ORGANIC EVOLUTION — PHYSICAL 45 



It (the organism) is during life subjected to all man- 

 ner of external conditions, "svhich tend to destroy it, 

 or so to maim and enfeeble it that it is easily destroyed ; 

 for, unlike plants and low animal organisms, the interde- 

 pendence of the parts as well as of the cells of a higher 

 animal organism is so great, that no one part can be in- 

 jured without all the otherparts being injuriously affected. 

 The ultimate death of all organisms with liighly siDecial- 

 ized cells is tbereforc inevitable. To put it another 

 way : all cells or aggregate of cells, which are so special- 

 ized as to be incapable of independent existence, cannot 

 continue the race, and must inevitably perish. In hiyh 

 animal oiganisms the germ cells alone are capable of 

 existing independently, and therefore they alone survive 

 in tlieir descendants. It is true that in the liighest 

 animals the germ cells after conjugating are still re- 

 tained within the body of, and supplied with the nutri- 

 ment by, one of the parents till development is con- 

 siderably advanced, but in the sense in which I write 

 they as truly lead a separate existence as the child 

 whom the mother suckles. 



But though conjugation is not universally necessary 

 to unending reproduction it is yet generally true ; and 

 the point I Avish to emijhasize is this — that in the 

 highest animals, as well as in tlie lowly infusorians, 

 unless conjugation occurs after a certain number of cell- 

 generations, cell-multiiilication ceases, and the race 

 perishes. 



The germ cell, like the unicellular organism, on con- 

 jugating, divides and redivides many times Avithout con- 

 jugation ever occurring again among its descendant 

 cells, the successive generations of Avhicli may be com- 

 pared to successive non-conjugating generations of 

 infusorians, so that the body of a multicellular organism 

 in the successive stages of its ontogeny is comparable to 

 later and later generations of infusorians ; but, unlike 



