226 
cing the development, often a very definite 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, 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 band 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 
indiyidual. 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, if 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 
