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.

This is all very well when the mother herself founds the dwelling, when
she lays the first rows of bricks. But, when she is in the presence
of an old nest, of which she is quite unable to alter the general
arrangement, how is she to make use of the few vacant rooms, the large
and the small alike, if the sex of the egg be already irrevocably fixed?
She can only do so by abandoning the arrangement in two consecutive
rows and accommodating her laying to the varied exigencies of the home.
Either she finds it impossible to make an economical use of the old
nest, a theory refuted by the evidence, or else she determines at will
the sex of t