Studies on variation and selection. 



151 



to build up. Thus, it is just as naive to call a certain genetic factor b, 

 because it is present in a black animal and not in a chocolate- 

 coloured one, as it would be, to call copper by the same letter, be- 

 cause of the fact, that copper-sulphate is blue and sulphur alone is not. 



II. Continuous and discontinuous variation. 



Darwin made no clear distinction between continuous an dis- 

 continuous variation. He observed that all organisms varied, that 

 every character studied varied, and that the variations constituted 

 mostly a continuous series. To that which later has come to be 

 called discontinuous variation, he did not give much of his attention. 



If we study any quality of living organisms, we see at once, 

 that this quality is not identically the same in all; there is always 

 a commonest grade of it, and the more the plants or animals have 

 this character in a grade different from the commonest, the rarer 

 they are. Practically every character varies, and its variation follows 

 the laws of chance, and can be rendered graphically in a more or 

 less normal Quetelet's curve. 



Even to this day there survives a whole class of scientists, the 

 Biometricians, who concern themselves with the study of this kind 

 of variation, or rather with the recording of it, for it is a curious 

 fact, that it is commonly taken for granted by them, that the causes 

 underlying variation defy analysis. 



With Bateson originates the distinction between continuous and 

 discontinuous variation. In his work "Materials" he pointed out a 

 class of cases in which another type of variation was seen, not giving 

 an unbroken series of grades, graduating one into the other. In 

 some cases variation is distinctly discontinuous, such as where in a 

 population of cattle some have horns and some are hornless, or 

 where some chickens have tails and others none, or where in a 

 sowing of teazels some are normal and some are twisted. 



DE Vries, in his "Mutationstheorie" has further elaborated this 

 distinction between continuous and discontinuous variation, and it is 

 chiefly de Vries who is responsible for the fact, that nowadays con- 

 tinuous variation has somehow become practically synonymous with 

 non-inheritable variation, and discontinuous variation with inheritable. 



de Vries emphasized the fact, that the differences causing dis- 

 continuous variation are more important than those resulting in 

 continuous variation. In the first days of Mendelism, the genetic 



