516 THE AMERICAN NATURALIST [Vol. LI 



of each other at the time when the members of each pair 

 separate (segregate). 



But between the year 1866, when Mendel published his 

 paper, and the present year, 1917— an interval of fifty- 

 one years— much water has run under the Mendelian mill. 

 In consequence we can now add certain further attributes 

 to the rather formal characterization of the gene as de- 

 ducible from Mendel's law alone. But before I discuss 

 the evidence for these postulated attributes, I must pause 

 for a moment to call attention to a movement that was in 

 certain important respects a forerunner of our present 

 standpoint. 



I refer to the views of Koux and of Weismann, both of 

 whom assumed that the germ plasm is made up of par- 

 ticles or determiners, as Bonnet, Spencer, Darwin and 

 others had done before them. Their argument was largely 

 speculative, and not of the same kind as the more recent 

 evidence derived from Mendelian analysis. Moreover in 

 all of Weismann 's earlier and best known writings his 

 idea of the units in heredity was more involved than are 

 our present ideas. He thought that whole germ plasms 

 were the units that segregated, germ plasms that dif- 

 fered in one or many determiners, whereas the factorial 

 view tliat we follow since Mendelism came to the front 

 as-niii('< that the units that segregate are themselves only 

 part> of a whole which is the sum total of all the units. 

 In his latest Ijook, however, Weismann accepted the evi- 

 dence from ;\rendelism and modified his ideas accordingly. 

 , We owe to Weismann the popularization of the view 

 that the hereditary material is carried by the chromatin, 

 jbut especially we owe to Weismann the development of 

 the idea that the sorting out of the hereditary materials 

 Itakes place at the time of the maturation process in the 

 jegg and sperm. 



On the other hand, it must be emphatically pointed out 

 that the earlier idea of Roux, adopted by Weismann, that 

 one of the hereditary complexes is sorted out during the 



