This seems to indicate that even in
the mid-Tertiary some millions of years after the first
appearance of the ant, the social life which we admire in the
ants today had not yet been developed. The Tertiary bees, on the
other hand, are said to show some traces of the division of
labour (and modification of structure) which make the bees so
interesting; but in this case the living bees, rising from a
solitary life through increasing stages of social co-operation,
give us some idea of the gradual development of this remarkable
citizenship.
It seems to me that the great selective agency which has brought
about these, and many other remarkable activities of the insects
(such as the storing of food with their eggs by wasps), was
probably the occurrence of periods of cold, and especially the
beginning of a winter season in the Cretaceous or Tertiary age.
In the periods of luxuriant life (the Carboniferous, the
Jurassic, or the Oligocene), when insects swarmed and varied in
every direction, some would vary in the direction of a more
effective placing of the eggs; and the supervening period of cold
and scarcity would favour them. When a regular winter season set
in, this tendency would be enormously increased.
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