THE ORIGIN OF LIFE 161 



be had to seedlings. Hence a process of nuclear reorgani- 

 zation seems, in most cases, at least, to be essential for 

 the restoration of vitality and the continuance of life. Whether 

 this need of periodic renewal is absolutely universal, we can- 

 not sa5^ The banana has been propagated for over a century 

 by the somatogenic method, and there are a few other in- 

 stances in which there appears to be no limit to this type of 

 reproduction. Nevertheless, the tendency to decline is so 

 common among living beings that the rare exceptions serve 

 only to confirm (if they do not follow) the general rule. 



In cytogenic reproduction three kinds of rejuvenation by 

 means of nuclear reorganization are known: (1) amphimixis 

 or syngamy; (2) automixis or autogamy; (3) endomixis. In 

 amphimixis or syngamy, two gametic (haploid) nuclei of dif- 

 ferent parental lineage are commingled to form the diploid 

 nucleus of the zygote, which is consequently of biparental ori- 

 gin. In automixis or autogamy, two reduced or haploid nuclei 

 of the same parental lineage unite to form a diploid nucleus, 

 the uniting nuclei being daughter-nuclei derived from a com- 

 mon parent nucleus. In endomixis, the nucleus of the ex- 

 hausted cell disintegrates and fuses with the cytoplasm, out 

 of which it is reformed or reconstructed as the germinal nu- 

 cleus of a rejuvenated cellular series. Endomixis occurs as 

 a periodic phenomenon among the protists, and it appears to 

 be homologous with parthenogenesis among metists. In cer- 

 tain ciliates, like the Paramoecium, endomixis and syngamy 

 are facultative methods of rejuvenation. This has been proved 

 most conclusively by Professor Calkins' work on Uroleptus 

 mohilis, an organism in which both endomixis and conjugation 

 are amenable to experimental control. Nonsexual reproduc- 

 tion in this protozoan (by binary fission) is attended with 

 a gradual weakening of metabolic activity, which increases 

 with each successive generation. The initial rate of di- 

 vision and metabolic energy can, however, be restored 

 either by conjugation (of two individuals), or by endomixis, 

 which takes place (in a single individual) during encyst- 



