THE DARWINIAN THEORY 



553 



THE NORMAL CURVE OE VARIATION 



AS SHOWN BY A PAILFUL OF SCALLOPS 



76 SHELLS 



30 SHELLS 



5 SHELLS 



20 SHELL' 



■=4 SHELLS 



15P„\YS 18 RAYS 17 RAYS WRAYS 19 RAYS 



Fig. 307. — Variation in scallop shells. 



Specimens taken at random from a species and measured for any feature usually show 

 a curve of variation of this general type, although a study of the descendants of single 

 individuals may resolve the group into a number of pure lines, each with its curve of fluctuat- 

 ing variations (c/. Figs. 241 and 308). If there appeared in a population, such as is repre- 

 sented by this figure, individuals whose shell rays fluctuated between limits below fifteen 

 or above nineteen, and if such individuals bred true, this would be a clearly recognizable 

 case of mutation. (Photo, by courtesy American Museum of Natural History.) 



