THE OSMLE 243 



their lesser capacity, and female eggs in the old female 

 cells of more spacious dimensions. 



This presence of both sexes at a time, even when 

 there are but two cells free, one spacious and the other 

 small, proves in the plainest fashion that the regular 

 distribution observed in the complete nests of recent pro- 

 duction is here replaced by an irregular distribution, 

 harmonizing with the number and holding-capacity of 

 the chambers to be stocked. The Mason-bee has before 

 her, let me suppose, only five vacant cells : two larger and 

 three smaller. The total space at her disposal would do 

 for about a third of the laying. Well, in the two large 

 cells, she puts females; in the three small cells she puts 

 males. 



As we find the same sort of thing in all the old nests, 

 we must needs admit that the mother knows the sex of 

 the eggs which she is going to lay, because that egg is 

 placed in a cell of the proper capacity. We can go fur- 

 ther and admit that the mother alters the order of suc- 

 cession of the sexes at her pleasure, because her layings, 

 between one old nest and another, are broken up into 

 small groups of males and females according to the 

 exigencies of space in the actual nest which she happens 

 to be occupying. 



Here then is the Chalicodoma, when mistress of an 

 old nest of which she has not the power to alter the ar- 

 rangement, breaking up her laying into sections compris- 

 ing both sexes just as required by the conditions imposed 

 upon her. She therefore decides the sex of the egg at 

 will, for, without this prerogative* she could not, in the 



