NATURAL SELECTION. 85 



and Alfred Russel Wallace made to the theory of evolu- 

 tion, was to show how natural selection, acting upon in- 

 definite variations, might give rise to new species and 

 adaptations. "Natural selection has been the main, but 

 not the exclusive, means of modification." 



The Darwinian theory may be thus summarised : All 

 organisms produce offspring on the whole like themselves, 

 but also exhibiting new and individual features. As the 

 result of the severe struggle for existence, only a small per- 

 centage of offspring survive to become reproductive adults. 

 The survivors are those whose variations enable them to gain 

 some advantage over their fellows in the struggle for food, 

 mates, and other conditions of well-being. A fit variation not 

 only secures the survival of its possessors, but is transmitted 

 from parents to offspring, and is intensified from generation 

 to generation. By this process of natural selection of ad- 

 vantageous variations, continued for many generations, the 

 modification of species has been effected. 



There is no doubt that the variations on which Darwin 

 mainly relied were constitutional or congenital variations, 

 whose transmissibility is quite certain. He rarely specu- 

 lated as to their origin, but believed it to be a fact that 

 " spontaneous," " indefinite," " fortuitous " variations con- 

 stantly occurred. 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, and require no particular emphasis. Nor 

 is the importance of sexual selection 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, by the geographical separation of the members 

 of a species, or by a reproductive variation causing mutual 

 sterility between two sections of a species living on a com- 

 mon area. " Without isolation, or the prevention of free 

 intercrossing, organic evolution is in no case possible. Iso- 

 lation has been the universal condition of modification. 

 Heredity and variability being given, the whole theory of 



