No. 604] SHORTER ARTICLES AND DISCUSSIONS 251 



ferences. But modification of the germ plasm by selection is 

 impossible, if that agency acts only as "a mechanical sorter of 

 existing diversities. ' ' 



The present note is not concerned with the causes of variation. 

 It refers only to the first-mentioned phase of the selection prob- 

 lem. It accepts the statements in the quotation below as sub- 

 stantially correct, and attempts in brief compass to evaluate the 

 arguments by which Dr. Pearl supports his position regarding 

 them. 



By transposition of a few phrases his ideas may be expressed 

 in his own words as follows: 



The mere fact of elimination and survival ... is capable, in theorj- 

 at least, of bringing about evolutionary changes of a progressive sort, 

 ... if the elimination be selective, and the survivors transmit to their 

 progeny those differences that mark them off from the eliminated. 

 The theory that these two rules are always and everj'where in opera- 

 tion, taken together with the observed fact that living creatures do die, 

 is the Darwinian theory of Natural Selection as a factor in organic 



If, as is implied, Darwin gratuitously assumed the intolerable 

 burden involved in the use of the words, always and everywhere, 

 it is immaterial. It is not a vital issue whether the form in 

 which he expressed himself will bear literal interpretation, but 

 merely whether his idea is correct that natural selection effects 

 notable changes in the course of evolution. Hence it seems suffi- 

 cient to say, that if the "Dance of Death" is governed in gen- 

 eral, or even in part, by the joint action of the two principles 

 enunciated in the preceding paragraph, the changes described 

 there should follow as surely, although more slowly, than if the 

 conformity were complete. 



It is stated by Dr. Pearl, as one of three broad facts on ac- 

 count of which natural selection is no longer regarded as a 

 primary, or perhaps even a major factor in evolution, that even 

 when selective elimination on the basis of somatic characters 

 does occur, it does not follow generally and regularly that the 

 somatic differences on which the selection acted will reappear in 

 the progeny, . . . actual experience having abundantly demon- 

 strated that a very great many of such somatic differences are 

 not inherited. 



This may refer, first, to the fact that a sitiirle phenotype may 

 include members of different gonotyiies. Yet. even so, no strict 

 placed upon the possibility of clianging the char- 

 lixed population by selection based upon somatic 

 plus variants of an inferior strain seem superior to 



