HERITABLE VARIATIONS AND THE RESULTS OF 



SELECTION IN THE FISSION RATE OF 



STYLONYCHIA PUSTULATA 



AUSTIN RALPH MIDDLETON 



From the Zoological Laboratory of the Johns Hopkins University 



SEVENTEEN FIGURES 



CONTENTS 



Introduction 451 



Technique 454 



Experiments to test the effect of selection on the fission rate within a 



single clone 456 



1. First series: Long continued opposite selection, followed by balanced 



selection, mass culture, and reversed selection 456 



2. Second series: Repetition among progeny of a single individual from 



series 1 487 



3. Third series: Repetition among progeny of single individual not related 



to those of series 1 and 2; also, effects of conjugation 488 



Discussion and conclusions 497 



Summary 501 



List of literature 502 



INTRODUCTION 



Organisms multiplying without admixture of two parents that 

 differ in hereditary constitution have been found remarkably 

 constant in their inherited characteristics. Most recent work 

 agrees that in such uniparental reproduction inherited varia- 

 tions occur rarely or not at all, and that selection has practically 

 no effect in altering racial characteristics. These results have 

 given origin to the concept of the Genotype (Johannsen), as a 

 designation for the permanent heritable constitution of the race. 

 In view of the importance of these relations for the problem of 

 the method of evolution, much further study of this matter is 

 required. The present paper deals with the inheritance of varia- 



451 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 19, NO. 4 



