MUTATION OF WAAGEN 



961 



way and within this horizon the derivatives of this species will 

 become modified in certain definite directions. 



As an illustration may be chosen the Linnsean species of brachi- 

 opod Spirifer mucronatus of the Middle Devonic of eastern North 

 America (Fig. 253). This is represented by at least five distinct 

 mutations in successive horizons, or in distinct basins. Each of 

 these five mutations dififers from the others in certain more or less, 

 constant characters, which, however, are the result of definite 

 modifications of the preceding more primitive types, chiefly by the 

 appearance of new characters.* Thus these mutations are readily 



Fig. 253. Spirifer mucronatus, a. primitive mutation — b.-d. mutation thed- 

 fordensc. b. Long-winged retarded submutation, shell index 

 1.7. c. The most accelerated submutation (shell index 0.73). d. 

 The same drawn with curvature eliminated so as to show full 

 length. The more transverse character (higher shell index) of 

 the younger stages is shown in each. 



recognizable and separable from one another with comparative 

 ease. Within each mutation, however, there is a long series of vari- 

 ants, which are modified by a progressive change in the relative 

 proportion of width and height — a modification or change of quan- 

 titative rather than qualitative character — a type of change to which 

 Osborn has applied the term allouictric, while the resulting char- 

 acters are allomctrons. The change in proportion in each of these 

 successive mutations is from broad-winged to short-winged types, 

 or allometrons. Expressed in shell indices, derived by dividing the 

 entire width along the hinge-line by the height measured on the 

 curvature, the change is from a high shell index to a lower one. 

 In each mutation the change is in the same direction, and in each 

 a dominant type can be designated which, as a sub-mutation, repre- 

 sents, for tlie mutation to which it belongs, that index to which the 



* Termed, by Osliorn, Rectigradations. (See beyond.) 



