FACTORS IN ORGANIC EVOLUTION 913 



factor in evolution will vary according to stringency of the 

 eliminating process, and it must be noted that the " struggle 

 for existence " varies in intensity within wide limits, that 

 it requires to be investigated for each case, and cannot be 

 postulated as a force of nature. 



The importance of the factor will also depend on the 

 number, nature, and limits of the variations which occur. 

 Thus a new species might arise, either by the occurrence of 

 a discontinuous variation of considerable magnitude, or by 

 the eliminating process acting for many generations on a 

 series of minute continuous variations. 



Darwin also believed in the importance of sexual selec- 

 tion, in which the females choose the more attractive males, 

 which, succeeding in reproduction better than their neigh- 

 bours, tend to transmit their qualities to their numerous 

 male heirs. But this and other forms of reproductive selec- 

 tion may be regarded as special cases of natural selection. 



2. *' Isolation^ — Under this title, Romanes, Gulick, and 

 others include the various ways in which free intercrossing 

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

 graphical separation, or by a reproductive variation 

 causing mutual sterility between two sections of a species 

 living on a commor area. Without some ** isolation " 

 tending to limit the range of mutual fertility within a 

 species, or bringing similar variations to breed together, a 

 new variation is liable, they say, to be " swamped " by 

 intercrossing. But definite facts as to this " swamping," 

 and in many cases as to the alleged " isolation," are hard to 

 find, nor can we say that a strong variation will not persist 

 unless it be " isolated." In fact, much evidence has been 

 gathered in recent years which shows that certain kinds of 

 variations are very strongly heritable and do anything but 

 " blend." Romanes' view, however, was that " 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 varia- 

 bility being given, the whole theory of organic evolution 

 becomes a theory of the causes and conditions which lead 

 to isolation." It must be admitted that some forms of 

 isolation lead to inbreeding, and this to *' prepotency," 

 which often implies the persistence of individual variations. 



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