VARIATION 



41 



the two sets of individuals may be so great that it is not filled 



out by variations of both groups under the influence of the 



environment in the widest 



sense, and in other instances 



it will not be large enough 



to keep the variation from 



being continuous. In a popu- 



lation of exclusively black 



animals or exclusively hairy 



plants, there may be indi- 



viduals with and others with- 



out a factor which influences 



colour or hairiness so little, 



that the difference altoge- 



ther escapes notice. There 



are numerous genetic fact- 



ors which we can never hope 



to study, notwithstanding 



the fact that they do in- 



fluence the development. 



We have seen, that vari- 

 ation falls more or less clearly 

 into two different kinds, 

 continuous variation and 

 discontinuous variation. Con- 

 tinuous variation is obvious- 



ly often the result of the fact gous is the same in the three instan- 



that the normlation which ces> but the remainin S genotype is 

 tnat tne popuiatl I wmcn different where the curves overlap, 



shows it, is influenced by individuals, represented by squares, 



have been drawn in black. By pla- 

 cing the original variation -curves 

 closer together, the composite curve 

 becomes two-tipped, and a one-top- 



Mendelian 3: 1 segregation in the 



several non-inherited en- 

 vironmental factors. But we 

 have seen that part of the 

 variability of such a group fore the modes of the original curves 



may be due to the presence fal1 to g ether - 



or absence of inherited developmental factors. It may even 



happen, that a number of genetic factors exist, each of 



ped variation-curve results long be- 

 fore 



