774 THEOR Y OF £ VOL UTION. 



The importance of natural selection as a secondary 

 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 selection, 

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

 succeeding in reproduction better than their neighbours, 

 tend to transmit their qualities to their numerous male 

 heirs. But this and other forms of reproductive selection 

 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 

 geographical separation, or by a reproductive variation 

 causing mutual sterility between two sections of a species 

 living on a common 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." Romanes' view, however, was 

 that, "without isolation, or the prevention of free inter- 

 crossing, 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." It may also be noted that some 

 forms of isolation may lead to inbreeding, and this to 

 "prepotency," which often implies the persistence of in- 

 dividual variations. 



