I.] THE DURATION OF LIFE. 1 3 



destroy numbers of the helpless young, and in winter many 

 succumb in the struggle against cold and hunger, or to the 

 numerous dangers which attend migration over land and sea, 

 dangers which decimate the young birds. 



It is impossible directly to ascertain the exact number which 

 are thus destroyed ; but we can arrive at an estimate by an 

 indirect method. If we agree with Darwin and Wallace in 

 believing that in most species a certain degree of constancy 

 is maintained in the number of individuals of successive gene- 

 rations, and that therefore the number of individuals within 

 the same area remains tolerably uniform for a certain period 

 of 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 200 eggs which are laid during the ten years, which con- 

 stitute 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 bringing 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 

 however to enforce the fact that the number of j'-oung destro3^ed 

 must reach in birds a very high figure as compared with the 

 number of those which survived 



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 fife is the only means by which the main- 

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

 length of fife is proved to be an absolute necessity for birds. 



^ See Appendix, note i, p. 36. 



