14 THE DURATION OF LIFE. [I. 



I have already mentioned that these animals demonstrate 

 most clearly that physiological considerations do not by any 

 means suffice to exj)lain 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 

 greatl}' 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 compare 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 lives about thirty-five years, the wild 

 boar twenty-five, the sheep fifteen, the fox fourteen, the hare 

 ten, the squirrel and the mouse six years ^; but the golden 

 eagle, though it does not weigh more than from 9-12 pounds, 

 and is thus intermediate as regards weight between the hare 

 and the fox, attains nevertheless an age which is ten times as 

 long. The explanation of this difference is to be found first in 

 the much greater fertility of the smaller Mammalia, such as the 

 rabbit or mouse, and secondly in the much lower mortality 

 among the young of the larger Mammalia. The minimum 

 duration of life necessary for the maintenance of the species is 

 therefore much lower than it is among birds. Even here, how- 

 ever, we are not yet in possession of exact statistics indicating 

 the number of young destroyed ; but it is obvious that Mam- 

 malia possess over birds a great advantage in their intra-uterine 

 development. In Mammalia the destruction of young only 

 begins after birth, while in birds it begins during the develop- 

 ment of the embryo. This distinction is in fact carried even 

 further, for many mammals protect their young against enemies 

 for a long time after birth; 



It is unnecessary to go further into the details of these cases, 

 or to consider whether and to what extent every class of the 

 animal kingdom conforms to these principles. Thus to con- 

 sider all or even most of the classes of the animal kingdom 

 would be quite impossible at the present time, because our 

 knowledge of the duration of life among animals is very incom- 

 plete. Biological problems have for a long time excited less 

 interest than morphological ones. There is nothing or almost 



* See Appendix, note 2, p. 38. 



