198 



REV. J. T. GULIOK 01^ DIYERaENT EVOLUTIOIS- 



theory is to show the connection of these facts with divergent 

 evolution. 



Though many divergences appear in our method of treating 

 the subject, the fundamental theory underlying my Segregate 

 Fecundity and Mr. Eomanes's Physiological Selection seems to be 

 very similar, if not the same. The most important differences I 

 have noticed are, (1) that he seems to regard mutual sterility as 

 sufficient to account for the separate propagation of species and 

 varieties thus characterized, without calling in the aid of any 

 other form of segregation, while I regard it as a Negative form o£ 

 Segregation that would result in the general destruction of all 

 life if not associated with what I call Positive forms of Segre- 

 gation ; and (2) that he maintains that " Physiological selection 

 is almost exclusively a theory of the origin of species, seeing that 

 it can but very rarely have had anything to do with the formation 

 of genera, and can never have had anything at all to do with the 

 formation of families, order, or classes. Hence, the evidence which 

 we have of the evolutionary influence of physiological selection, 

 unlike that which we have of the evolutionary influence of natural 

 selection, is confined within the limits of specific distinctions," * 

 while I maintain that Segregation of some form is a necessary 

 condition for all divergent evolution, and that in fact Segregate 

 Fecundity in many cases prevents the intercrossing of divergent 

 forms that, though descended from a common stock, now belong 

 to different families and orders. 



The first of these differences, though of considerable importance, 

 is, I think, due to the method of presentation, rather than to any 

 fundamental discrepancy in the theories. The Positive forms of 

 Segregation are, I judge, assumed to be present, though their 

 co-operation is not distinctly recognized as a necessary condition 

 for the breeding of forms that are mutually sterile. 



I must, however, confess that I do not see how to reconcile his 

 statement that " Physiological selection can never have had any- 

 thing at all to do with the formation of families, orders, or 

 classes " with what I believe to be the facts concerning Segre- 

 gate Fecundity ; and if physiological selection is to be understood 

 as including Seasonal and perhaps other forms of Segregation, 

 this passage seems to be still more opposed to the principles of 

 divergent evolution as I understand them. He certainly could 

 not have intended to say that mutual fertility between allied 

 * Linn. Soc. Journ., Zoology, vol. xix. p. 396. 



