158 K. S. LASHLEY 



dence- bearing upon the stability of the unit character. The 

 first attempt to test the general application of Johannsen's 

 conclusions was made by Elise Hanel. In 1908 she published 

 the results of two years of experimental study of inheritance in 

 the asexual reproduction of the fresh-w^ater polyp, Hydra grisea, 

 and drew conclusions which were quite in accord with the results 

 obtained with self-fertilized lines of beans. Her results were 

 soon called in question, however, by the work of Hase ('09) 

 and the criticisms of Pearson ('10), who obtained such con- 

 flicting results that a re-investigation of the subject became 

 necessary. The results of such a renewed, independent study of 

 variation and inheritance in Hydra are j)resented in the present 

 paper. 1 In order to make clear the questions at issue and the 

 bearing of the results presented here upon these and the general 

 problems of genetics, it will be necessary to review briefly the 

 work of Hanel and the criticisms raised against it, especially as 

 the latter are complicated and not clearly valid. 



Before this is undertaken, however, some explanation of the termi- 

 nology of the present paper may not be out of place in view of the past 

 confusion in genetic literature. Johannsen ('13), following Shull, 

 has employed the word 'clone' to describe a familj- descended from a 

 single individual by asexual reproduction. The name 'biotype' applies 

 to any group of organisms which show the same hereditary constitution 

 ('genotype'). In the statistical terminology the 'distribution' is the 

 natural arrangement of individuals showing variation ('variates') in 

 classes. The '^mean' is the arithmetical average of the variates. The 

 'standard deviation' (a) is an expression of the average extent of variation 

 from the mean shown by the variates. The 'coefficient of variation' 

 expresses the average percentage of variation from the mean in terms 

 of the mean as unity. The 'coefficient of correlation' (r) represents 

 an arbitrary measurement of the average degree of resemblance be- 

 tween correlated members of two series of variates. The 'coefficient of 

 regression' (R) expresses the average extent to which the variations of 

 one series follow the variations of the correlated series. 



The nomenclature of the genus Hydra has been subject to frequent 

 changes. The names which have been used most frequently by ex- 



^ This was undertaken at the suggestion of Prof. H. S. Jennings, to whom I 

 wish to express my indebtedness for his assistance and kindly criticism during 

 the course of my work. I wish also to thank Prof. B. E. Livingston for the use 

 of the facilities of the Laboratory of Plant Physiology of the Johns Hopkins 

 University during the sunnner of 1912. 



