It is well known how the invention of machinery in the middle of the eighteenth century brought about that industrial revolution, the social and political effects of which are still developing at this hour. Well, I venture to put it forward as a proposition which applies to human evolution, so far back as our evidence goes, that history is the history of great inventions. Of course, it is true that climate and geographical conditions in general help to determine the nature and quantity of the food-supply; so that, for instance, however much versed you may be in the art of agriculture, you cannot get corn to grow on the shores of the Arctic sea. But, given the needful inventions, superior weapons for instance, you need never allow yourselves to be shoved away into such an inhospitable region; to which you presumably do not retire voluntarily, unless, indeed, the state of your arts--for instance, your skill in hunting or taming the reindeer--inclines you to make a paradise of the tundra. Suppose it granted, then, that a given people's arts and inventions, whether directly or indirectly productive, are capable of a certain average yield of food, it is certain, as Malthus and Darwin would remind us, that human fertility can be reckoned on to bring the numbers up to a limit bearing a more or less constant ratio to the means of subsistence. At length we reach our more immediate subject--namely, social organization. In what sense, if any, is social organization dependent on numbers? Unfortunately, it is too large a question to thrash out here. I may, however, refer the reader to the ingenious classification of the peoples of the world, by reference to the degree of their social organization and culture, which is attempted by Mr. Sutherland in his _Origin and Growth of the Moral Instinct_. He there tries to show that a certain size of population can be correlated with each grade in the scale of human evolution--at any rate up to the point at which full-blown civilization is reached, when cases like that of Athens under Pericles, or Florence under the Medici, would probably cause him some trouble. For instance, he makes out that the lowest savages, Veddas, Pygmies, and so on, form groups of from ten to forty; whereas those who are but one degree less backward, such as the Australian natives, average from fifty to two hundred; whilst most of the North American tribes, who represent the next stage of general advance, run from a hundred up to five hundred. At this point he takes leave of the peoples he would class as "savage," their leading characteristic from the economic point of view being that they lead the more or less wandering life of hunters or of mere "gatherers." He then goes on to arrange similarly, in an ascending series of three divisions, the peoples that he terms "barbarian." Economically they are either sedentary, with a more or less developed agriculture, or, if nomad, pursue the pastoral mode of life. His lowest type of group, which