6 THE DURATION OF LIFE. 



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 con- 

 tained 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 ordinary 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 years. The largest animals also attain the greatest 

 age. Thus there is no doubt that whales live for some hundreds 

 of years. 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 days 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 hun- 

 dredth part of the weight of a pig. 



It is therefore evident that length of life cannot be determined 

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

 between these two attributes. A large animal lives longer than n 

 small one because it is larger ; it would not be able to become even 

 comparatively large unless endowed with a comparatively long dura- 

 tion of life. 



