68 CROSSING. 



was essentially the spontaneous creation of new pangenes, the 

 cytologists had made remarkable progress in the technique of 

 studying chromosomes. 



Most of the genes studied by Mendel in the pea, and by the 

 first Geneticians who verified Mendel's work with all kinds of 

 animals and plants, were of such a nature that they influenced 

 the development considerably, and at different points. The 

 material was carefully selected with this point in view. And 

 it was easy to believe, that the mutual independence of the 

 characters studied, depended directly upon a corresponding 

 mutual independence of its genes, and further, that a certain 

 character would be present, when its determinant was present, 

 and absent when it was absent. 



And after the idea had once become established that each 

 character had its own representant in the germ, its own factor, 

 the cases, which were discovered a little later, in which we saw 

 two or three seperately transmitted genes influence one iden- 

 tical quality, were looked upon as complications. 



Even nowadays, we read about characters which have a mul- 

 tiple representation in the germ, and it is evident that some 

 authors consider such cases to be exceptions from a rule, that 

 every character of an organism usually has its own single 

 determiner. 



And for this reason it is not strange, that some authors have 

 even tried to explain those cases, in which more than one gene 

 were found to influence a certain quality, by the assumption 

 that here a single gene was somehow split up into component 

 ones, that such sets of genes had more in common than their 

 effect on the development, that they were fundamentally of 

 the same nature. We think, that the facts as we know them, do 

 not at all force us to the assumption, that in those cases, in 

 which we know more than one gene influencing the same char- 

 acter, these genes must be fundamentally analogous. If the 

 only thing we know about three genes is, that they all tend to 

 make grain-colour darker, or that they tend to make a plant 

 grow taller, it is easy to be led to believe, that they must be in 



