92 Trans. Acad. Sci. of Si. Louis. 



are precisely alike, but show countless differences, " varia- 

 tions " as Darwin called them. It has been thoroughly es- 

 tablished by observation that all parts of the organism are 

 subject to variation, and furthermore, that any variation in 

 the parent tends to be transmitted to the offspring. Should 

 any of the innumerable variations of the body be of utility to 

 the possessor in the struggle for existence, that is, should it be 

 a determining factor in deciding whether the possessor is to 

 survive or perish, it will be naturally selected, and the off- 

 spring will tend to receive the same advantage. In this way 

 Darwin explained the origin of adaptations, those exquisite 

 adjustments of the organism to its environment which before 

 his work seemed so purposeful that a supernatural Intel- 

 ligence was thought necessary to account for them. But 

 Darwin showed that many, if not all, adaptations could be 

 satisfactorily explained by the inheritance of those accidental 

 or fortuitous variations which have been selected naturally 

 from among innumerable indifferent variations by reason of 

 their life-preserving value in the struggle for existence. By 

 a slow and gradual process, useful variations once established 

 would be perfected by further transmission of additional im- 

 provements along the same lines, until adaptations, as we see 

 them now, in all their intricate complexity and perfect adjust- 

 ment would result. 



Early in the history of the doctrine the objection was urged, 

 and for a long time strongly pressed, that until a variation 

 had at least reached a considerable degree of development it 

 could not be useful and hence could not determine survival ; 

 and moreover, that a single favorable variation would soon be 

 lost by the swamping effect of cross-breeding. As an answer 

 to this argument, it has been shown since Darwin's time (and 

 this has been one of the most important of the later addi- 

 tions to the theory) that it is not necessary for a variation to 

 be of profound significance at first, but that a gradual advance 

 may take place by raising the general average of each genera- 

 tion even by a slight amount. It has been clearly shown that 

 variations occur around a mean, and that with regard to any 

 particular character animals or plants arrange themselves 

 maintyinto groups — those above and those below the average. 



