the nerves
of the head, and of the legs must have taken place parallel with the
enlargement of the jaws. HARMONIOUS ADAPTATION (coadaptation) has here
been active in a high degree, and yet these "soldiers" are sterile!
There thus remains nothing for it but to refer all their adaptations,
positive and negative alike, to processes of selection which have taken
place in the rudiments of the workers within the egg and sperm-cells
of their parents. There is no way out of the difficulty except the one
Darwin pointed out. He himself did not find the solution of the riddle
at once. At first he believed that the case of the workers among social
insects presented "the most serious special difficulty" in the way of
his theory of natural selection; and it was only after it had become
clear to him, that it was not the sterile insects themselves but their
parents that were selected, according as they produced more or less
well adapted workers, that he was able to refer to this very case of the
conditions among ants "IN ORDER TO SHOW THE POWER OF NATURAL SELECTION"
("Origin of Species", page 233; see also edition 1, page 242.). He
explains his view by a simple but interesting illustration. Gardeners
have produced, by means of long continued artificial selection, a
variety of Stock, which bears entirely double, and therefore infertile
flowers (Ibid. page 230.). Nevertheless the variety continues to be
reproduced from seed, because in addition to the double and infertile
flowers, the seeds always produce a certain number of single, fertile
blossoms, and these are used to reproduce the double variety. These
single and fertile plants correspond "to the males and females of an
ant-colony, the infertile plants, which are regularly produced in large
numbers, to the neuter workers of the colony."

This illustration is entirely apt, the only difference between the two
cases consisting in the fact that the variation in the flower is not
a useful, but a disadvantageous one, which can only be preserved
by artificial selection on the part of the gardener, while the
transformations that have taken place parallel with the sterility of the
ants are useful, since they procure for the colony an advantage in the
struggle for existence, and they are therefore preserved by
natural selection. Even the sterility itself in this case is not
disadvantageous, since the fertility of the true females has at the same
time considerably increased. We may therefore