C. DOBELL 169 



The line bred in hay infusion refused to conjugate : whereas the parallel 

 line in beef extract conjugated. He concludes that " the determining 

 feature was the medium used." '' Conjugation is induced by external 

 conditions." 



101. Jennings (IJ)IO) has come to the conclusion that "the con- 

 ditions determining conjugation differ greatly in different races of 

 Paramecium {aurelia or caudatuin). Some races conjugate frequently, 

 and under conditions readily supplied in experimentation. Others, 

 under the same conditions, conjugate very rarely or not at all." 

 Calkins and Gregory (1913) also share the view that there are con- 

 jugating and non-conjugating lines of Faramecmiii. Such a hypothesis 

 would help us to comprehend the extraordinarily different observations 

 made by different workers : but there is a very serious difficulty in the 

 way of accepting it — namely, we have absolutely no proof that any race 

 exists, which, under suitable conditions, is unable to conjugated Un- 

 doubtedly the weightiest evidence for the existence of such a race was 

 that furnished by Woodruff (§ 39), whose pedigree line of P. aurelia 

 consistently refused to conjugate for over 4000 generations. We 

 now hear, however, that after more than this number of generations, 

 conjugations have at last taken place (Woodruff, 1914). The likelihood 

 that anybody will ever succeed in demonstrating with any plausibility 

 that any given race of ciliates is incapable of conjugating, seems there- 

 fore immeasurably remote. 



102. We may now pass to a consideration of the effects of con- 

 iugation. We have already seen that Maupias considered the chief 

 result of conjugation to be " karyogamic rejuvenation " (§ 17). His 

 work has frequently been misinterpreted as showing that conjugation 

 reinvigorates the stock in which it occurs — that after a number of 

 asexual generations the stock becomes weakened with age and divides 

 more slowly, but may be restored to its former vigour and rate of 

 reproduction by means of conjugation^. The work of Maupas does 

 not show this. Calkins (1904), however, has adopted this standpoint, 

 concluding from his work "that conjugation does actually rejuvenate 

 and overcome the conditions of so-called ' old age '." He appears to be 



• I do not, of course, herebj' controvert the general statement of Jennings (1910) that 

 "the conditions for conjugation are different in different races." 



'■' This view was first definitely advocated by Biitschli (1875, 1876) who believed that 

 conjugation resulted in "eine erhohte Teilungsfahigkeit," which he interpreted as a sign 

 that "Verjiingung" had been brought about. Balbiani's views were — for a time, at least 

 — similar. 



