PRESENT I'ROHLEMS IN EVOLUTION AND HEREDITY. 350 



sioUj but with ;ui entirely different sii]»])<)siti<)n as to tlicnahue of tlie 

 material reduced or eliminated, 



Maiipas on ('onjngatioii among the Infusoria.* — Among the newer 

 researches which tlirow light upon this old problem, those of Mani)as 

 are certainly the most brilliant. After a most exact and arduous re- 

 search, extending o\er several years, he collected his results in two 

 memoirs, published in 1S89 and 1S!)(). 



His exi)eriments were first directed n])on the laws of direct mnlti- 

 plication by fissicm, which revealed a complete cycle of life in the single- 

 celled Infusoria and showed that after a long j)evio<l this mode of 

 reproduction becomes less vigorous, then declines, and finally ceases 

 altogether unless the stock is reiuvenated by conjugation of individ- 

 uals from different broods. In other words, these broods of minute 

 organisms grow old and die unless they are enabled to fertilize each 

 other by an exchange of hereditary substance altogether analagous to 

 that observed in the higher multicellular organisms. 



The cultures were made in a dro[» of water upon a slide, and feeding 

 was adapted either to the herbiAorous or carnivorous habits of the 

 species. Under these conditions it was found that the late of fission 

 or direct multiplication varied directly with the temperature and food, 

 rising in some species {GJaneoina ftciiitilUois) to five bipartitions daily. 

 With the o]»timum of conditions this rate, if sustained for thirty-eight 

 days, would produce from a single individual a mass of protoplasm 

 equivalent to the volume of the sun. This rate is however found to 

 be steady for a time, and then the offs])ring decline into ''senescence," 

 in which they appear at times only one fourth the original size, with 

 reduced buccal w^reaths and degenerate nu(;lear apparatus. This is 

 reached sooner in some species than in others; Stylonichia pustulata 

 survives three hundred and sixteen generations or fissions, while Leu- 

 cophryH p<it}ila persists to six hundred and sixty generations. Finally, 

 even under the most favorable conditicms of environment, death ensues. 



Not so where conjugation is brought about by mingling the offspring 

 of different broods in the same fluid, as in tlu^ natural state, Maupas 

 soon discovered that exhaustion of food would induce conjunction be- 

 tween members of mixed broods. He thus could watch every feature 

 of the conjugation process, and determine all the ])hases in the cycle of 

 life. These differed, as in the longevity of the species. In Stylonicliia, 

 for example, "immaturity" extended over the first oiu' hundred bipar- 

 titions: "])uberty,^' or the earliest phase favorabh^ to conjugation, set 

 in with the one hundred ami thirtieth bipartition; '-engamy," or (he 

 most favorable conjugation phase, extended to the one hundred and 

 seventieth; then "senescence" set in, characterized by a sexual hyper- 

 a^.sthesia in wiiich conjugation was void of result or rejuvenesence, 

 owing apparently to the destruction of the essential nuclear ai)paratus. 



*8nr]ji uuiltiplicaf ioii dos IiiCnsoires Cilios, Ardiir. dc ZoUloijlv cxperimentala, Sor. 

 2, vol. VI., pp. 1(55-273; Le I'ajcinnsscment luiri/of/aiiiitiiic dies Ics Cilie's, vol. A'll, pp. 

 149-517. See also Ilartog, Quurl. Jour. Microscop. Scioicc, December, 1891. 



