THE ORIGIN OF LIFE 155 



multicellular, originate exclusively by reproduction, that is, 

 by generation from living parents of the same organic type or 

 species. This is the law of genetic vital continuity, which, by 

 the way, Aristotle had formulated long before Harvey, when he 

 said: "It appears that all living beings come from a germ, and 

 the germ from parents." ("De Generatione Animalium," lib. I, 

 cap. 17.) All reproduction, however, is reducible to a process 

 of cell-division. That such is the case with unicellular or- 

 ganisms is evident from the very definition of a cell. That 

 it is also true of multicellular organisms can be shown by 

 a review of the various forms of reproduction occurring among 

 plants and animals. 



§ 4. Reproduction and Rejuvenation 



Reproduction, the sole means by which the torch of life 

 is relayed from generation to generation, the exclusive process 

 by which living individuals arise and races are perpetuated, 

 consists in the separation of a germ from the parent organ- 

 ism as a physical basis for the development of a new organ- 

 ism. The germ thus separated may be many-celled or one- 

 celled, as we shall see presently, but the separated cells, be 

 they one or many, have their common and exclusive source in 

 the process of mitotic cell-division. In a few cases, this di- 

 visional power or energy of the cell seems to be perennial 

 by virtue of an inherent inexhaustibility. In most cases, 

 however, it is perennial by virtue of a restorative process 

 involving nuclear reorganization. In the former cases, which 

 are exceptional, the cellular stream of life appears to flow 

 onward forever with steady current, but as a general rule it 

 ebbs and flows in cycles, which involve a periodic rise and 

 fall of divisional energy. The phenomena of the life-cycle 

 are characteristic of most, perhaps all, organisms. The com- 

 plete life-cycle consists of three phases or periods, namely: 

 an 'adolescent period of high vitality, a mature period of 

 balanced metabolism, and a senescent period of decline. Each 

 life-cycle begins with the germination of the new organism 



