THE DURATION OF LIFE. 13* 



time ; it follows that, if we know the fertility and the average 

 duration of life of a species, we can calculate the number of those 

 which perish before reaching- maturity. Unfortunately the average 

 length of " life is hardly known with certainty in the case of any 

 species of bird. Let us however assume, for the sake of argument, 

 that the individuals of a certain species live for ten years, and that 

 they lay twenty eggs in each year ; then of the zoo eggs which 

 are laid during the ten years, which constitute the lifetime of an 

 individual, 198 must be destroyed, and only two will reach maturity, 

 if the number of individuals in the species is to remain constant. 

 Or to take a concrete example ; let us fix the duration of life in 

 the golden eagle at 60 years, and its period of immaturity (of which 

 the length is not exactly known) at ten years, and let us assume 

 that it lays two eggs a year; then a pair will produce 100 eggs 

 in 50 years, and of these only two will develope into adult birds ; 

 and thus on an average a pair of eagles will only succeed in bring- 

 ing a pair of young to maturity once in fifty years. And so far 

 from being an exaggeration, this calculation rather under-estimates 

 the proportion of mortality among the young ; it is sufficient how- 

 ever to enforce the fact that the number of young destroyed must 

 reach in birds a very high figure as compared with the number of 

 those which survive 1 . 



If this argument holds, and at the same time the fertility from 

 physical and other grounds cannot be increased, it follows that 

 a relatively long life is the only means by which the maintenance 

 of the species of birds can be secured. Hence a great length 

 of life is proved to be an absolute necessity for birds. 



I have already mentioned that these animals demonstrate most 

 clearly that physiological considerations do not by any means suffice 

 to explain the duration of life. Although all vital processes take 

 place with greater rapidity and the temperature of the blood is 

 higher in birds than in mammals, yet the former greatly surpass 

 the latter in length of life. Only in the largest Mammalia, the 

 whales and the elephants is the duration of life equal to or 

 perhaps greater than that of the longest lived birds. If we com- 

 pare the relative weights of these animals, the Mammalia are 

 everywhere at a disadvantage. Even such large animals as the horse 

 and bear only attain an age of fifty years at the outside ; the lion 



1 See Appendix, note i, p. 36. 



