5 8 THE DURATION OF LIFE. [I. 



disturbed and are therefore prompted to seek other breeding- 

 places. From the small island of St. Kilda, oft' Scotland, 20,000 

 young gannets (Sn/a) and an immense number of eggs are 

 annually collected ; and although this bird only lays a single 

 egg yearly and takes four years to attain maturity, the numbers 

 do not diminish \ 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'-. 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 clifts 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 clifts 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 of Life among Mammals. 



The statements upon this subject in the text are taken from 

 many sources ; from Giebel's ' Saugethierc,' from Oken's * Na- 

 turgeschichte,' 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 lengthen- 

 ing 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 bj' freezing (Anabiosis : Preyer''j, 



' Okcn, ' Naturgeschichte,' Stuttgart, 1837, Bd. IV. Abth. i. 

 ■-' Brchm, ' Lebcn dcr VOgel,' p. 278. 



' ' Naturwissenschaftlichc Thatsachcn und Probleme,' Populare Vor- 

 tragc, Berlin, 1880 ; vide Appendix. 



