IV. 

 THE EVOLUTION OF DEATH. 



Mortality was formerly regarded as the necessary end- 

 phenomenon of life. It was not until our own times that 

 it appeared probable to us that so-called natural death does 

 not occur with aU organisms. 



The development of the higher plants and animals begins 

 with the fertilized ovum. By continued division such an egg 

 produces the cells which form the plant or animal, as the case 

 may be. Many years ago Huxley defended the thesis that 

 aU cells which arise from a single ovum belong together and 

 constitute a cycle. He further "prdpb&d to regard all the 

 ceUs of a single cycle as constituting the individual proper. 

 The problem of individuality, however, which formerly 

 often occupied thinkers, has lost much in interest and signifi- 

 cance, owing to the progress of biology. In the higher animals 

 as in the unicellular, we encounter real individuals , but in tlie 

 lower multicellular animals we recognize on the contrary no dis- 

 tinct individualities. Thus, for example, in the case of corals 

 and sponges, we cannot speak of individuals. Under these 

 conditions Huxley's conception of the cycle was very seduc- 

 tive to biologists. It could apparently be very well applied 

 to the unicellular organisms because in many of them con- 

 jugation had been observed. Conjugation is a phenomenon 

 closely related with sexual reproduction. It was assumed 

 that conjugation served to excite the cell division of unicellu- 

 lar organisms. If conjugation and the fertilization of the ovum 

 are homologous phenomena, then we are justified in regard- 



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