38 THE DURATION OF LIFE. 



and although this bird only lays a single egg yearly and takes 

 four years to attain maturity, the numbers do not diminish 1 . 

 30,000 sea-gulls' eggs and 20,000 terns' eggs are yearly exported 

 from the breeding-places on the island of Sylt, but in this case 

 it appears that a systematic disturbance of the birds is avoided 

 by the collectors, and no decrease in their numbers has yet taken 

 place 2 . The destruction of northern birds is not only caused by 

 man, but also by various predaceous mammals and birds. Indeed 

 the dense mass of birds which throng the cliffs is a cause of 

 destruction to many of the young and to the eggs, which are 

 pushed over the edge of the rocks. According to Brehm the foot 

 of these cliffs is ' always covered with blood and the dead bodies of 

 fledglings.' 



Such birds must attain a great age or they would have been 

 exterminated long ago : the minimum duration of life necessary for 

 the maintenance of the species must in their case be a very 

 high one. 



Note 2. THE DURATION or LIFE AMONG MAMMALS. 



The statements upon this subject in the text are taken from 

 many sources ; from Giebel's ' Saugethiere,' from Oken's ' Natur- 

 geschichte,' from Brehm's ' Illustrirtem Thierleben,' and from an 

 essay of Knauer in the ' Naturhistoriker,' Vienna, 1880. 



Note 3. THE DURATION OF LIFE AMONG MATURE INSECTS. 



A short statement of the best established facts which I have been 

 able to find is given below. I have omitted the lengthening of 

 imaginal life which is due to hybernation in certain species. In 

 almost all orders of insects there are certain species which emerge 

 from the pupa in the autumn, but which first reproduce in the 

 following spring. The time spent in the torpid condition during 

 winter cannot of course be reckoned with the active life of the 

 species, for its vital activity is either entirely suspended for a time by 

 freezing (Anabiosis : Preyer 3 ), or it is at any rate never more than 

 a vita minima, with a reduction of assimilation to its lowest point. 



1 Oken, ' Naturgeschichte,' Stuttgart, 1837, Bd - IV - Abth - r - 

 3 Brehm, ' Leben der Vogel,' p. 278. 



8 ' Naturwissenschaftliche Thatsachen und Problems,' Populate Vortriige, Berlin, 

 1880; vide Appendix. 



