Man is unique among gregarious animals in the size of the major unit upon which natural selection and its supposedly chief instrument, war, is open to act unchecked. There is no other animal in which the size of the unit, however laxly held together, has reached anything even remotely approaching the inclusion of one-fifth or one-quarter of the whole species. It is plain that a mortal contest between two units of such a monstrous size introduces an altogether new mechanism into the hypothetical “struggle for existence” on which the conception of the biological necessity of war is founded. It is clear that that doctrine, if it is to claim validity, must contemplate at any rate the possibility of a war of extremity, even of something like extermination, which shall implicate perhaps a third of the whole human race. There is no parallel in biology for progress being accomplished as the result of a racial impoverishment so extreme, even if it were accompanied by a closely specific {129} selection instead of a mere indiscriminate destruction. Progress is undoubtedly dependent mainly on the material that is available for selection being rich and varied. Any great reduction in the amount and variety of what is to be regarded as the raw material of elaboration necessarily must have as an infallible effect, the arrest of progress. It may be objected, however, that anything approaching extermination could obviously not be possible in a war between such immense units as those of modern man. Nevertheless, the object of each of the two adversaries would be to impose its will on the other, and to destroy in it all that was especially individual, all the types of activity and capacity which were the most characteristic in its civilization and therefore the cause of hostility. The effect of success in such an endeavour would be an enormous impoverishment of the variety of the race and a corresponding effect on progress.