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REV. J. T. GULIOK ON DIVERGENT EVOLUTION 
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 of 
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. 
