distributed in such a way as utterly to upset the idea of an invariable
succession first of females and then of males, the idea which occurs
to us on examining the new nests. If this rule were a constant one, we
should be bound to find in the old domes at one time only females, at
another only males, according as the laying was at its first or at its
second stage. The simultaneous presence of the two sexes would then
correspond with the transition period between one stage and the next and
should be very unusual. On the contrary, it is very common; and, however
few cells there may be, we always find both females and males in the old
nests, on the sole condition that the compartments have the regulation
holding-capacity, a large capacity for the females, a lesser for the
males, as we have seen.

The old male cells can be recognized by their position on the outer
edges and by their capacity, measuring on an average the same as a
column of sand 31 millimetres high in a glass tube 5 millimetres wide.
(1.21 x.195 inches.--Translator's Note.) These cells contain males of
the second or third generation and none but males. In the old female
cells, those in the middle, whose capacity is measured by a similar
column of sand 45 millimetres high (1.75 inches.--Translator's Note.),
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 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