THE DEVELOPMENT OF ORGANISMS 759 



failing, ageing, and dying. A common animal life-curve shows — 

 embryonic development, tender youth (whether of larval period or 

 infancy), adolescence, love-making, maturity, parentage, full 

 strength, ageing, senescence, death. Parallel to that may be pictured 

 the normal life-curve of a higher plant — embryonic development 

 within the seed, sprouting, growing, leafing, flowering, fruiting, 

 seeding, withering, and dying. The first unifying idea that we wish 

 to suggest is that, on general grounds, every life of a multicellular 

 organism must illustrate something of this ascent and descent, for 

 there is a continual see-saw between waste and repair, nutrition 



AATURITY 



ADOLESCENCE— i---'^ ^^ m^ENTAGE 



YOUTH— —^'^^''^ ^^\^ 



INFANCY-/^' \ DECLINE 



ANTE- / \ 

 natal/ AMinALS Y SE NESCENCE 



DEATH 

 FLOWERING 

 IZMltiO-^a^-^ ^""V,— FRUITING 

 GROWING-^''' ^* SEEDING 



-V/ITHERIMO 

 PLANTS 



^ 



Fig. 121. 



Diagram of Typical Life-Curve in Plants and Animals. Tn the animal curve 

 there is indicated a succession of phases: ante-natal, infancy, youth, 

 adolescence, maturity, parentage, decline, senescence, and death. In 

 the plant curve, similarly, there are indicated : sprouting, growing, leafing, 

 flowering, fruiting, seeding, and withering. 



and reproduction, work and rest, the issue of which is that processes 

 of senescence, slowly or quickly, gain on the processes of rejuvenes- 

 cence. Except in the Protozoa, which seem to be largely immune 

 from Natural Death, senescence always gains on rejuvenescence. 

 The second unifying idea is that the various arcs on the typical 

 life-curve may be thought of as capable of elongation and of shorten- 

 ing in the various types. Thus, one organism may have a prolonged 

 youthful period, and die immediately after reproduction; while 

 another may have a very rapid youth and a long-drawn-out maturity. 

 Moreover, in Backboned Animals (Vertebrata) we know that this 

 lengthening out and telescoping down may be controlled by varia- 

 tions in the hormone-producing endocrinal glands. The cretin's 

 abnormal prolongation of infancy is a sad instance of thyroid 

 deficiency. Thirdly, the strangely diverse life-histories, in some 

 degree unified by these two ideas, have then to be interpreted in 



