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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
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