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such where two are necessary but where either is absent and 

 so on, because any one quality of an organism is only reached 

 by development under the innuence of a great number of factors. 

 And among these factors it can hardly be said that sonie are 

 more important than otliers. In the case of our bakery, it is 

 just as important that water or yeast are to be had as that the 

 baker is sober or the oven heated. 



It is very well possible to study the genetic and the non- 

 genetic factors in heredity separately as such, but whenever 

 we want to study the qualities of an organism ; we must take 

 both kinds into account. 



It has long been a question whether a good distinction 

 was possible between these two kinds of factors, a question to 

 which Mendelism has undoubtedly given a positive answer. We now 

 know that genetic factors can only be either present or absent, non- 

 genetic ones may each vary in intensity. As selection in a group, 

 whose members exhibit a continuous variability for any quality 

 has repeatedly been shown to shift the mean of the variation- 

 curve in the desired direction, it is clear that either continuous 

 Variation can depend upon genotypic differences within the group, 

 or eise, that ; if this Variation should depend solely on a Variation 

 of intensity of the non-genetic factors influencing the quality, that 

 these non-genetic factors must have transmittable innuence. 



By the experiments ofjohannsen and Nilsson-Ehle, 

 it has been conclusively shown, that continuous Variation may 

 depend on genotypic differences between the members of the 

 population ; and that in a group of organisms with identical genetic 

 factors, the modincations by the non-genetic factors are not trans- 

 mitted. Johanns en's well-known experiments with beans have 

 shown that selection within a biotype has absolutely no effect. 

 The author recently concluded a series of selection-experiments 

 with dandelion, which fully corroborated Johannsen's work. 

 The experiments of Nilsson-Ehle on the colour of wheat- 

 grains, have conclusively shown that several different genetic 

 factors can contribute to the development of the members of one 

 biotype, which all of them tend to innuence this development in 

 the same direction. In the work with the colours of mice, and 

 above all with cavies, evidence has been found for the same 

 fact. (Miss Sollas). A population of which the members differ 

 in respect to the possession or non-possession of one or more of 



