1088. The territorial limits to vegetation are determined in general by three different causes : - 1. By sandy deserts, which seeds cannot pass over either by means of winds or birds, as that of Sahara, in Africa; 2. By seas too vast for the seeds of plants to be drifted from one shore to the other, as in the ocean , while the Mediterranean sea, on the contrary, exhibits the same vegetation on both shores ; and, 3. By long and lofty chains of mountains. To these causes are to be attributed the fact, that similar climates and soils do not always produce similar plants. Thus in certain parts of North America, which altogether resemble Europe in respect to soil, climate, and elevation, not a single European plant is to be found. The same remark will apply to New Holland, the Cape of Good Hope, Senegal, and other countries, as compared with countries in similar physical circumstances, but geographically different. The separation of Africa and South America, Humboldt considers, must have taken place before the developement of organised beings, since scarcely a single plant of the one country is to be found in a wild state in the other.
