Sl8 POLITICAL SCIENCE. ing, for instance, must be presupposed before the favorable causes can have effect. Other situations begin to manifest their influence on the political forms at a yet later period than these. The choice of a place near a good harbor for a city implies that already navigation is profitable ; and the selection of it at a little distance from the coast implies already the danger of hostile invasions from the sea. Insular situations and the defence that islands furnish to their inhabitants, help colonies there to grow in security from all attacks but those of pirates. That navigation has made progress implies the con- centration of various useful arts in one place, and here we have two sources of city life. Even a barren rock may be turned into a city, if navigation and land transportation across a continent furnish supplies for the masses gathered together. 4. Races.—Some of the leading physiologists have consid- ered the human race to have a unity beyond that of a com- mon nature with its faculties, sentiments, and desires—a unity implying descent from common ancestors.* But the differ- ences of race cannot now be assigned to their causes, and the differences of language are yet greater and quite as inex- plicable. To a considerable extent, again, races must have become mixed in remote ages, so that the characteristics of race are made yet more difficult of definition. Race, again, may coexist with peculiarities of climate in producing differ- ences where it may be unsafe to assign to each factor its share in the result. Thus much, however, may be said : that race, which must have something to dp with external nature, is attended with characteristics which are, if not unalterable, at least slowly changeable. Within the same race, again, there are subordinate characteristics, and in each nation of the subor- dinate divisions of the race, national characteristics which are not all due to external nature. How strikingly unlike the Celts, the Slavonians, and the Greeks of the one Indo-Euro- pean family. How unlike the Frenchman and the English- *Conip. A. von Humboldt, Cosmos, vol. i., p. 361, Otte's trausL Load.,, 1848.