394 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



respects do they differ? \Miat are the causes of the resemblances 

 or differences, as the case may be? 



The attempt is made to treat these questions broadly, determin- 

 ing experimentally the dift'erent classes of causes concerned, without 

 prejudice as to their relative importance. External and internal fac- 

 tors are therefore equally considered, the purpose of the investiga- 

 tion being to give as complete an analysis of the phenomena of 

 resemblances and differences as possible. Our problem, then, requires 

 an analysis from this point of view of all things which may result 

 in producing, increasing or decreasing the similarities and dift'erences 

 between -individuals— reproduction, growth, conjugation, the effects 

 of environment, of selection, and the like. 



The investigation will be best introduced by proposing at once 

 what is really the central problem — that concerning heredity. Is 

 size inherited in Paraniecinm? 



How would heredity of size be shown? If certain individuals 

 differ in size, and the progeny of these individuals, under identical 

 conditions, show corresponding dift'erences. this is what would com- 

 monly be called heredity of size. " Heredity is a certain degree of 

 correlation between the abmodality of parent and offspring " (Daven- 

 port, 1899, p. 35). Do large individuals of Paramecium produce, 

 under the same conditions, larger progeny than do small ones? Is 

 it possible to obtain by selection large and small races of Paramecia? 



To study this question, we must first examine the variations in 

 size commonly found in Paramecium. 



II. PRELIMINARY STUDY OF VARIATION IN 

 PARAMECIUM. 



We owe our present knowledge of variation in Paramecium 

 mainly to Pearl and his co-workers (see Pearl, 1907; Pearl and 

 Dunbar, 1905). A more extensive work by Pearl on variation in 

 Paramecium has been mentioned as in prospect; I learn from per- 

 sonal communication, however, that this is not to appear. I shall 

 therefore publish my own results more fully than I should otherwise 

 have done. Certain points in connection with variation in Para- 

 mecium have been dealt with by Simpson (1902) and Pearson 



