NATURAL SELECTION 399 



to constitute new species; while De Vries claimed that new species 

 were formed by the sudden appearance by mutations of forms specifi- 

 cally distinct from the parents. That mutants were new species! 



It seems evident that Darwin did not regard " saltatory evolution " 

 as the common method, while De Vries did. 



Darwin believed that individual, usually small, variations fur- 

 nished the material on which selection acts; while De Vries thought 

 that mutants, usually large variations, furnished the material. Both, 

 however, believed thoroughly that natural selection was a vera causa of 

 evolution. 



But things have changed greatly since 1904. The work of 

 Morgan, Castle, Jennings and a host of others has shown that many 

 mutations are so small, from a phenotypic standpoint, that they are 

 quantitatively no greater than the individual variations of Darwin; 

 and that they are heritable in the Mendelian way. 



Castle produced a perfectly graded series of hooded rats which 

 exhibits almost ideally the steps by which a new form might be 

 oroduced by natural selection. He says: 



"If artificial selection can, in the brief span of a man's lifetime, 

 mould a character steadily in a particular direction, why may not 

 natural selection in unlimited time also cause progressive evolution in 

 directions useful to the organism ?" 



Jennings says: 



"Sufficiently thorough study shows that minute heritable varia- 

 tions — so minute as to represent practically continuous gradations — 

 occur in many organisms: some reproducing from a single parent, 



others by biparental reproduction It is not established that 



heritable changes must be sudden large steps; while these may occur, 

 minute heritable changes are more frequent. Evolution according to 

 the typical Darwinian scheme, through the occurrence of many small 

 variations and their guidance by natural selection, is perfectly con- 

 sistent with what experimental and paleontological studies show us; 

 to me it appears more consistent with the data than does any other 

 theory." 



Many believers in mutation have been needlessly befuddled by 

 the diverse meanings of "variations" as used by Darwin and 

 De Vries. Darwin included in his "individual variations" both the 

 "fluctuating varieties" and the "mutations" of De Vries. Pheno- 

 typically they cannot even now be distinguished. De Vries nimself 

 candidly admits that this was Darwin's attitude, thus proving himself 



