Studies on variation and selection. igj 



and Indians. In this same book instances are given, which go to 

 prove that not all Europeans have children of the same colour from 

 parents of one single darker-coloured race. This also shows the active 

 influence of the lighter parent on the skin-colour of the children from 

 such unions. 



Certain authors have regarded the "multiple-factor-hypothesis" 

 as a new kind of dodge, a trick to provide a facile "Mendelian" 

 explication for cases which are evidently "non-Mendelian". We have 

 tried to show, that this hypothesis is not a new one at all, that 

 .simply the "one-factor-one-character-idea", which is still all too com- 

 mon, made the fact, that several genetic factors can influence the 

 same quality, look new. 



There is not the slightest fundamental difference between cases, 

 in which we are observing the segregation and redistribution of a few 

 genes which happen to influence one single quality, and such, in which 

 a number of genes are studied, which make their influence filt on as 

 many different points. 



VI. Rules and Laws in Genetics. 



A good example of the general looseness in terminology, which 

 has done so much to hinder the development of Genetics as an 

 inductive science is afforded by the use of the word "law" for some 

 generalizations. The difference between "rule" and "law", which is 

 a very definite one, has been lost sight of by quite a number of 

 authors. 



^ rtile results from observations. If we see that something 

 happens very frequently under given circumstances, we can state 

 that it is the rule that under such circumstances this happens. It 

 is a rule that objects, released at a certain distance from the earth's 

 surface fall down. It is a rule that the genes for which an organism 

 is heterozygous, are distributed over the gametes without influencing 

 each others distribution. Such rules simply state what generally 

 happens. We may occasionally find exceptions to rules. A rubber 

 bag, distended by a light gas, does not fall towards the earth, but 

 away from it, and as to the distribution of genes, cases of coupling 

 and of repulsion have been found. 



Natural laws are generalizations of another kind. Whenever we 

 find a general rule with a number of definite exceptions, we may, 



