248 Experime7ital Zoology 



Length 0] Life in Different Species 



In contrast to the method of studying growth and senescence 

 described in the preceding pages another method has been fol- 

 lowed by Weismann, viz. the method of comparison of the 

 length of life in different species. 



In his essay on "The Duration of Life," Weismann has 

 brought together some interesting data relating to the length of 

 life of animals. He shows how meager our information is con- 

 cerning the length of life of lower organisms, except in the case of 

 those that live only through a certain season of the year, as many 

 of the insects do. Weismann points out that it is often stated 

 that both the duration of the period of growth and the length 

 of life are longer for larger animals and shorter for smaller ones. 

 An elephant may Hve for 200 years, a horse for 40 years, and 

 since they both require a relatively long time to grow up, it fol- 

 lows that part, at least, of their life must be longer than that of a 

 smaller animal that may reach its full age in a few months. 

 On the other hand, other smaller vertebrates, such as certain 

 fish — the pike and carp — may live as long as an elephant ; 

 and a cat or a toad may Hve as long as a horse. 



Flourens thought that the length of life of an animal was equiv- 

 alent to five times its period of growth. Thus if man grows 

 for twenty years, he lives to 100; but on the other hand a horse 

 becomes mature in four years and may live to 40 years, or ten 

 times as long as its growth period. From this and from other 

 cases it is evident that no such ratio as that of Flourens will hold. 

 Neither does greater activity mean necessarily a shorter life, for 

 some of the most active birds are the longest hved, and may 

 live as long as do some of the sluggish amphibians. 



Weismann's conclusion is that neither size, activity, complexity 

 of structure, nor "constitution" can account for duration of fife, 

 but that it "is dependent upon adaptation to external conditions; 

 that its length, whether longer or shorter, is governed by the 

 needs of the species, and that it is determined by precisely 

 the same ^mechanical process^ of regulation as that by which 



