962 



PRINCIPLES OF STRATIGRAPHY 



majority of individuals of that fauna approximate. The dominant 

 sub-mutation of a higher mutation will be found to have a smaller 

 index than the dominant sub-mutation of a lower mutation. In 

 the same way, the most primitive sub-mutation of the higher muta- 

 tion, i. c, the one with the largest shell index, has a smaller index 

 than the most primitive sub-mutation of the lower mutation. In 

 like manner the most specialized sub-mutation of the higher (geo- 

 logic) mutation will have a smaller index than the most specialized 

 sub-mutation of the lower mutation. In other words, not only has 

 the dominant sub-mutation of the higher mutation advanced beyond 



75%- 



1.1 1.0 ass 0.73 



Fig. 254. Curves representing the range in shell index of two mutations of 

 Spirifer mucronatus. 



that of the lower mutation in the savic direction of modification of 

 proportions, but also the most primitive and the most specialized 

 sub-mutation and all the intermediate sub-mutations of the later 

 mutations are ahead of the earlier one. This may be expressed in 

 the accompanying diagram (Fig. 254), where the height of the 

 curve represents the percentage of individuals and the base the 

 decline in shell index from 3.0 to 0.5. 



Mutation Theory of De Vries. 



In 1901 Professor Hugo De Vries published his epoch-making 

 book "Die Mutations-theorie" (6) in which he recognized that the 

 Linnsean "species" was in reality a compound of innumerable 

 groups of more restricted relationship. These minor groups, which 

 have generally been classed together as a "species," are really enti- 

 ties composed of very definite associations of minute characters, 

 and to them the name elementary species applies. Of these ele- 

 mentary species there may be very many in a Linnaean species. 

 These elementarv species De A'ries thinks arose suddenly by a new 

 combination of the eleinents of which the characters of organisms 

 are made up. These elements (Einheitcn) are sharply separated 



