MR. a. J. EOMAKES ON PHYSIOLOGICAL SELECTION. 387 



stand, unquestionably prove the fact of ramification ; and it ap- 

 pears to me no less unquestionable that ramification, as often as 

 it has occurred, can only have been permitted to occur by the 

 absence of intercrossing with parent forms. But, apart from 

 geographical barriers (which, according to Mr." Darwin's argu- 

 ment, would be inimical to the divergence of character by natural 

 selection),' the ramification can only take place as a consequence 

 of physiological selection, or as a consequence of some change in 

 the reproductive system which prevents intercrossing with un- 

 changed (or differently changed) compatriots. But when once 

 this condition is supplied by physiological selection, I have no 

 doubt that divergence of character may then be promoted by 

 natural selection, in the way that is explained by Mr. Darwin. 



And this latter consideration is a most important one for us to 

 bear in mind, because it furnishes an additional reason for the 

 fact that when a section of a species has become physiologically 

 separated from the rest of its species, it forthwith begins to run 

 into variations of other kinds, and so eventually to differ from the 

 parent type, not only as regards the primary distinction of 

 sterility, but also as regards secondary distinctions which may 

 affect any part of the organism. The only reasons which I have 

 hitherto assigned for this fact are, first, that from the time when 

 overwhelming intercrossing with the parent form is prevented, 

 the varietal form is allowed to develop an independent course of 

 varietal history, as in the parallel case where intercrossing is 

 prevented by geographical barriers, or by migration ; and, second, 

 that when the primary variation takes place in the reproductive 

 system, it is apt to cause secondary variations in the progeny. 

 But now I may make this important addition to those reasons — 

 the addition, I mean, that when intercrossing with a parent form 

 is in any degree prevented by physiological selection, the varietal 

 form is free to develop diversity of character under the in- 

 fluence of natural selection, in the way that has been so ably 

 shown by Mr. Darwin. 



Erom which it wiU be seen that the theory of physiological 

 selection has this advantage over the theory of natural selection 

 in the way of explaining what Mr. Darwin calls diversification of 

 character, or what I have called the ramification of species. This 

 diversification or ramification has reference chiefly to the secon- 

 dary specific distinctions which, as we have seen, the theory of 

 natural selection supposes to be the first changes that occur, and 



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