248 Experimental Zoology 
Length of 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 live 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 live 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 lived, 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 life, 
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 
