REPRODUCTION. 



As has been already noticed, protoplasm, in whatever form, 

 after passing through certain stages in development, undergoes 

 a decline, and finally ^ies and joins the world of unorganized 

 matter ; so that the permanence of living things demands the 

 constant formation of new individuals. Groups of animals 

 and plants from time to time become extinct ; but the lifetime 

 of the species is always long compared with that of the indi- 

 vidual. Reproduction by division seems to arise from an exi- 

 gency of a nutritive kind, best exemplified in the simpler or- 

 ganisms. When the total mass becomes too great to be supported 

 by absorption of pabulum from without by the surface of the 

 body, division of the organism must take place, or death ensues. 

 It appears to be a matter of indifference how this is accom- 

 plished, whether by fission, endogenous division, or gemmation, 

 so long as separate portions of protoplasm result, capable of 

 leading an independent existence. The very undifferentiated 

 character of these simple forms prepares us to understand how 

 each fragment may go through the same cycle of changes as 

 the parent form. In such cases, speaking generally, a million 

 individuals tell the same biological story as one ; yet these 

 must exist as individuals, if at all, and not in one great united 

 mass. But in the case of conjugation, which takes place some- 

 times in the same groups as also multiply by division in its 

 various forms, there is plainly an entirely new aspect of the 

 case presented. We have already shown that no two cells, how- 

 ever much alike they may seem as regards form and the cir- 

 cumstances under which they exist, can have, in the nature of 

 the case, precisely the same history, or be the subjects of ex- 

 actly the same experiences. We have also pointed out that all 

 these phenomena of cell-life are known to us only as adapta- 

 tions of internal to external conditions ; for, though we may not 

 be always able to trace this connection, the inference is justi- 



