Il8 ELEMENTS OF BIOLOGY. 



ever, ultimately comes in the life of all organisms when mul- 

 tiplication by gemmation and fission becomes insufficient, 

 when it becomes necessary that the essential elements of 

 reproduction should be produced. The additional tax thus 

 imposed upon the organism is usually borne without injury for 

 a certain length of time ; but the losses thus caused, if slow, 

 are sure, and in some cases they are so great as to end in 

 the immediate extinction of the organism. There are, how- 

 ever, strong grounds for the belief that in this respect man's 

 position differs materially from that of all other animals. 



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