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cing the development, often a very dennite stage in it. Whe- 

 never this stage is not reached, for any reason, the factor 

 can not have any influence, but that in such a case it is trans- 

 mitted just the same can be seen from the fact that the indi- 

 vidual's gametes contain it 7 so that, if the zygote, to which they 

 contribute is such, or develops under such conditions, that the 

 necessary developmental stage is reached, the factor once more 

 asserts itself. 



Among both cathegories of factors in the development, on 

 one band the material, selfmultiplying, more concrete things which 

 are the genetic factors ; on the other hand the non-genetic factors, 

 conditions, interrelations, more abstract things, there are some 

 which are absolutely indispensable. So are for example the 

 presence of water, a constant atmospheric pressure, non-genetic 

 lactors which are indispensable. In the case of these non-genetic 

 factors we can study each one of them by modifying it at will, 

 or by creating factors antagonistic to the one we want to study. 



In our study of the genetic factors, we meet an enormous 

 limitation. We can not as yet take at will any given factor from 

 a germ. We therefore simply must limit our study to those cases 

 where a genetic factor has already been lost in at least one 

 individual. And this is not all, the only way to be sure that the 

 individual we observe has lost only one genetic factor, is to mate 

 it with another which still has it, and observe the second 

 generation from this mating. And this means, that in the case we 

 are dealing with a genetic factor, which is indispensable for even 

 a partial fertility of the individual to the development of which 

 it normally would contribute, our analysis becomes very difficult 

 and generally impossible. 



There are only a few cases in which the analysis can still 

 be made. For instance, if we find that one animal, it mated to 

 some others produces nothing but normal offspring, but if mated 

 to another gives one fourth the number of its offspring with a 

 certain defect or infertile (goats), we can with some certainty 

 say that here the difference between normal and abnormal, fertile 

 and infertile is caused by presence or absence of one genetic 

 factor. 



This limitation of the field of study has given rise to the 

 remarkable objection to Mendelism one hears rather often in 

 France, namely that we are studying the inheritance of all kinds 



