Variation and Heredity 467 



The symmetrical curve shows the highest frequency, or 

 mode, in the center, but not infrequently the mode is 

 considerably shifted from one side to the other, giving 

 skew curves. Again, multimodal curves occur, and many 

 other subsidiary forms have been found to prevail in cer- 

 tain species or races. 



283. Darwin's theory of natural selection. Through 

 his wide experience with living things Darwin was thor- 

 oughly conversant with the existence of fluctuations in 

 nature. It was apparent that growers select, isolate, and 

 breed desirable forms or individuals, excluding or destroy- 

 ing those undesirable. Such a process appears to have led 

 to the origination of new breeds and races. Darwin saw 

 in nature similar forces yielding similar results ; viewing 

 the problem, therefore, in the uncontrolled or natural 

 environment, he formulated the following ideas : (1) any 

 individual variation, slight or considerable, which enables 

 the organism possessing it to succeed or maintain itself 

 better than its neighbor will have a strong chance of be- 

 coming perpetuated ; (2) more seeds are produced than 

 can grow again unto seedage, more organisms enter upon 

 life than can be reared ; (3) those less well equipped for 

 life's struggle succumb, and there is manifest a powerful 

 process of Natural Selection. 



He would seem to have maintained that the main line of 

 evolutionary progress and change lies in the natural or 

 artificial selection of relatively minute variations ; that is, 

 natural selection lays hold upon fluctuating variations. 

 There is constant variation, hence there is constant change, 

 or evolution. Wide variations arising suddenly, termed 

 sports, or discontinuous variations, were apparently 



