IN RATS AND GUINEA-PIGS. 3 



Such changes are not the result of selection; they often appear, as it 

 seems, spontaneously, and they are permanent in the race, if isolated. 



De Vries maintains that all species-forming variations are of this sort; 

 that selection is unable to form new species, because it can neither call into 

 existence mutations nor permanently modify a race by cumulation of 

 abmodal fluctuations. Darwin, on the other hand, and the great majority 

 of his followers, while admitting that races are occasionally produced by 

 discontinuous or "sport" variation, ascribe evolutionary progress chiefly to 

 the cumulation through long periods of time of slight individual differences, 

 such as De Vries calls fluctuations. The issue between the two views is 

 sharp and clear. According to De Vries, if we rightly understand him, 

 selection is not a factor in the production of new species, but only in their 

 perpetuation, since it determines merely what species shall survive; accord- 

 ing to the Darwinian view, new species arise through the direct agency 

 of selection, which leads to the cumulation of fluctuating variations of a 

 particular sort. 



De Vries and the Darwinians differ not only as to the part which selection 

 plays in evolution, but also as to the nature of the material upon which 

 selection acts. According to De Vries, species are not modified by selection ; 

 mutations are new species and selection determines only what mutations 

 shall survive, fluctuations having no evolutionary significance. On the 

 Darwinian view, all species, whether arising by mutation or not, are subject 

 to modification by selection. 



A great deal can be said in favor of each of these contrasted views, but dis- 

 cussion is at present less needed than experimental tests of the views outlined. 

 To De Vries we owe much for showing that such tests are possible. 



It was our purpose to make tests of this sort when we undertook the 

 experiments described in this paper. The questions to which principally 

 attention has been directed are these: (i) Can discontinuous variations be 

 modified by selection alone? (2) Can discontinuous variations be modified 

 by cross-breeding? A negative answer to these questions will support the 

 view of De Vries ; an affirmative answer will support the Darwinian view, 

 because it will show that through selection new conditions of organic stability 

 can be obtained; that is, new species may be produced. 



The material used consisted of certain discontinuous variations in the 

 color- pattern of rats. The general result obtained is this: Various color- 

 patterns, like the several pigments found in the rodent coat, are mutually 

 alternative in heredity. Each group of individuals referred to the same 

 type of color- pat tern forms a continuous series fluctuating in accordance 

 with the laws of chance about a common modal condition. The different 

 types in general do not overlap ; they form a discontinuous series. Now, these 

 types may be modified in two different ways : (a) By selection of abmodal 



