DURATION OF ANIMALS. 3 



must take the average of the hves of a number of indivi- 

 duals, and rest satisfied with the approximation to truth 

 which can thus be obtained. 



There is but httle resemblance, in respect of longevity, 

 between the different classes, or even species of animals. 

 There is no peculiar structure, by which long-lived species 

 may be distinguished from those which are short-lived. 

 Many species, whose structure is complicated, live but for 

 a few years, as the rabbit, while some of the testaceous 

 mollusca, with more simple organization, have a more ex- 

 tended existence. If longevity is not influenced by struc- 

 ture, neither is it modified by the size of the species. 

 While the horse, greatly larger than the dog, lives to tAvice 

 its age, man enjoys an existence three times longer than the 

 former. 



The circvimstances which regulate the term of existence 

 in different species, exhibit so many peculiarities, corre- 

 sponding to each, that it is difficult to offer any general ob- 

 servations on the subject. Health is precarious, and the 

 origin of diseases generally involved in obscurity. The 

 condition of the organs of respiration and digestion, how- 

 ever, appears so intimately connected with the comfortable 

 continuance of life, and the attainment of old age, that 

 existence may be said to depend on the due exercise of the 

 functions which they perform. 



Whether animals have their blood aerated by means of 

 lungs or gills, they require a regular supply of oxygen 

 gas. But as this gas is extensively consumed in the pro- 

 cesses of combustion, putrefaction, vegetation and respi- 

 ration, there is occasionally a deficiency in particular pla- 

 ces for the supply of animal life. But, in general, where 

 there is a deficiency of oxygen, there is also a quantity of 

 carbonic acid or carburetted hydrogen present. These 

 gases not only injure the system by occupying the place of 



A 2 



