FISSION RATE OF STYLONYCHIA PUSTULATA 499 



a source of indefinite and unlimited variation. After a time the 

 possible combinations of factors would be exhausted, and such 

 constancy would result as Johannsen claims to have found in 

 his 'pure lines' of beans. Evolution thus could not make ex- 

 tended and continuous progress in this way. If on the other 

 hand the nuclear reorganization occurs in no precisely definable 

 way, but with indefinite and unlimited variations, then this 

 apparatus shows precisely the characteristics held to be common 

 to organisms by those who believe in continued evolutionary 

 progress through the accumulation of such indefinite and un- 

 limited variations. Some material- basis for such variations 

 would have to be assumed; the nucleus might furnish this as well 

 as any other portion of the organism. But, as we have seen, 

 the present evidence does not favor the idea that the hereditary 

 variations in Stylonychia are dependent at all on these nuclear 

 reorganizations . 



Our main result, that during vegetative reproduction among 

 the progeny of a single individual selection of small variations 

 produces cumulative hereditary effects, is in marked contrast 

 with the results of most investigators, who, following Johannsen 

 ('03, '09, '11), have found that 'pure lines' or 'clones' are heredi- 

 tarily constant under selection. Johannsen 's results were ob- 

 tained with self-fertilized lines of beans. Similar ineffectiveness 

 of selection has been found by Hanel ('08) and Lashley ('15) 

 as to the number of tentacles in Hydra multiply by budding, 

 by Jennings ('08, '09, '10) for size in infusoria; by Barber ('07) 

 (in the main), and by Winslow and Walker ('09), in bacteria; 

 by East ('10) in the vegetative reproduction of the potato, by 

 Agar ('13 and '14) in Cladocera and aphids multiplying partheno- 

 gerietically ; and by various other investigators on diverse organ- 

 isms. Some discordant results have been recorded, but most 

 of these are ill-defined or uncertain ; it is mainly in bacteria, with 

 their immense difficulties for precise technique in pedigree work, 

 that heritable variations or modifications have been described. 

 The immense preponderance of evidence has been that in uni- 

 parental reproduction heritable variations do not occur (save as 

 rare mutations of marked character), and that selection of slight 



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



