770 THEORY OF EVOLUTION. 



eliminated, while the relatively more fit will tend to 

 survive. As many variations re-appear generation after 

 generation, and may become gradually increased in amount, 

 the continuance of the selective or eliminating process will 

 work towards the origin of new adaptations and new species. 



The importance of Natural Selection as a secondary factor 

 in evolution will vary according to stringency of the elimin- 

 ating 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 neigh- 

 bours, 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, and require no particular emphasis. Nor is the 

 importance of sexual selection or preferential mating 

 admitted by so great an authority as Wallace. 



(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 

 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's 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 



