The Mother Decides the Sex 



measured by a similar column of sand 45 

 millimetres' high? are females and none but 

 females. 



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 production 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 egg which she 

 is going to lay, because that egg is placed 

 in a cell of the proper capacity. We can go 

 further and admit that the mother alters the 

 order of succession of the sexes at her plea- 

 sure, because her layings, between one old nest 

 and another, are broken up into small groups 



'1.75 inches. — Translator's Note. 



129 



