ORGANIC EVOLUTION 287 



purpose, and to continue this selection generation after generation. It 

 was found that this continuous selection gradually built up the selected 

 characters, until the desired result was obtained. This could well be 

 called the origin of new forms by artificial selection; that is, selection 

 directed by -man. 



Darwin concluded that there is a process similar to this going on in 

 nature. Innumerable variants are constantly appearing, in numbers 

 beyond any possibility of their continuance. The more suitable ones 

 are selected by nature for survival, the means of selection being " the 

 struggle for existence." This selection continuing from generation to 

 generation, the favorable variations would be perpetuated and increased, 

 and eventually the variation might become so great that it could be re- 

 garded as standing for a new species. The very appropriate name given 

 to this process is natural selection, and its method consists in the slow 

 building up of small variations, in a given direction determined by the 

 environment, to one great enough to cross the boundary of the parent 

 species. 



Although natural selection is certainly operative in the destruction of 

 certain forms and the preservation of others, it is thought by many 

 to be doubtful whether this process can result in the production of new 

 species. Some of the reasons for this doubt that have been urged are 

 as follows: 



(1) It is generally believed that acquired characters are not inherited; 

 and if so, it is thought that the small variations exhibited by individuals 

 would not be passed on to their progeny with any certainty. An ac- 

 quired character is one that is " taken on " by the individual during its 

 lifetime, and is no part of its parental inheritance. The variations 

 claimed to be used by natural selection, however, are probably inherited 

 for the most part, and can hardly be included among acquired characters; 

 so that this objection is not a serious one. 



(2) It is claimed that the slight variations used by the theory of nat- 

 ural selection cannot be extended by continuous selection beyond the 

 boundary of the species; in other words, that there is a limit of variation 

 for each species, which cannot be passed by variations of this type. 

 Such variations are commonly spoken of as fluctuating variations, and 

 the amount of fluctuation varies in different species. It is claimed that 

 with all the centuries of artificial selection by plant and animal breeders, 

 the species- boundary has never been crossed by this process. 



(3) It is recognized that the forms improved by artificial selection 



