6 THE DURATION OF LIFE. [I. 



One is at first tempted to seek the answer by an appeal to 

 the differences in morphological and chemical structure which 

 separate species from one another. In fact all attempts to 

 throw light upon the subject which have been made up to the 

 present time lie in this direction. 



All these explanations are nevertheless insufficient. In 

 a certain sense it is true that the causes of the duration of life 

 must be contained in the organism itself, and cannot be found 

 in any of its external conditions or circumstances. But 

 structure and chemical composition — in short the physiological 

 constitution of the body in the ordinar}^ sense of the words — 

 are not the only factors which determine duration of life. This 

 conclusion forces itself upon our attention as soon as the 

 attempt is made to explain existing facts by these factors alone : 

 there must be some other additional cause contained in the 

 organism as an unknown and invisible part of its constitution, 

 a cause which determines the duration of life. 



The size of the organism must in the first place be taken into 

 consideration. Of all organisms in the world, large trees have 

 the longest lives. The Adansonias of the Cape Verd Islands 

 are said to live for 6000 3'ears. The largest animals also attain 

 the greatest age. Thus there is no doubt that whales live for 

 some hundreds of 3'ears. Elephants live 200 years, and it 

 would not be difficult to construct a descending series of 

 animals in which the duration of life diminishes in almost exact 

 proportion to the decrease in the size of the body. Thus a 

 horse lives forty years, a blackbird eighteen, a mouse six, and 

 many insects only a few daj's or weeks. 



If however the facts are examined a little more closely it will 

 be observed that the great age (200 years) reached by an 

 elephant is also attained by many smaller animals, such as the 

 pike and carp. The horse lives forty years, but so does a cat 

 or a toad ; and a sea anemone has been known to live for over 

 fifty years. The duration of life in a pig (about twenty years) 

 is the same as that in a crayfish, although the latter does not 

 nearly attain the hundredth part of the weight of a pig. 



It is therefore evident that length of life cannot be deter- 

 mined by the size of the body alone. There is, however, some 

 relation between these two attributes. A large animal lives 

 longer than a small one because it is larger ; it would not be 



