exceptionally favourable for growth; but, in spite of appearances,
there is no reason for admiring the marvellous foresight and
extraordinary sureness of instinct; nearly everything depends on a
fortuitous circumstance, a chance. This becomes very evident if we
study another related beetle; it is called the _Sitaris colletis_, and
lives at the expense of the hymenopterous _Colletes_, as its relative
at the expense of the _Anthophora_. But these two species of the same
genus are very unequally aided by chance. The one whose history we
have just traced attaches itself to an insect whose egg floats above a
store of honey; the second chooses a victim who attaches its egg to
the walls of a chamber. (Fig. 15.) This almost insignificant
difference has a considerable influence on the parasite's evolution.
In the first case it is alone, and may develop with certainty; in the
second, on the contrary, several _Sitaris_ penetrate the chamber and
climb up to attack the egg, which in this case also must be their
first food. This rivalry causes a struggle to the death. If one of the
larvae is notably more vigorous than its rivals, it may free itself
from them and survive. Let us consider the fate in store for the two
species. The first is much more favoured, since a happy chance permits
each germ to produce an individual; in the second, each individual
which completes its evolution deprives several of its brothers of
life. And even this only happens in the most favourable cases, for it
may be that not one _Sitaris_ in the chamber may reach the adult
state. If the first arrival begins to absorb the egg of the
_Colletes_, a second hungry one may kill it in the midst of its repast
and take its place. But the conqueror finds the provisions already
reduced and insufficient to enable it to reach the moulting stage, at
the end of which it could profit by the honey. Ill-nourished and
weakened, it cannot support this crisis, and its corpse falls beside
that of its fellow whom it had sacrificed. Three or four parasites may
thus succeed to the same feast, and the victory of the last is useless
to him. His first struggle for life and his first triumph are followed
by irreparable defeat. These two examples show very well how a slight
difference may favour a species, and how a happy quality is capable of
being perpetuated by heredity, since by its very nature it is destined
to be extended to more numerous beings.

[71] "Hypermetamorphoses et M