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 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. Just now, in the new nest, we saw the Mason-bee arranging her total laying into series first of females and next of males; and here she is, mistress of an old nest of which she has not the power to alter the arrangement, breaking up her laying into sections comprising 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 chambers of the nest which she owes to chance, deposit unerringly the sex for which those chambers were originally built; and this happens however small the number of chambers to be filled. When the nest is new, I think I see a reason why the Mason-bee should seriate her laying into females and then males. Her nest is a half-sphere. That of the Mason-bee of the Shrubs is very nearly a sphere. Of all shapes, the spherical shape is the strongest. Now these two nests require an exceptional power of resistance. Without protection of any kind, they have to brave the weather, one on its pebble, the other on its bough. Their spherical configuration is therefore very practical. The nest of the Mason-bee of the Walls consists of a cluster of upright cells backing against one another. For the whole to take a spherical form, the height of the chambers must diminish from the centre of the dome to the circumference. Their elevation is the sine of the meridian arc starting from the plane of the pebble. Therefore, if they are to have any solidity, there must be large cells in the middle and small cells at the edges. And, as the work begins with the central chambers and ends with those on the circumference, the laying of the females, destined for the large cells, must precede that of the males, destined for the small cells. So the females come first and the males at the finish.