8 THE DURATION OF LIFE. [I. 



a few vigorous efforts.' 'The inertness of the Amphibia is, 

 on the other hand, accompanied by relatively great length 

 of life.' 



There is certainly some truth in these observations, and yet 

 it would be a great mistake to assume that activity necessarily 

 implies a short life. The most active birds have very long lives, 

 as will be shown later on : they live as long as and sometimes 

 longer than the majority of Amphibia which reach the same 

 size. The organism must not be looked upon as a heap of 

 combustible material, which is completely reduced to ashes in 

 a certain time the length of which is determined by size, and 

 by the rate at which it burns ; but it should be rather compared 

 to a fire, to which fresh fuel can be continually added, and 

 which, whether it burns quickly or slowly, can be kept burning 

 as long as necessity demands. 



The connection between activity and shortness of life cannot 

 be explained by supposing that a more rapid consumption of 

 the body occurs, but it is explicable because the increased rate 

 at which the vital processes take place permit the more rapid 

 achievement of the aim and purpose of life, viz. the attainment 

 of maturity and the reproduction of the species. 



When I speak of the aim and purpose of life, I am only using 

 figures of speech, and I do not mean to imply that nature is in 

 any way working consciously. 



When I w^as speaking of the relation between duration of 

 life and the size of the body, I might have added another factor 

 which also exerts some influence, viz. the complexity of the 

 structure. Two organisms of the same size, but belonging to 

 different grades of organization, w^U require different periods 

 of time for their development. Certain animals of a very lowly 

 organization, such as the Rhizopoda, may attain a diameter of 

 •5 mm. and may thus become larger than many insects' eggs. 

 Yet under favourable circumstances an Amoeba can divide 

 into two animals in ten minutes, while no insect's egg 

 can develope into the young animal in a less period than 

 twenty-four hours. Time is required for the development of 

 the immense number of cells which must in the latter case 

 arise from the single egg-cell. 



Hence we may say that the peculiar constitution of an animal 

 does in part determine the length of time which must elapse 



