374 Appe7idix C 



mental biography, of course, — especially those of a more prac- 

 tical character, which bring out human points of view. In the 

 other work, we have the systematic exposition of the theory of 

 Physiological Selection, which is possibly Romanes' most origi- 

 nal and interesting single contribution to natural history. 



This theory has two main features ; features which should be 

 taken separately, I think, and which only lose in force and serve 

 to introduce confusion when brought under a single point of view, 

 as Romanes does. The real novelty of physiological selection 

 consists in the hypothesis that congenital variations toward 

 infertility might lead to relative segregation in a group of ani- 

 mals living together, and the development of the groups thus 

 segregated away from one another in divergent lines. No one 

 who appreciates the problem of inter-specific infertility can, I 

 think, fail to see the force of this hypothesis, nor fail to agree 

 with Romanes — quite apart from the evidence of fact — in the 

 hypothesis that specific differences may be secondary to sexual 

 variations, rather than the reverse, as Darwin supposed. I can- 

 not help thinking, however, that Romanes places too much 

 confidence in the so-called * principle ' of Weismann (amixia) 

 and Delbceuf, that any slight average difference between differ- 

 ent groups must develop itself. That would seem to depend 

 upon circumstances ; and at any rate it is purely hypothetical. 

 Romanes weakens his case by making it a sort of corner-stone 

 to his structure ; for whatever the causes be of the subsequent 

 divergent evolution — say Wallace's pure utility view — the 

 original segregation by physiological selection would lose none 

 of its value, if it be true, especially in cases of absolute infer- 

 tility. The value of physiological selection as producing diver- 

 gent species would seem to rest, in cases of relative or partial 

 infertility, largely upon the sort of variations which were cor- 

 related' with the infertility — a point which the theory of 

 Reproductive Selection of Professor Karl Pearson covers. 



V As to whether partial infertility alone, without any regular correlations, 

 would produce divergent results, seems very doubtful, except as it tended to 

 result (by accumulation of variations) in absolute infertility. This latter result 

 Romanes himself supposes. 



