212 THE PHYSIOLOGY OF REPRODUCTION 



at the conclusion that in this protozoon there is a definite 

 tendency for like individuals to mate with like, since there is a 

 considerable degree of homogamic 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 Paramoecium 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 variabihty induced by environmental in- 

 fluences ; or, in other words, to preserve a relative stabihty of 

 type. This conclusion is antagonistic to Weismann's hypothesis 

 referred to above. (See footnote i, p. 192.) 



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 In- 

 fusoria (Paramoecium, Stylonychia, &c.) 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 physio- 

 logically comparable to the period of sexual maturity in multi- 

 cellular organisms. If conjugation were prevented from occur- 

 ring, the individuals gradually ceased to divide and underwent 

 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 vitahty.^ 



Maupas' observations have been confirmed by Joukowsky ^ 



' Maupas, " Recherches exp^rimentales sur la Multiplication des Infusories 

 Cilids," Arch, de Zool, Exp. et Oen., vol. vi., 1888. " Le E6gennissement 

 Karjogamique chez les Cili^s," Arch, de Zool. Exp. et Oen., vol. vii., 1889. 



2 Joukowsky, "Beitrage zur Frage naoh den Bedingungen der Ver- 

 mehrung und des Eintrittes der Konjugation bei den Ciliaten," Verh. 

 Nat. Med. Ver. , Heidelberg, vol. xxvi., 1898. 



