8 THE DURATION OF LIFE. 



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

 be shown later on : they live as long 1 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 was 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 struc- 

 ture. Two organisms of the same size, but belonging to different 

 grades of organization, will require different periods of time for 

 their development. Certain animals of a very lowly organization, 

 such as the B/hizopoda, 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 before 

 reproduction begins. The period before reproduction is however 

 only part of the whole life of an animal, which of course extends 

 over the total period during which the animal exists. 



Hitherto it has always been assumed that the duration of this 

 total period is solely determined by the constitution of the ani- 

 mal's body. But the assumption is erroneous. The strength of 



