THE DEVELOPMENT OF ORGANISMS 8oi 



Yet in spite of this, attempts at generalisation have been made along 

 various lines. 



(i) There is a widely accepted view that large animals, like 

 elephants, have long lives; while small animals, like shrews, have 

 short ones. There is probably a grain of truth in this, for large size 

 usually means considerable capitalisation of energy; so a wealth of 

 reserves should tend, other things equal, towards the prolongation 

 of life. But the generalisation will not work. A cat or a toad may live 

 as long as a horse, say forty years; and a crayfish as long as a pig, 

 say twenty years. 



(2) Another generalisation, suggested by Flourens, is that the 

 length of life is normally about five times the period of growth. 

 Here, again, there is a grain of truth; for a long-drawn-out growing 

 period implies laying very secure foundations on which a stable 

 adult life may be based. But this theory will not fully work either. 

 The common eel grows for five to eight years; but it seems to die 

 abruptly after its first and last spawning. A horse is mature in about 

 four years, yet may live to be forty. Some insects go on slowly 

 growing as larvae for several years (indeed, the larva of the famous 

 American Cicada may live for seventeen years), and yet die in the 

 first year of maturity. 



(3) Another of these general ideas, also with some sense in it, is 

 that very active animals wear themselves out quickly, while those 

 that take things easily live long. Worker-bees, victims of an exagger- 

 ated instinct for industry, do not usually live for more than six weeks 

 in the summer season — a short life, and not a very merry one. But 

 a sea-anemone, which lives without sound or fury, may last longer 

 than man; olive-trees may survive for very many centuries, and 

 giant Sequoias of Yosemite have been growing quietly for the past 

 three thousand years. The unexcitable carp may live as long as the 

 strenuous elephant ; but a sluggish snail does not live so long as the 

 active thrush. The fact is that by choosing suitable types one can 

 argue for or against any of these generalisations. 



This has led many back to Weismann's view that the duration of 

 life has been punctuated from without, rather than from within, 

 and has been determined by natural selection. Let us take an 

 illustration. 



The Golden Eagle, weighing nine to twelve pounds, is inter- 

 mediate as regards weight between hare and fox. All three are very 

 strenuous; all three are intricate masterpieces; all three hold their 

 own. But while the hare lives ten years and the fox fourteen, the 

 Golden Eagle attains to sixty! Weismann's interpretation is that the 

 Golden Eagle needs to live so long, if it is to keep its foothold in the 

 struggle for existence. It must live for a long time, so to speak, for 

 it takes ten years to mature, and then has only two eggs in the 

 year. The two mammals are much more fertile than the bird; 



VOL. II F 



