102 ANIMAL LIFE 



but a year, ascending the rivers in numbers when young in 

 the spring, the whole mass of individuals dying in the fall 

 after spawning. 



Naturalists have sought to discover the reason for these 

 extraordinary differences in the duration of life of different 

 animals, and while it can not be said that the reason or 

 reasons are wholly known, yet the probability is strong that 

 the duration of life is closely connected with, or dependent 

 upon, the conditions attending the production of offspring. 

 It is not sufficient, as we have learned from our study of 

 the multiplication of animals (Chapter III), that an adult 

 animal shall produce simply a single new individual of its 

 kind, or even only a few. It must produce many, or if it 

 produces comparatively few it must devote great care to 

 the rearing of these few, if the perpetuation of the species 

 is to be insured. Now, almost all long-lived animals are 

 species which produce but few offspring at a time, and 

 reproduce only at long intervals, while most short-lived ani- 

 mals produce a great many eggs, and these all at one time. 

 Birds are long-lived animals ; as we know, most of them 

 lay eggs but once a year, and lay only a few eggs each time. 

 Many of the sea birds which swarm in countless numbers 

 on the rocky ocean islets and great sea cliffs lay only a 

 single egg once each year. And these birds, the guillemots 

 and murres and auks, are especially long-lived. Insects, on 

 the contrary, usually produce many eggs, and all of them 

 in a short time. The May-fly, with its one evening's lifetime, 

 lets fall from its body two packets of eggs and then dies. 

 Thus the shortening of the period of reproduction with the 

 production of a great many offspring seem to be always 

 associated with a short adult lifetime ; while a long period 

 of reproduction with the production of few offspring at a 

 time and care of the offspring are associated with a long 

 adult lifetime. 



There seems also to be some relation between the size 

 of animals and the length of life. As a general rule, 



