AN ESSA Y ON LONGEVITY. 39 



when growth has ceased or nearly so, that repro- 

 duction sets in. 



The effect of this additional tax is to start the 

 organism more rapidly down the incline towards the 

 termination of the road of life, the length of time 

 occupied in the downward run depending no doubt 

 on the height of the hill which has been mounted, 

 and on the friction, inclination, and additional acce- 

 leration, if any, of the descending body. An accident 

 on the way may bring the imaginary rider over 

 some precipice to the bottom of the course at once, 

 and it is little likely that he will succeed in avoiding 

 the many dangerous corners and pitfalls which in- 

 crease towards the end of the road, and finally 

 expend the full amount of impulse in traversing the 

 whole course. 



Some organisms may continue to grow and pro- 

 duce young throughout their life ; but the earlier 

 reproduction is commenced, and the more rapidly 

 it is carried on, the sooner must the increase of the 

 organism's bulk be stopped, and so waste and death 

 ensue. Fish, molluscs, and trees are the extreme 

 cases of this protracted period, which was explained 

 as due to small personal expenditure. A test of the 

 superabundance of the matter of life is seen in the 

 reproduction of lost parts which Salamandroid Am- 

 phibians, and also Crustacea, exhibit during a con- 

 siderable period of, life,^ though it may be questioned 



' The power is possessed by the larvm only of insects. 



