C. DOBELL 157 



than the original race to sudden changes of temjDerature. The original 

 culture was derived from a single individual; and after conjugation an 

 exconjugant was similarly isolated and cultivated, so that Jollos's belief 

 that he has assisted at the production of a " mutation " appears' to be 

 justified. 



71. Jollos (1913) has further recorded experiments with arsenic 

 compounds. He finds that Paramecium may acquire an increased 

 resistance to arsenic if treated with it for some time-. An arsenic- 

 resistant race so produced remains resistant for a considerable time if 

 bred further in an arsenic-free medium. But the increased resistance 

 gradually diminishes in the course of time, until it is finally lost. The 

 loss is more rapid if the animals are subjected to sudden changes in 

 temperature and nutrition. For such a partially permanent change 

 Jollos proposes the name of " enduring modification " (Dauermodifika- 

 tion). (See also infra, § 113.) 



72. Estabrook (1910) has found no evidence that a race of Para- 

 mecium of a given mean size can be transformed by chemicals (§ 54) 

 into a larger or a smaller race. Temporary changes in size were 

 observed, but regarded as due to " variations in the nutritive and 

 other conditions of the normal environment." Concerning another 

 ciliate, however, Prowazek (1909) has published a peculiar observation. 

 Leucophr-ys patula was said by Maupas to be dimorphic — some organisms 

 being large, others small ^ Prowazek has confirmed this : and he adds 

 that he has been able, by treating cultures of the smaller form with 

 minute doses of quinine, to extract from them races of the larger form. 

 These, however subsequently reverted to the smaller form. 



73. Among the records of experimentally produced variations the 

 researches of Popoff occupy a prominent place. From studies relating 

 to the karyoplasmic ratio (|§ 45 — 49) he surmised that variations in 

 size might originate through alteration of the relative proportions of 

 nuclear and cytojDlasrnic matter. Thus if an organism with a given 

 karyoplasmic ratio ;*■ chanced to divide into two new individuals of 

 unequal size, but each with a normal ratio («) ; then it might be 

 supposed that the now individuals would give rise to new races of 



' His paper is only a preliminary note, without adequate evidence for his statements. 



2 Compare the similar but more definite results previously obtained by others with 

 trypanosomes — reviewed in an earlier paper in this Journal (Vol. ii. p. 201, 1912). 



3 Maupas thought the small forms were probably sexual individuals. Prowazek never 

 saw them conjugate, and appears not to share this view : but what his own interpretation 

 of the dimorphism may be (? sexual) I cannot comprehend. It is certain, at all events, 

 that the large and small individuals are not females and males (§ 14). 



Journ. of Gen. iv H 



