JC2 Arend. L. Hagedoorn and A. C. Hagedoorn. 



factors studied were all such, as rather intensely influenced the 

 development of the organisms which carried them, and therefore a 

 group of plants or animals, of which some have such a gene in their 

 germ and others not, shows a discontinuous variation. It is however 

 easily seen, that the difference is one of degree only. For even in 

 such a case, where two contrasted types result, one if a certain 

 genetic factor cooperates to the development, the other if it does 

 not, both types vary, and it is only because the limits commonly 

 reached by the variation of each type do not overlap, that the 

 variation may be called discontinuous. To give an example. 



In mice, we know a gene which is present in black ones but 

 absent from a certain kind of yellow, tortoise. If we have a group 

 of mice, of which some have this gene and others lack it, but which 

 for the rest all have such a genetic constitution, that if they all had 

 this gene they would be black, this group will present a discontinuous 

 variation, some animals being tortoise, others being black. 



But in another group of mice, we may be dealing with an 

 identical situation, excepting that here the gene absent from some 

 animals is another one, that one namely, which commonly disting- 

 uishes deeply coloured animals from fadely coloured ones. In such 

 a population the darkest animals without this factor may be blacker, 

 darker, than the lightest with it, and in such a case we may there- 

 fore be deahng with a continuous variation. 



If therefore a group of organisms shows continuous variation, 

 this not necessarily means that these organisms have all the same 

 genotype. 



Nilsson-Ehle has published the results of a series of experiments 

 with wheats, showing the existance of several genes, which all by 

 their cooperation to the development of a plant tend to darken the 

 colour of the grain produced. When the seed is sown, produced by 

 a plant, impure for a few of these genes, the resulting population 

 varies in graincolour, and this variation is a typically continuous one, 

 which can be graphically expressed by a typically normal variation- 

 curve. We will show other similar cases in this paper. 



It is gradually beginning to be understood, that, apart from the 

 genetic factors which influence the development in a very striking 

 way, genes, whose study has given rise to the "unit-character-idea", 

 there are many others, or rather, there are probably far more genes, 

 which only have comparatively little influence on the development 

 of the germ in which they find themselves, and sometimes none at 



