334 THE STUDY OF ANIMAL LIFE CHAP. 



furthermore have very inexpensive modes of multiplica- 

 tion. The same may be more or less true of simple 

 animals like Hydra and Planarian worms. " Natural 

 death is incident (1) on the complexity of the bodily 

 machinery, which makes complete recuperation well- 

 nigh impossible, and almost forces the organism to accu- 

 mulate arrears, to go into debt to itself ; (2) on the limits 

 that are set to the multiplication and renewal of cells 

 within the body, thus the number of nerve-cells in higher 

 animals cannot be added to after an early stage in develop- 

 ment ; and (3) on the occurrence of organically expensive 

 modes of reproduction, for reproduction is often the 

 beginning of death. At the same time, it seems difficult 

 to rest satisfied with these and other physiological reasons, 

 and we fall back on the selectionist view that the dura- 

 tion of life has been, in part at least, punctuated from 

 without and in reference to large issues ; it has been 

 gradually regulated in adaptation to the welfare of the 

 species. 1 



Retrospect. Just as there are many novels but only 

 a few plots, so amid an apparent multiplicity of life- 

 histories we discern but a few main types. The details 

 may seem very different, but they are often interpretable 

 as due to a lengthening out here and a condensation 

 there, to a changing of the time of the tune. This seems 

 to be a guiding idea in the study of life-histories. 



Just as there are in organisms architectural variations 

 which find expression in spatial rearrangement of materials 

 (comparable to those we see a schoolboy effecting with 

 his " meccano," out of which he constructs now a crane 

 and again a bridge, to-day a railway truck and to-morrow 

 an aeroplane), so there are temporal variations which find 

 expression in changes in the rate of growth and develop- 

 ment, or in alterations in the rhythm or punctuation of 

 life. In this connection it is interesting to remember 

 that in the internal secretions (of backboned animals at 

 least) there is a means by which the rate of growth and 

 development can be automatically regulated. How sug- 

 gestive, too, is the result of Gudernatsch's experiments 

 1 The author's Wonder of Life, p. 442. (Melrose, London, 1914.) 



