452 AUSTIN RALPH MIDDLETON 



tions and the effect of selection in the case of a most dehcately 

 poised and readily modifiable physiological character, the rate 

 of fission of an infusorian multiplying without conjugation. 



Most studies of the fission rate in infusoria have consisted in 

 observations on the rate in given species, for descriptive pur- 

 poses ; or in examinations of the direct effect of changed physical 

 and chemical conditions; or in the study of the effect of conju- 

 gation on the rate of multiplication. Review of these studies 

 is not demanded here. 



A certain number of observations have been made which bear 

 on the inheritance of the fission rate and on the question whether 

 there are differences among the progeny of a single individual 

 in respect to these. The great papers of Maupas ('88 and '89) 

 contain some of the most important of these. Maupas made 

 extensive studies of the effects of temperature and of other con- 

 ditions on the fission rate in many species of infusoria; among 

 the.rest in Stylonychia pustulata, the subject of the present study. 

 Maupas became convinced, as a result of his studies, that under 

 given conditions the rate of fission is uniform in all the progeny 

 of a given individual; that inherited variations in the rate do 

 not occur in such a 'pure race' or to use the modern term, within 

 a single clone. On this matter the conclusions of Maupas are 

 in complete harmony with those of the 'pure line' workers and 

 .upholders of the constancy of the genotype, in more recent times. 

 He gives detailed observations on the matter for a number of 

 species, and resumes his results as follows: 



In all my cultures I have always seen all the normal descendants of 

 the same ancestor, grow and multiply with the most perfect uni- 

 formity. I have become convinced of the integral transmission of the 

 faculty of development from one generation to another, and the most 

 complete physiological equivalence must exist among all the normal 

 individuals, produced in the successive generations ('88, p. 203). 



In long and numerous experiments on fifteen to twenty species, I 

 have never observed anything which permits belief in the existence 

 of morphological and physiological differences, not merely between the 

 two products of a given fission, but even among all those which have 

 descended from such a fission by regular and continuous generations 

 ('88, p. 176). 



