much foresight and rerum prudentia to a simple 

 bird. May not the cause of these latebrce being left 

 unfinished arise from their meeting in those places 

 with strata too harsh, hard, and solid for their pur- 

 pose, which they relinquish, and go to a fresh spot 

 that works more freely ? Or may they not in other 

 places fall in with a soil as much too loose and 

 mouldering, liable to founder, and threatening to 

 overwhelm them and their labours? 



One thing is remarkable — that, after some years, 

 the old holes are forsaken and new ones bored ; per- 

 haps because the old habitations grow foul and fetid 

 from long use, or because they may so abound with 

 fleas as to become untenantable. This species of 

 swallow moreover is strangely annoyed with fleas: 

 and we have seen fleas, bed-fleas {Pulex irritans), 

 swarming at the mouths of these holes, like bees on 

 the stools of their hives. 



The following circumstance should by no means 

 be omitted — that these birds do not make use of 

 their caverns by way of hybernacula, as might be 

 expected ; since banks so perforated have been dug 

 out with care in the winter, when nothing was found 

 but empty nests. 



The sand-martin arrives much about the same 

 time with the swallow, and lays, as she does, from 

 four to six white eggs. But as this species is crypto- 

 game, carrying on the business of nidification, incu- 

 bation, and the support of its young in the dark, it 



17 



