SWALLOWS: THEIR HIBERNATION. 183 



not being able to fly, and subsist upon a diet which they 

 pick up from the ground. 



But this does not explain the moulting of those swal- 

 lows and martins, few or many, which have been proved 

 to remain torpid in northern countries. Do these come 

 out in the spring only to die, or do they perish in their 

 winter retreats and never revive ? If they are destined 

 to perish here, why has Nature provided them with an 

 instinct which answers no purpose whatever in their 

 economy ? If this submersion is only a method of 

 suicide, why do they not perish immediately, instead 

 of lingering along during the whole winter to die at 

 the end of this season ? And if they do not . perish at 

 this time, but awake and revive like bats and dormice, 

 the most important question is, not where and when they 

 moult, but why Nature has provided migration for a part 

 of each swallow family, and a torpid sleep under water, 

 and in crevices of rocks, for the remainder of the same 

 families. I cannot but conclude that there is yet the 

 greatest burden of proof remaining with those who main- 

 tain the theory of migration. 



