222 THE PHYSIOLOGY OF REPRODUCTION 



ultimately in death, could be tided over and the race carried through 

 further cycles of activity by having recourse to artificial stimuli in 

 the medium surrounding the culture. In a series of experiments, 

 which Calkins conducted for twenty-three months with a single race 

 of Paramoecium, it was found that periodic reductions of vitality 

 occurred at intervals of about six months. At such times the race 

 under cultivation would have died out entirely had it not been for 

 the application of stimuli in the form of extracts of various food 

 substances (beef, pancreas, brain, etc.). With the assistance of these 

 restoratives, on three separate occasions, this particular race of 

 Paramoecium was carried through four cycles of activity and 742 

 generations without the occurrence of conjugation. It thus appears 

 that a change in the environment may result in a rejuvenescence of 

 the race. 



As a consequence of these experiments, Calkins has suggested 

 that the purpose of conjugation may be to bring about the union of 

 individuals which have lived in different environments, and so to 

 produce a renewal of vitality in the same kind of way as a change in 

 the environment itself. 



Calkins differs from Maupas in stating that diverse ancestry is 

 not essential in order that conjugation may occur, since he obtained 

 as large a percentage of successful endogamous as exogamous 

 pairings, and carried one endogamous ex-conjugant through 379 

 generations. On the other hand, there is some evidence that 

 conjugation does not result in rejuvenescence when both gametes 

 have lived for a long time in the same medium, so that their chemical 

 composition is too similar. 1 



According to Enriques, however, conjugation in Colpoda steini 

 only takes place under certain environmental conditions (e.g. if the 

 layer of the water is not thicker than two millimetres) and does not 

 occur at all if the conditions of life are stationary, the infusorians 

 going on multiplying indefinitely and the number of divisions from 

 the last conjugation making no difference. 2 According to Woodruff, 3 



1 Cull, "Rejuvenescence as a Result of Conjugation," Jour, of Exp. ZooL, 

 vol. iv., 1907. Blackman ("The Nature of Fertilisation," British Assoc. Reports, 

 York Meeting, 1906) is of opinion that the rejuvenescence theory of fertilisation 

 is difficult to apply generally in view of the large number of plants in which 

 the fusing cells or nuclei are closely related. The force of this objection must 

 be admitted. If, however, the conjugating cells have been subjected to slightly 

 different environmental influences, this near relationship is not necessarily a 

 difficulty. 



2 If water from a culture in which conjugation is " epidemic " be added to a 

 normal culture, it is stated to induce conjugation. Conversely, if water from 

 a normal culture is added to a "conjugation culture" it inhibits it. 



:! Woodruff, "The Life-Cycle of Paramoecium when Subjected in Varied 

 Environment," Jour, of Exp. ZooL, vol. xlii., 1908. Jennings, "The Effect of 

 Conjugation in Paramoecium," Jour, of Exp. Zool., vol. xiv., 1913. 



