C. DOBELL 153 



length — giant individuals of this species and Oiiychodronius resulting 

 from cannibalism. 



61. We may now pass to a consideration of variations and their 

 hereditary transmission — a matter to which the foregoing facts inevit- 

 ably lead us. The nature of " heredity " in the asexual reproductive 

 process of a ciliate should be clearly realized, and I would therefore 

 remind the reader of a few important facts before introducing this 

 subject. It is often assumed that " inheritance " in a ciliate is a very 

 simple matter — one organism merely dividing into two exactly similar 

 organisms. A moment's thought, however, will make it clear that when 

 a complex creature with morphologically and physiologically differ- 

 entiated ends divides transversely across the middle, the acquisition 

 of the parent's form by the two daughter-individuals is by no means 

 so simple a matter as at first sight it might ajjpear. Indeed, it is 

 probably true that all the complex buccal and appendicular structures 

 — in fact the entire ciliary coat and all its derivatives — of the parent 

 are resorbed or undifferentiated during division and a set of corre- 

 sponding organs formed by new growth and differentiation in each 

 daughter organism. Details of this process in certain complex forms 

 have been worked out especially by Wallengren (1901) and Griffin 

 (1910). In no known case is the differentiated protoplasm of the 

 parent passively parcelled out or " handed down " to the offspring in 

 the fashion contemplated by a priori speculators. Only certain 

 inclusions' (food, symbiotic algae, etc.), behave thus. The nuclei, of 

 course, divide during division of the organism as a whole, but in the 

 meganucleus undiff'erentiation and subsequent re-differentiation often 

 occur during fission. 



62. Simpson (1902) has shown that when a Paramecium divides 

 into two, the products — measured at given later moments — may differ 

 from one another in size. But Jennings (1909) has shown that "these 

 'variations ' are mere temporary fluctuations, without effect in heredity " 

 (§ 63). 



63. Jennings (1908 a, 1909, 1911) has now demonstrated, by long 

 series of careful measurements and observations, the important fact 

 that the species of Paramecium called caudatum and aurelia are not 

 homogeneous — that each consists of an assemblage of distinct races 

 (Jordan's " little species," Johannsen's " pure lines ") which differ inter 

 se, but which in se are constant. These races differ from one another 

 not only in size and form but also in physiological respects. The 



' But see § 81 et seq., infra. 



