THE DEVELOPMENT OF ORGANISMS 765 



life in one great bound, skipping the whole juvenile period. What 

 a contrast between this and the prolonged youth of the elephant, 

 for the young giant sometimes remains for ten years beside its 

 mother. The elephant is, of course, a slow-growing and slowly 

 reproducing type, which may reach an age of a hundred years, but 

 even then the youthful period is long in proportion. In many 

 mammals there is not only prolonged youth but prolonged maturity; 

 and most come to a violent end without any visible trace of senes- 

 cence in the tissues. 



A cockchafer may have a larval period of four years: contrast 

 that with the exceedingly rapid larval development of types like 

 bluebottles, which do not require more than three days. Many 

 nesting birds, e.g. the Golden Eagle, have a prolonged nurture in 

 the cradle; what a contrast to birds of, say, the plover type, with 

 little more than an apology for a nest, for in these types the newly 

 hatched young creature is extraordinarily precocious, more fully 

 finished than a chick, active and at home almost immediately after 

 hatching. 



These instances must serve to illustrate our thesis that the life- 

 histories of different animals differ in the tempo of the various parts 

 of the general life-curve. For the plant world the same holds true, 

 for not only are there annual, biennial, perennial, century plants, 

 and so on, but there are instances of prolonged leafing and long- 

 delayed flowering, of withering after seed-scattering, and of per- 

 sistent nutritive vigour unaffected by reproduction. The flower of 

 the common garden Day-Lily (Hemerocallis) literally deserves that 

 old name, while a single flower of a Lady's Slipper orchid (Cypri- 

 pedium) may last three months. 



FACTORS DETERMINING LIFE'S TRAJECTORY.— The factors 



that alter the average or normal form of the life-curve may be 

 grouped as environmental, functional, or organismal. 



{a) Environmental. — In cold waters the rate of protein-meta- 

 bolism is slowed, and therefore, since there is less chromatin pro- 

 duced, there are fewer cell- divisions in a given time. The eggs of the 

 salmon in the cold rivers mav take three months to hatch. In cold 

 surroundings the duration of life tends to be prolonged; there are 

 thus more generations living at the same time; hence the plankton 

 is denser in northern seas than at the Equator. There are indications 

 that life is very slow in the eternal winter of the great abysses; in 

 tropical water it is often hurried. Stimulating food hastens develop- 

 ment, as is seen in maggots amid the flesh and in some internal 

 parasites. Linnseus remarked that a flesh-fly and its progeny may 

 devour the carcase of an ox more rapidly than a lion can. But 

 uncongenial food may retard development, and Planarian worms 

 misfed may have offspring that are "born old". The conditions of 



