castle: role of selection in evolution 369 



GENETICS. — The role of selection in evolution. 1 W. E. Castle, 

 Bussey Institution. 



Up to the year 1900 those who believed in organic evolution 

 almost without exception believed in selection as its efficient 

 cause. Then came a period of doubt, inaugurated by DeVries' 

 Mutation Theory and strongly supported by Johannsen's Pure 

 Line Theory. In the minds of many biologists at the present 

 time selection is an obsolete agency in evolution and an 

 adequate explanation of evolution is to be found only in muta- 

 tion and pure lines. I believe this to be a mistaken view, not 

 because mutation and pure lines are false, but because their 

 applicability is very limited as compared with the broad field of 

 organic evolution. To universalize them is to hide the world 

 by holding a small object close to the eye. For even if we con- 

 cede the strongest possible claim for mutation as an agency in 

 evolution, viz, that it produces all new and heritable variations, 

 it is still unable to produce evolution without the aid of selec- 

 tion. The production of new variations produces no racial 

 change unless those variations persist, but their persistence 

 depends wholly upon selection. This is admitted by DeVries, 

 the author of the mutation theory, but overlooked by many 

 of those who have adopted the term mutation, as a scientific 

 shibboleth. 



But it is idle to enter upon a discussion of either selection or 

 mutation without carefully defining these terms, since both 

 are often used quite ambiguously, the latter in particular being 

 used in several different senses, and so being a cause of 

 misunderstanding where no genuine difference of view exists. 



Ever since DeVries' original attack in 1900, it has become 

 increasingly common among biologists to refer with disrespect 

 to "Darwinian selection." But Darwin understood by selec- 

 tion any agency which would cause one organism to survive 

 rather than another, and it is not clear that any theory of evo- 

 lution can dispense with such an agency. Since more organisms 

 are born than can survive, some must perish. In a state of 



1 A lecture delivered before the Washington Academy of Sciences, April 13, 

 1917. 



