2G0 THE EVOLUTION THEORY 



to-day, by division into two. It follows, therefore, that there is no 

 natuTdl death amono- them, for, if there were, the species would die 

 out as the individuals ^rew old; but this does not happen. The two 

 dau(>-hter oriranisuis which arise from the binary fission of an Infu- 

 sorian are in no way different in regard to their power of life ; each 

 of them possesses an equal power of doubling- itself again by division, 

 and so it goes on, as far as we can see, for an unlimited time. Thus 

 the unicellular organisms are not subject to natural death ; their body 

 is indeed used up in the course of ordinary life so that the formation 

 of new cilia and so on is necessary, but it is not worn away in the 

 same sense in which our body is and that of all Metazoa and Meta- 

 phj'tes, where, through functioning, the organs are gradually worn away 

 initil they become incapable of function. Our body grows old, and 

 can at last no longer continue to live ; but among unicellular organisms 

 there is no growing old, and no death in the normal course of the 

 development of the individual. The unicellulars are, as we may say, 

 immortal ; that is, while individuals may be annihilated, by external 

 agencies, boiling heat, poisons, being crushed, or eaten, and so on, at 

 every period some individuals escape such a fate, and perpetuate 

 themselves through succeeding ages. For, strictly speaking, the 

 daughter-individual is only a continuation of the mother-individual ; 

 it contains not only half of the substance, but also the organization, 

 and life is continued directly from mother to daughter. The daughter 

 is simply half of the mother, which is subsequently regenerated ; and 

 the other half of the mother lives on in the other daughter, so that 

 nothing dies in this multiplication. It may be said that the daughter 

 has to develop the other half of its body anew, and that therefore it is a 

 new individuality, and not merely a continuation of the old, and that 

 therefore the unicellular animals are not immortal. The ' immorta- 

 lity ' of the Protozoa may be scoffed at ; the idea may seem absurd 

 that the ' immortal ' Protozoa are still the same individuals which 

 lived upon the earth millions of years ago, but all such objections 

 mean no more than doctrinaire quibbling with the concepts of 

 ' individual ' and ' immortality,' which do not exist in nature at all, 

 but are mere human abstractions, and therefore only of relative value. 

 My thesis as to the potential immortality of the Unicellulars aims at 

 nothing more than impressing on Science the fact that the occurrence 

 of physiological, that is, natural, death is causally associated with the 

 transition from single-celled to many-celled organisms ; and this is 

 a truth which will not be overthrown by any sophisms. It is the 

 Volvocineee which show us, so to speak, the exact point at which 

 natural death set in, at which it was introduced into the world of life. 



