The Background of Genetics 5 



vive, while the rest perish in the struggle. The one 

 hundred survivors are ''litter" than were the others of 

 their generation because they happened to have certain 

 useful organs or processes somewhat more fully de\'el- 

 oped. In the following generation, some of the progeny 

 will have the organs in question still more fully de\'eloped 

 than did their parents, while on the other hand there 

 will also be some that have them less fully developed. 

 The former group will again be chosen by nature to sur- 

 vive and perpetuate the species, and thus progress will 

 be made in the direction of better development of useful 

 organs until a degree of development has been attained 

 which may be said to represent a new species. The wa}^ 

 in which nature manipulates these quantitative, con- 

 tinuous variations of Darwin's to bring about this 

 progressive evolution or adaptation can be \'isualized 

 more concretely from this simple diagram. 



(More poorly 

 adapted forms 



perish in 

 competition) 



^ ^ 6 



^ ^ 



Environment -> 



(favors 



variations in this 



direction) 



^ 



8 



The numerous objections to Darwin's theory cannot 

 be discussed here; suffice it to say that these objections 

 were directed mainly at the adequacy of the Darwinian 

 variations in accounting for the results of evolution 

 rather than at the idea of natural selection. The ''sur- 

 vival of the fittest" is a rather generally accepted idea. 

 The question whether the Darwinian variations are 



