Evolution of Evolution-Theory. 235 



individuals". But one fails to find as yet sufficient war- 

 rant for this supposition that numerous similar, fortuit- 

 ous, indefinite, indeterminate variations should occur. 

 For Lamarckians, or for believers in progressive varia- 

 tion along definite lines, the supposition is natural, but 

 not for Darwinians. 



Another answer to the difficulty applicable to certain 

 cases might perhaps be found in the fact, which 

 breeders allege, that certain strongly marked (ger- 

 minal and racial) variations are by no means readily 

 swamped, even in the absence of isolation. We might 

 perhaps venture to speak of a struggle for existence 

 within the fertilized ovum, wherein the physical basis, 

 corresponding, let us say, to a strongly marked pater- 

 nal characteristic, asserts itself even without co-opera- 

 tion from the maternal substance. 



But the answer which has been within recent years 

 suggested by Romanes, Gulick, and others is an elabo- 

 rate theory of "Isolation". Under this title they 

 include a variety of ways in which free intercrossing is 

 prevented between members of a species, e.g. by geo- 

 graphical barriers, by change of habit, by a reproduc- 

 tive variation causing mutual sterility between two sec- 

 tions of a species living on a common area, and so on. 



According to Romanes : ' ' Without isolation, or the 

 prevention of free intercrossing, organic evolution is in 

 no case possible. Isolation has been the universal 

 condition of modification. Heredity and variability 

 being given, the whole theory of organic evolution 

 becomes a theory of the causes and conditions which 

 lead to isolation." 



There is still, however, a lack of sufficiently precise 

 evidence in regard to the supposed swamping without 

 isolation, and in regard to the supposed general preven- 

 tion of free intercrossing. 



(e) So-called "Organic Selection". Prof. Weismann 

 suggested in one of his essays that individual modifica- 

 tions, though not transmissible, might co-operate with 

 progressive congenital variations in effecting adapta- 

 tions of importance, and this hint has been developed 

 by Prof. C. Lloyd Morgan, Prof. Mark Baldwin, and 



