Again, where the population is dense, and all the land is required to grow food for man, it is not possible to set aside regions for the pasturage of domestic animals, whose numbers must necessarily then be small. This is true of China and Japan, where domestic hoofed animals are few in number. The contrary condition is of course seen in new countries, like Australia and the Argentine, where there are far fewer men than sheep and cattle. The United States is beginning to pass from this condition, and there have been published already warnings to the community that it cannot go on giving up much of its fertile land to the growth of crops for its domestic animals, instead of for its citizens directly. Of the domestic animals which have assisted man in the work of transport there can be no doubt that the horse is by far the most valuable. It is the strongest, the swiftest, and the best fitted for man's purposes. The camel, of which so much has been written, is in point of fact a poor substitute for the more valuable animal, rendered necessary by desert conditions. Among the other animals which have played their part as beasts of burden or of traction are many kinds of ox, including the yak of Tibet; the llama of South America; the elephant; the reindeer; the dog; and the animals which rank as horses to the zoologist though not to the owner--that is the ass and the mule. Just as the use of beasts of burden is an enormous improvement upon human porters, and that of wheeled vehicles upon beasts of burden, so is the use of steam an enormous improvement upon wheeled vehicles drawn by animals. Railways have practically revolutionised the problem of land transport, though their cost, especially in countries of marked relief, is a great drawback to their universal use. The last few years have seen in the development of motor-driven vehicles a new change, which has given back to the roads their old importance, and which will probably in the future greatly aid the development of new countries, and take from the railways some of their importance. It is too soon yet to say whether the last development of means of transport, the aeroplane, is destined to affect greatly man's movements and methods of exchanging commodities.