THE DARWINIAN THEORY 555 



denly take new directions. Hence, there is the possibihty of 

 changes of all sorts, some progressive, others degenerative as in 

 organisms that have been selected as parasites. Under natural 

 selection the environment has been compared to a sieve that 

 selects a certain type of individual. This sieve may change and 

 produce new lines of evolution. It was such a mechanism in 

 nature that Darwin conceived to be most important in directing 

 the course of evolution. 



Present Status of Natural Selection. — It is ahnost self-evident 

 that the process of Natural Selection is one of the major causes 

 of evolution; although the selective process has obvious limita- 

 tions. It has sometimes been urged that natural selection is 

 inadequate because it does not explain variation and heredity, 

 which are the fundamental causes of evolution. Thus it is said 

 that the " origin " of species is to be sought in the origin of varia- 

 tions that are inherited, and that selection merely determines what 

 variations, and hence what incipient species, shall survive after 

 they have originated from other causes. This is perhaps a fair 

 criticism of the title of Darwin's great book, " The Origin of Species 

 by Natural Selection or the Preservation of Favored Races in the 

 Struggle for Life"; but it fails to recognize that the argument for 

 selection is: given heredity and variation, certain things will follow. 

 Heritable variations are known to exist, just as reproduction in 

 excessive numbers is known to exist, and it is as unfair to criticize 

 the selection theory because it does not explain heredity and varia- 

 tion as it would be to criticize it because it does not explain repro- 

 duction. Heredity and variation demand investigation by reason 

 of their intrinsic interest as well as their importance in evolution, 

 but we do not need to know how heritable variations arise and are 

 perpetuated in order to be convinced that selection exercises a 

 directive influence upon the course of evolution. 



Another criticism of selection is that it explains only the exist- 

 ence of useful or adaptive characters, while organisms seem to 

 have many characters which must have arisen during evolution and 

 which are non-useful without being particularly injurious. This 

 limitation is recognized by all who understand the theory, since 

 the nature of the process is such that only useful characters can be 

 selected. Conversely, injurious characters are ehminated by 

 selection whenever they are sufficiently important. Selection 

 involves the elimination of the unfit as well as the preservation of 



