It has sometimes been maintained--never more energetically than to-day, especially among the nations which most eagerly entered the present conflict--that war is a biological necessity. War, we are told, is a manifestation of the "Struggle for Life"; it is the inevitable application to mankind of the Darwinian "law" of natural selection. There are, however, two capital and final objections to this view. On the one hand it is not supported by anything that Darwin himself said, and on the other hand it is denied as a fact by those authorities on natural history who speak with most knowledge. That Darwin regarded war as an insignificant or even non-existent part of natural selection must be clear to all who have read his books. He was careful to state that he used the term "struggle for existence" in a "metaphorical sense," and the dominant factors in the struggle for existence, as Darwin understood it, were natural suitability to the organic and inorganic environment and the capacity for adaptation to circumstances; one species flourishes while a less efficient species living alongside it languishes, yet they may never come in actual contact and there is nothing in the least approaching human warfare. The conditions much more resemble what, among ourselves, we may see in business, where the better equipped species, that is to say, the big capitalist, flourishes, while the less well equipped species, the small capitalist, succumbs. Mr. Chalmers Mitchell, Secretary of the London Zoological Society and familiar with the habits of animals, has lately emphasised the contention of Darwin and shown that even the most widely current notions of the extermination of one species by another have no foundation in fact.[1] Thus the thylacine or Tasmanian wolf, the fiercest of the marsupials, has been entirely driven out of Australia and its place taken by a later and higher animal, of the dog family, the dingo. But there is not the slightest reason to believe that the dingo ever made war on the thylacine. If there was any struggle at all it was a common struggle against the environment, in which the dingo, by superior intelligence in finding food and rearing young, and by greater resisting power to climate and disease, was able to succeed where the thylacine failed. Again, the supposed war of extermination waged in Europe by the brown rat against the black rat is (as Chalmers Mitchell points out) pure fiction. In England, where this war is said to have been ferociously waged, both rats exist and flourish, and under conditions which do not usually even bring them into competition with each other. The black rat (_Mus rattus_) is smaller than the other, but more active and a better climber; he is the rat of the barn and the granary. The brown or Norway rat (_Mus decumanus_) is larger but less active, a burrower rather than a climber, and though both rats are omnivorous the brown rat is more especially a scavenger; he is the rat of sewers and drains.