STRUCTURE OF ORGANISMS. 31 



ie soma dies ; but the germ-cells live on, passing from 

 one mortal parent to another. As pointed out by Weis- 

 mann, germ-cells, like the unicellular organisms just 

 mentioned, are potentially immortal. Since the first 

 appearance of living substance on this earth death has 

 never interrupted the main streams of life. Death is, 

 so to speak, a by-product of multicellular organisms. 



But even the soma might conceivably go on living for 

 ever, provided only it continue growing, or the wear and 

 tear of life be perpetually repaired. We are familiar with 

 plants which can be propagated by cuttings for an indefi- 

 nitely long period, and we know of others, such as trees, 

 which, preserving vigorous embryonic cells at their grow- 

 ing surfaces, may prolong their natural life for thousands 

 of years. Even in the best regulated organism, however, 

 those essential proportions between surface and volume 

 can hardly be so well preserved ; that nice co-ordination 

 of parts, that power of repairing injuries and waste, can 

 hardly be so accurately adjusted as to continue working 

 smoothly for ever. Especially is this the case in animals, 

 where both size and shape are almost always definite and 

 limited. If one may be allowed to use metaphorical 

 language rather than go on resisting the wear and tear 

 of life, nature sacrifices the old and battered soma, and 

 trusts the germ-cells to start a new individual afresh. 



So the length of life of an organism may become defi- 

 nitely adapted to its needs ; as in the case of annual 

 plants and most animals, where the energies of the in- 

 dividual are exhausted in securing the success in life of 

 the next generation. Thus a large number of animals 

 in the colder regions live only for one season, leaving 

 behind them their eggs to survive the winter anjl develop 

 next year. Frequently the male sex dies as soon as fer- 

 tilisation has been accomplished. A definite relation 

 can be traced between the length of life of the individuals 

 of a species and the number of young produced, and the 

 care required to bring them up. 



