14 THE DURATION OF LIFE. 



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 1 ; -but the golden eagle, though it does not weigh more 

 than from 91 2 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, however, we are 

 not yet in possession of exact statistics indicating the number of 

 young destroyed ; but it is obvious that Mammalia 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 development 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 consider 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 incomplete. Biological 

 problems have for a long time excited less interest than morpho- 

 logical ones. There is nothing or almost nothing to be found in 

 existing zoological text books upon the duration of life in animals ; 

 and even monographs upon single classes, such as the Amphibia, 

 reptiles, or even birds, contain very little on this subject. When 

 we come to the lower animals, knowledge on this point is almost 

 entirely wanting. I have not been able to find a single reference 

 to the age in Echinodermata, and very little about that of worms, 

 Crustacea, and Coelenterata 2 . The length of life in many mol- 

 luscan species is very well known, because the age can be deter- 

 mined by markings on the shell 3 . But even in this group, any 

 exact knowledge, such as would be available for our purpose, is still 



1 See Appendix, note 2, p. 38. 

 a See Appendix, note 4, p. 54. 

 3 See Appendix, note 5, p. 55. 



