GENERATION, SEX AND OXTOOKXY 2U 



long lives — the small sonjz; l)ir(l.s from oi^^lit to ci^litccn years, 

 and the great eagles and vultures u{) to a liundred years or 

 more. On the other hand, among all tlie thousands of species 

 of insects, tlie individuals of very few indeed hve more than a 

 year; the adult life of most insects being l)ut a few days or weeks, 

 or at best months. Even among tlie higher animals, some are 

 very short-lived. In Japan is a small fish (Sahnix) which prob- 

 ably lives 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 the.se 

 extraordinary differences in the duration of life of different 

 animals, and while it cannot be said that the reason or reasons 

 are wholly known, yet the probability is strong that the dura- 

 tion of hfe is closely connected with, or dejx'ndent upon, the 

 conditions attending the production of offs})ring. It is not 

 sufficient that an adult animal shall i)roduce simj:)ly a single 

 new" individual of its kind, or even only a few. It must })roduce 

 many, or if it produces comparatively few it nnist devote great 

 care to the rearing of these few, if the perpetuation of the 

 species is to be insured. Now, almost all long-livcnl animals 

 are species which produce but few offspring at a time, and 

 reproduce only at long intervals, wliile most short-lived animals 

 produce a great many eggs, and these all at one time. I>irds 

 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 nmrres 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 life- 

 time; while a long period of reproduction with the production 

 of few offspring at a time and care of the offspring are as,so- 

 ciated with a long adult lifetime. 



At the end comes death. After the animal has completed 

 its Hfe cycle, after it has done its share toward insuring the 

 perpetuation of its species, it dies. It may meet a violent 



