These are some of the ways in which Nature affects those economic, social, and political conditions of the life of man the changes in which make up history. As we have seen, that which Nature gives to man is always the same, in so far as Nature herself is always the same--an expression which is more popular than accurate, for Nature herself--that is to say, not the laws of Nature, but the physical environment of man on this planet--is in reality always changing. It is true that this environment changes so slowly that a thousand years may be too short a period in which man can note and record some forms of change--such, for instance, as that by which the temperature of Europe became colder during the approach of the glacial period and warmer during its recession--while ten thousand years may be too short to note any diminution in the heat which the sun pours upon the earth, or in the store of oxygen which the earth’s atmosphere holds. [Sidenote: Progress of Modern Invention] [Sidenote: Man Cannot Disregard Nature] But as we have also seen, the relation to man of Nature’s gifts differs from age to age as man himself becomes different, and as his power of using these gifts increases, or his need of them becomes either less or greater. Every invention alters those relations. Water power became less relatively valuable when steam was applied to the generation of motive force. It has become more valuable with the new applications of electricity. With the discovery of mineral dyes, indigo and cochineal are now less wanted than they were. With the invention of the pneumatic tyre for bicycles and carriages, caoutchouc is more wanted. Mountains have become, since the making of railways, less of an obstacle to trade than they were, and they have also become more available as health resorts. Political circumstances may interfere with the ordinary and normal action of natural phenomena. A race may be attracted to or driven into a region for which it is not physically suited, as Europeans have gone to the West Indies, and negroes were once carried into New York and Pennsylvania. The course of trade which Nature prescribes between different countries may be hampered or stopped by protective tariffs; but in these cases Nature usually takes her eventual revenges. They are instances which show, not that man can disregard her, but that when he does so, he does so to his own loss. It would be easy to add further illustrations, but those already given are sufficient to indicate how multiform and pervading is the action upon man of the physical environment, or in other words, how in all countries, and at all times, geography is the necessary foundation of history, so that neither the course of a nation’s growth, nor its relations with other nations, can be grasped by one who has not come to understand the climate, surface, and products of the country wherein that nation dwells. [Sidenote: There is no Unmixed Race left]