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these genetic factors presents a continuous Variation between 
light and dark colour. But nevertheless by germinal analysis it 
can be shown tbat it are not the genetic factors themselves 
which Vary in intensity^ only the qualities produced by their 
Cooperation witb tbe other factors. This explanation of the effect 
of selection on a population, would seem to make it unnecessary 
to assume that ever the effects of the non-genetic factors wonld 
become hereditary^ and the question naturally poses itself : Could^ 
under circumstances selection or any other non-genetic influence 
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 author 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 handreds I am not 
yet prepared to state how many genetic factors can constitute 
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 made 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 influence 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 tliis medium 
before as well as after birth, the tail gets correspondingly longcr. 
It might be conceived that sometimes this influence the 
environment on the taillength would be accompanicd by one on 
the gonads, so that e. g. a genetic factor, normal ly present, 
would get lost. We have reason to assume that somclimes >iK'h 
