i26o LIFE: OUTLINES OF GENERAL BIOLOGY 



BIOLOGY AND HUMAN LIFE 



The Trajectory of Life: Its Sequence of Risks and Rewards. 

 — From a bio-sociological outlook, we come back again to 

 the idea that human life is like a trajectory, a curve of ascent 

 and descent: a curve on which there are successive arcs — 

 infancy, childhood, adolescence, love-making, maturity, ageing, 

 senescence, dying — which can be lengthened out or cut down 

 according to nurture: arcs, each with its triumphs and its joys 

 (even in senescence!) and each with its tragedies and risks. The 

 underlying practical idea, with which we are all familiar, is that our 

 task is to make the most and the best of each of these arcs on our 

 life curve, and that success depends upon our endeavour to gain 

 positive health both of body and mind. The idea of life as a curve 

 or trajectory is very old, though there are few books in which this 

 simple graph finds expression. It seems strange that such a pic- 

 turesque and telling image should not be used oftener: the idea of 

 a bridge over which life has to make its way, ascending, then for a 

 while on the level, and then descending. For it is plain that in every 

 life there must be a period of developing, growing, strengthening, 

 improving; and then comes the decline, the facilis descensus, the 

 downgrade towards senescence. As previously explained, the plant 

 kingdom has arcs similar to the animal kingdom — sprouting, 

 growing, leafing, flowering, fruiting, seeding, withering — the familiar 

 story of the trajectory of the higher plants. It is interesting to follow 

 this out in a little detail, noting how the form of the curve differs in 

 different types, and in relation to different conditions of life. 



For instance, in the case of the mayfly, there is a long youth, two, 

 three, or sometimes four years, then two or three evenings of active 

 adult aerial life, reproducing without feeding, and then the sharp 

 fall to death. There is one species of mayfly which has actually a 

 single hour of adult life ! 



In contrast to that, think of an animal like an elephant, which 

 sometimes remains beside its mother for ten years, a long period 

 without great responsibility. Or, again, take the curious case of the 

 common eel. The young eels or elvers which come into the British 

 rivers from the Atlantic are already two and a half or two and three- 

 quarter years old. In the course of their journey to our shores from 

 a stretch of sea towards the Bermudas they have undergone a meta- 

 morphosis from knife-blade-like glass-eels to cylindrical elvers; 

 they continue to develop in the quiet waters of the rivers; they 

 become reproductively mature in five to eight years, and then make 

 for the open sea — a long journey — ending in death after spawning. 

 This is another case where the infantile and adolescent life is greatly 

 prolonged, but the adult life stops abruptl}^ 



