-GEOLOGY 



63 



upon a large number of specimens which, though they exhibit a wide 

 range of forms, make up a continuous series. From these Carruthers 

 selected samples typical of various phases in the sequence and called 

 them Z. delanoiiei {s. str.), Z. parallela, Z. consiricta, Z. disjuncta (early, 

 typical and advanced) respectively (Fig. i). Between these types there 

 occurred every gradation of form. In the next place these specimens 

 were collected from a succession of known horizons in the Lower 

 Carboniferous rocks of Scotland. Though some of these horizons were 

 separated by relatively wide intervals the range of variation exhibited by 

 the specimens collected at different levels overlapped to such an extent 



S T AG E5 isi ^ 



V, 



EMBRyOhJIC 



I TRANSITIONAL 

 STAGES 



I N 



NEANIC 

 DEVELOPMENT 



ADULT 



I'iG. I. — Diagram showing typical representatives of the gens Zaphrentis 

 delanouei, the order of their appearance in time and the main stages in 

 their development. (Modified from Carruthers.) 



that the continuity in the sequence of forms, from the bottom to the 

 top, was not broken. Further, the frequency of occurrence of each of 

 the types was recorded, and when plotted produced a curve which con- 

 formed to the normal unimodal frequency distribution curve. When 

 the curves for successive levels were compared it was found that the 

 mode shifted with the passage of time from Z. delanouei {s. str.) at the 

 bottom to advanced forms of Z. disjuncta at the top, thus showing that 

 the stock was undergoing a corresponding evolutionary change during 

 the period of its existence. The evolutionary character of this sequence 

 was further supported by the very close similarity of the developmental 

 stages of the later to those of the earlier types. 



For the purposes of making a comparison between the development of 



