224 



to the other extreme, holdmg that the characters of the orgauisms 

 are exclusively the result of the reactions of a non-specialized 

 germ on the different conditions it encounters during growth. Not 

 all the biomechanists have gone to this extreme, which will have 

 to be looked upon as a reaction after Weismann's self-satisfied 

 and research-stining revival of the ancient evolution idea. Roux 

 notably has from the beginning distinguished between the ty- 

 pischen Determinationsfaktoren and Realisations- 

 und Alterationsfaktoren in ontogenetic development, he 

 being the first to show by undisputable experiment that such a 

 distinction was called for. 



De Vries's modification of the Darwin-Weismann 

 conceptions of inheritable determinants had still the drawback of 

 assuming that the characters of an organism depend immediately 

 from determining particles, and the fact that from the rediscovery 

 of Mendel's work this conception of de Vries has been grafted 

 upon Mendelism has undoubtedly done much to discredit it in 

 the eyes of many biologists. It is only after it had been clearly 

 shown by the work of Bateson, Miss Durham and Cuenot, 

 that hereditary characters were not called forth each by a 

 corresponding genetic factor, but that factors could, under cir- 

 cumstances, be present in the germ without their presence making 

 itself feit, that it was possible clearly to distinguish between 

 genetic, transmitted factors, and the characters to the differentiating 

 of which they contribute. 



I think the fact that these two things, the characters of the 

 individuals, and the genetic factors transmitted through the 

 gametes, have been mixed up by several authors, has given rise 

 to much critisism of Mendelism by conservative thinkers, whose 

 attitude would not have been hostile, had it only always been 

 clear that we are studying the behaviour of one special group 

 of factors in the development of the organisms, and that the 

 generalizations, the laws of segregation and independancc of these 

 factors only indirectly concern the inheritance of characters. 



Such terms as dominance, latency, etc. which have been 

 invented to express the behaviour of the (abstract) qualities of 

 organisms in hybrid families, must always rigorously been reserved 

 for this use, and it must not be forgotten, that all the evideoce 

 points to it, that a gamete can only be in one of two possible . 

 conditions in respect to any genetic factor, it can either contain 



