FERTILISATION 221 



to mate with like, since there is a considerable degree of hornogamic 

 correlation both between the lengths of the conjugant pairs and also 

 between their breadths. Evidence is presented to show that the 

 homogamic correlation arises through the necessity for the anterior 

 ends and mouths of the two individuals to " fit " reasonably well in 

 the act of successful conjugation. If this is so, the necessity for 

 assortative mating in Paramcecium is purely mechanical, and the 

 principle involved is not of general application to other gametic 

 organisms. Pearl states, also, that there is no evidence that 

 conjugation tends to produce increased variability in ex-conjugants. 

 On the contrary, there are indications that conjugation tends to 

 restrict the existing variability induced by environmental influences ; 

 or, in other words, to preserve a relative stability of type. This 

 conclusion is antagonistic to Weismann's hypothesis referred to 

 above (see footnote, p. 198). 



As already mentioned, the reproductive processes in the Protozoa, 

 like those in the Metazoa, tend to run in cycles, each cycle beginning 

 and ending with an act of conjugation. Maupas' observations 

 showed that in various genera of Infusoria (Paramcecium, Stylonychia, 

 etc.) a long period, during which the animals multiply by simple cell 

 division, is succeeded by a period when conjugation is of very 

 common occurrence. This marks the commencement of a new cycle, 

 being physiologically comparable to the period of sexual maturity 

 in multicellular organisms. If conjugation were prevented from 

 occurring, the individuals gradually ceased to divide and under- 

 went changes which invariably led to death. As a result of 

 these experiments, Maupas arrived at the conclusion that the 

 purpose of conjugation is to counteract the tendency towards 

 senile degeneration, and to bring about a rejuvenescence or renewal 

 of vitality. 1 



Maupas' observations have been confirmed by Joukowsky 2 and 

 Simpson, 3 and more particularly by Calkins. 4 The last investigator 

 found, further, that the periodic seasons of " depression " or loss of 

 vitality which invariably occurred if conjugation were prevented, 

 and which normally resulted in the cessation of cell division and 



1 Maupas, " Recherches experimentales sur la Multiplication des Infusories 

 Cili3S," Arch, de Zool. Exp. et Gen., vol. vi., 1888. " Le Eajeunissement 

 Karyogamique chez les Cilies," Arch, de Zool. Exp. et Gen., vol. vii., 1889. 



2 Joukowsky, " Beitrage zur Frage nach den Bedingungen der Vermehrung 

 und des Eintrittes der Konjugation bei den Ciliaten," Verh. Nat. Med. Ver., 

 Heidelberg, vol. xxvi., 1898. 



3 Simpson (J. Y.), " Observations on Binary Fission in the Life-History of 

 the Ciliata," Proc. Roy. Soc. Edin., vol. xxiii., 1901. 



4 Calkins, " Studies on the Life-History of Protozoa," IV., Jour, of Exp. 

 Zool., vol. i., 1904. References to earlier papers are here given. See also Biol. 

 Bidl, vol. xi., 1906, and Amer. Nat., vols. xlix. and 1., 1915 and 1916.) 



