NATURAL HISTORY OF SELBORNE 145 



a different color from that which lay loose and bleached in 

 the sun. 



In what space of time these little artists are able to mine and 

 finish these cavities I have not been able to discover, for rea- 

 sons given above ; but it would be a matter worthy of obser- 

 vation, where it falls in the way of any naturalist to make his 

 remarks. This I have often taken notice of, that several holes 

 of different depths are left unfinished at the end of summer. 

 To imagine that these beginnings were intentionally made in 

 order to be in the greater forwardness for next spring is allow- 

 ing perhaps too much foresight and rerum prudentia to a 

 simple bird. May not the cause of these latebra being left 

 unfinished arise from their meeting in those places with strata 

 too harsh, hard, and solid for their purpose, which they re- 

 linquish, 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 flounder, and threatening to 

 overwhelm them and their labors ? 



One thing is remarkable that, after some years, the old 

 holes are forsaken and new ones bored ; perhaps 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\ swarm- 

 ing at the mouths of these holes, like bees on the stools of their 

 hives. 1 



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 cryptogame, carrying on the business of 

 nidification, incubation, and the support of its young in the 

 dark, it would not be so easy to ascertain the time of breeding, 

 were it not for the coming forth of the broods, which appear 

 much about the time, or rather somewhat earlier than those of 

 the swallow. The nestlings are supported in common like 

 ii 



