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these genetic factors presents a continuous Variation between 

 light and dark colonr. But nevertheless by germinal analysis it 

 can be shown that it are not the genetic factors themselves 

 which vary in intensity, only the qualities produced by their 

 Cooperation with the other factors. This explanation of the effect 

 of selection on a population ; wonld seem to make it unnecessary 

 to assume that ever the effects of the non-genetic factors wonld 

 become hereditary, and the question natnrally poses itself : Could, 

 under circumstances selection or any other non-genetic inflnence 

 change the Constitution of the germ? 



The experiments of Castle on the amount of black in the 

 coat of hooded rats have been interpreted by their anthor to show 

 that selection on continuous Variation within a strain can shift 

 the mean of the variation-curve. I am repeating these experi- 

 ments, and as I have only bred some few hundreds I am not 

 yet prepared to state how many genetic factors can coustitute 

 the difference between a dark rat and a light one. But I find, 

 that selection has effect only in so far as one chooses between 

 individuals differing in genetic Constitution, but is without effect 

 when the choice is macle between individuals with the same 

 genetic factors, but differing through the effect of non-genetic ones. 



How should we have to picture a possible inheritance of 

 modifications ? Theoretically spoken, it is impossible to conceive 

 of such an inffuence of a non-genetic factor on an organism, that 

 one or more of the genetic factors going into its gametes are so 

 changed as to produce in the offspring a change in the same 

 direction as that produced in the parent by the non-genetic 

 factor in question. 



Thus, life in a warm environment affects the taillength of 

 developing mice. The experiments of Przibram have shown 

 that the tail gets longer if the animals are subjected to the 

 changed environment from birth upwards, also, that the taillength 

 is similarly affected by the same influences if the individuals are 

 subjected to them from the moment of fertilization until the 

 moment of birth. And if an individual grows in this medium 

 before as well as after birth, the tail gets correspondingly longer. 

 It might be conceived that sometimes this influence of the 

 environment on the taillength would be accompanied by one on 

 the gonads, so that e. g. a genetic factor, normally present, 

 would gct lost. We have reason to assume that sometimes such 



