an explana- 

 tion, and not merely a re-statement of fact. For, if not, where has 

 been the need of all that has been written for the purpose of 

 endeavouring to explain the association ? If it has ever before 

 been recognized that species are the effects of variations in the 

 reproductive systems of ancestry, I cannot understand why this 

 should not have been clearly stated ; and still less can I under- 

 stand why, with so simple an explanation before the mind, any 

 naturalist should have cast about for other causes of a collateral 

 kind. What I can understand is that more evidence should be 

 demanded of the truth of the present explanation ; but this is 

 not the point with which the objection before us is concerned. 



The real standing of the matter is simply this. Evolutionists 

 have hitherto regarded mutual sterility as one among the effects 

 of specific differentiation, and they have therefore been led to 

 seek for causes which might be held adequate to account for this 

 effect. My theory, on the other hand, regards the sterility, 

 wherever it occurs, as itself the cause of specific differentiation ; 

 and this whether the sterility be spontaneous or induced by 

 changes going on in other parts of the organism, as previously 

 explained. Evolutionists have hitherto failed to find the causes 

 of which they have been in search ; and, according to my view, 

 necessarily so, inasmuch as there are no such causes to be found. 



