102 Hagedoorn. 



serious error, for it cannot be said tliat one factor by its presence 

 produces a "unit-character". The genetic factors, whenever present, 

 can have a certain definite influence upon the development of the 

 organism. When present in a certain combination of other factors, 

 genetic as well as non-genetic ones, they may influence the develop- 

 ment in such a way that the final result is something different from 

 what it should have been without their cooperation. Thus, black 

 colour of a mouse or rabbit, does not necessarily result from the 

 presence of a certain genetic factor "Black" in the germ. If it was 

 thus, we should be fully justified in calling this factor B, or z, or N 

 or S, according to our nationality. But we will see that the same 

 factor may be present in a germ, from which an animal with quite 

 another colour develops. 



For instance a certain other, frequently studied genetic factor 

 may also be present, in which case the animal will be coloured like 

 the wild mouse or rabbit. Or again, other factors, which were present 

 in the germ of our black and wildcoloured animals may be absent, 

 in which case the animals, though containing the factor "Black", 

 will be white or yellow or blue, according as to which other factor 

 is absent. This factor "Black" therefore does not produce black 

 colour, other than in a very definite combination of other factors. 

 We must say: In the case five known genetic factors are present in 

 a mousegerm, two others being absent, this germ can develop either 

 into a chocolate or into a black individual. If in this case the factor 

 of which we are treating be present, the colour will be black, if not 

 it will be brown. In the one hundred and twenty seven other com- 

 binations of the remaining factors studied however, the presence of 

 our factor will not determine black colour. The naming of a genetic 

 factor after a character, which, under certain circumstances, and with 

 numerous others, it helps to produce, may be likened to the naming 

 of a chemical element after some particular property of a drug into 

 the composition of which it enters. 



It must be remembered that the symbols we use in our formulae 

 do not stand for "unit-characters", but exclusively for the genetic 

 factors in the development of the organism with which we are dealing. 

 It must also be well understood that our formulae are by no means 

 meant to be complete representations of the constitution of the germs 

 of ou organisms. We have at present no indication as to the probable 

 number of genetic factors contained in the germ of the diverse organisms. 

 The number may, for as far as we know, be several hundreds or several 



