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 lessclearly 

 into two different kinds, 

 continuous variation and 

 discontinuous variation . Con- 

 tinuous variation is obvious- 

 ly often the result of the fact 

 that the population which 

 shows it, is influenced by 

 several non-inherited en- 

 vironmental factors. But we 

 have seen that part of the 

 variability of such a group 



Fig. 5. 

 Diagram to illustrate the shape of 

 variation curves in similar cases of 

 Mendelian 3: 1 segregation in the 

 ofl-spring of heterozygotes. The gene 

 for which the parents are heterozy- 

 gous is the same in the three instan- 

 ces, but the remaining genotype is 

 different. Where the curves overlap, 

 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- 

 ped variation-curve results long be- 

 fore the modes of the original curves 

 fall together. 



may be due to the presence 



or absence of inherited developmental factors. It may even 



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



