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THE AMERICAN NATURALIST [Vol. LI 



ularly and frequently, if not indeed usually, and in high 

 correlation with somatic characters ; second, that mixing 

 of germ plasms in fertilization alters hereditary deter- 

 miners mutually and hence is, in and of itself, a cause of 

 genetic variations, and, therefore, third, that a purely ex- 

 ternal agent, the continued selection of personal somatic 

 qualities, will alter the germ plasm.^^ 



The other side believes first, that the germ plasm is 

 fundamental and remarkably conservative, basing this 

 belief on such observations^^ on the one hand as those of 

 Walcott that pre-Cambrian annelids, snails, crustaceans, 

 and algae were in many cases so like forms living to-day 

 as to belong to the same genera, though a period of time 

 variously estimated at from 60 to 200 million years has 

 elapsed ; and, on the other hand, those of Wheeler on ants 

 enclosed in amber two million years ago but morpholog- 

 ically identical with forms living to-day. It believes, 

 second, that when the germ plasm changes it does so as 

 a result either of wholly internal physiological causes, 

 or of very extraordinary environmental stresses acting 

 directly upon the germ cells ; third, that mixing of germ 

 plasms, in and of itself, does not mutually alter hereditary 

 determiners, basing this belief on the regularity, con- 

 stancy and cleanness of t\T)ical Mendelian segregation; 

 and fourth, that selection only acts as a mechanical sorter 

 of existing diversities in the germ plasm and not as a 

 cause of alteration in it. 



The alternative views have been presented. In the 

 present state of knowledge nothing is to be gained by 

 mere assertions of opinion as to which more nearly repre- 

 sents the truth. But one may at least advance the view, 



fis In his last paper MaeDowell has justly emphasized the scientific futility 

 of defining and codifying the opponent 's position in a controversy, so that 



this point, and therefore have been at great pains in the preceding sentence 



86 Cited from Loeb, J.. ' ' The Organism as a Whole. ' ' New York, 1916. 



