42 VARIATION. 



which tends to change the development a little in such a way 

 that the same quality is slightly changed by each of them. A 

 population of animals or plants which is impure for a number 

 of such factors, may show a typically normal continuous varia- 

 tion, even if the conditions, the environment, under which the 

 organisms grow up, does very little to change the quality in 

 question. 



Such cases are that of human skin-colour, of the coherence 

 of pea-seeds, of ear-length of rabbits, of colour of wheat and 

 flax, of weight. 



Each of these qualities may show a typically normal con- 

 tinuous variation. Therefore we can say, that continuous vari- 

 ation may be caused by a variation of the environment, but also 

 by a variation in the constellation of inherited, transmittable 

 developmental factors, variation of the genotype. And we have 

 seen that discontinuous variation is very often the result of 

 the difference caused by presence or absence of a gene, of in- 

 herited, developmental factors, but it may be often wholly or 

 partly due to a difference in the action of one or more non- 

 genetic factors. 



As therefore both continuous variation and discontinuous 

 variation may be caused by a variation of the genotype, and 

 by a change in the complex of non-genetic factors, the envir- 

 onment, it is very clearly not possible to imagine that only 

 continuous variation or only discontinuous variation can be 

 ; concerned with evolution. Darwin thought that discontinuous 

 variation, the production of "sports" was a rare occurence, 

 and his theory of evolution is wholly concerned with the in- 

 fluence of selection on continuous variation. De Vries, on the 

 other hand, thinks, that continuous variation is wholly caused 

 by variations of the environment, and that only discontinuous 

 variation therefore can be the cause of evolution. 



We have seen that there are essentially four kinds of varia- 

 tion: 



A. Discontinuous geno-variation, discontinuous heritable 

 variation. 



