As all bee-keepers are aware, the disease known as foul-brood works more havoc among their bees than all the insectivorous creatures put together. Similarly throat disease among wood-pigeons does more towards keeping their numbers down than all the efforts of predacious birds. A check on multiplication not mentioned by Darwin is that which is sometimes imposed by the individuals of the species on one another. Thus, in some animals, as, for example, the hyæna, the male occasionally devours his own young ones. A check of a similar nature results from the habit which the Indian House Crow (_Corvus splendens_) has of interrupting the pairing operations of its neighbours. Attributes of Successful Species We are now in a position to sum up briefly the more important requisites for success in the struggle for existence. These are not so much specialised structure as courage, a good constitution, mental capacity and prolificacy. Few animals possess all these characteristics in a pre-eminent degree, for, to use the words of Mr Thompson Seton, “Every animal has some strong point or it could not live, and some weak point or the other animals could not live.” Courage may be of two kinds—active courage, like that of the Englishman, or passive courage, like that of the Jew. As D. Dewar has said: In the struggle for existence, “An ounce of good solid pugnacity is worth many pounds of protective colouration.” It is of course possible for an animal to possess too much courage. An excessive amount of courage will often cause a creature to fight unnecessary battles, which may lead to its premature death. This is perhaps the reason why the pugnacious black form of the leopard is not more numerous. Under a good constitution we must include the power of resisting the rigours of climate, more especially damp, the ability to resist disease, and the enjoyment of a good digestion. When from any cause the normal food of a species becomes scarce, the members of that species will have to starve or supplement the normal diet with food of an unusual nature; and those that are endowed with a good digestion will be able to digest the new food and thus survive, while those which cannot assimilate food to which they are unaccustomed will become emaciated and perish. We see this in every hard winter in England, when the redwing, which, unlike other thrushes, cannot thrive on berries, is the first to die. Most of the more successful birds—the crows and gulls, for example—are omnivorous—that is to say, they are able to digest all manner of food.