CELL CONSTANCY IN THE GENUS EORHYNCHUS 291 



degree when compared with the range of variation in the more 

 plastic groups of animals. This tendency toward uniformity 

 of body characters has led to, or possibly has resulted from, a 

 corresponding stability in the numbers of cells which go to make 

 np the organs of the body in the genus Eorhynchus. It is true 

 that within this genus the parasitic habit has contributed toward 

 the elimination of physical factors which might induce radical 

 changes. The result of this is that variability is practically con- 

 fined to minor variations such as those of body size, and in general 

 those which concern slight rearrangements of the component 

 cells, rather than those involving the production of entirely new 

 types of structures. The future of such a group of organisms is 

 very clear. The stability which has been produced serves as an 

 effective barrier to evolution. Under present conditions those 

 organisms displaying marked cell constancy are incapable of 

 producing new creations in the lines of organic progress, for the 

 elimination of variability has precluded the possibility of pro- 

 gressive evolution. 



I wish to express my indebtedness to Prof. H. B. Ward for the 

 many helpful suggestions and kindly criticism of methods and of 

 general outline of the work, rendered throughout the period of 

 its preparation. 



