158 WONDERS OF ORGANIC LIFE. 



evils — namely, cold and the want of food. So 

 strong, so dominant is this impulse, that migra- 

 tory birds kept in aviaries, defended from the 

 cold, and plentifully supplied with food, are 

 decidedly agitated when the season of their 

 southern migration comes on. What does the 

 bird require ? As far as warmth and food go, 

 nothing. But that impulse which the Creator 

 implanted in it disturbs its frame ; it strives to 

 escape from the bars of its prison-house, and 

 failing this, it dies. Hence the great difficulty 

 of keeping the nightingale, the black cap, the 

 wheatear, and others, in confinement. Eeptiles 

 awake from torpidity when the temperature is 

 even lower than that of the time in which they 

 retired to their winter-quarters. Our migra- 

 tory spring birds visit our shores when the cold 

 is lower than it is in autumn, when they take 

 their departure. At the period of their return, 

 however, the vital principle has acquired as 

 it were accumulated force by repose ; their 

 animalization seems to be at its highest point ; 

 and in like manner, their insect prey, roused 

 from torpidity, or emerging from the pupa state, 

 is ready for their acceptance. Throughout the 

 whole chain of nature, vitality is refreshed by 

 repose, and this repose is experienced in some 

 animals by hybernation, in others by change of 

 residence. 



As some birds, which in the north are migra- 

 tory, may be regarded as stationary in our lati- 

 tudes, so some species, which are migratory 

 in our island, remain stationary in southern 



