WEISMANWS ESSA YS UPON HEREDITY. 5 



low unicellular algae, and the more highly organized 

 unicellular infusoria. They can be killed, but they 

 do not die naturally. In this sense they are 

 immortal. And, as Professor Weismann puts it, 

 " Each individual of any such unicellular species 

 living on the earth to-day is far older than man- 

 kind, and is almost as old as life itself." 



Assuming, then, that the higher forms are evolved 

 from the lower, when did natural death appear, and 

 how could it have been for this is Weismann's 

 contention a gain to the species, brought about 

 by natural selection ? 



The answer is that when we pass from the 

 unicellular to the multicellular organisms we are 

 at first as far as ever from death. Multicellular 

 organisms, like unicellular, are capable of being 

 killed, but so long as the cells are homogeneous, 

 there is no death. For the dissolution of the cell- 

 colony is not death, since the separated cells still 

 live. But when among the multicellular organisms 

 we reach those in which a division of labour has 

 taken place among the cells that is to say, when 

 we pass from homoplastides to heteroplastides a 

 new fact appears. The previously identical cells 

 have become dissimilar, some being perishable, the 

 somatic cells, the others, the reproductive cells, 

 retaining the potential immortality, which belonged 

 alike to unicellular organisms and to the cells of 

 the homoplastid colony : 



