OLD AGE AND DEATH 369 



grow old at very different times. An attempt to explain 

 the diversity of longevity by reference to the immediate 

 conditions of life also fails. We have to face the fact that 

 while the rate at which the wheels go round is doubtless 

 very important for individual cases, it cannot be the key 

 to an understanding of the diversity in the average dura- 

 tion of life in more or less similar species. Many sluggish 

 animals, such as molluscs, are short-lived ; many active 

 animals, such as birds, are long-lived. Nor can we find 

 the solution in considering the diverse environments, diets, 

 industries, and so on. We are thus prepared for the next 

 step in the argument. 



Weismann's theory is that in the course of time, after 

 many experiments, so to speak, the length of life in a species 

 has been regulated in reference to the rate of multiplication 

 and the average mortality. The punctuation is from 

 without, not from within ; it is Natural Selection that has 

 determined and that slowly, after grave consideration 

 as it were where the stops come in : the semicolon of 

 maturity, the colon of senescence, and the full stop of 

 natural death. In other words, the internal constitutional 

 arrangements which secure that an elephant can survive 

 for two hundred years and a freshwater sponge for less 

 than a year, have been gradually wrought out or regulated 

 in relation to the welfare of the species. 



Let us take a concrete case : the golden eagle, weighing 

 9 to 12 lb., is intermediate as regards weight between hare 

 and fox ; all three are very active ; all three are very 

 complex. But while the hare lives ten years, and the fox 

 fourteen, the golden eagle lives sixty. Weismann says 

 that this has been in the course of time regulated in refer- 

 ence to various big facts of life, e.g. that the two mammals 

 are more fertile than the bird, that there is less mortality 

 among the young mammals than among the young birds, 

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