NATURAL SELECTION.
In his introduction, Mr. Darwin tells us that "when on board H. M. S. Beagle as naturalist, I was much struck with certain facts in the distribution of the organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to throw some light on the origin of species." In the chapter on Geographical Distribution, he says that "neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions." Thus, the plants and animals of South America, between latitudes 25° and 55°, are very different from those of Australia and South Africa; and yet the physical conditions of these three countries, within these limits, are very similar, while, notwithstanding the great differences of the physical conditions north of 25° and south of 35°, the plants and animals of these parts of South America are very similar. The existence of lofty mountain-chains, great deserts, etc. acts as a barrier to the free dispersion of plants and animals, and, therefore, is of great importance in reference to Geographical Distribution; thus, the life of opposite mountain-chains is often quite different. This is equally true of the ocean life on opposite sides of a continent: thus, the marine animals of the north side of the Isthmus of Panama are very different from those of the south side, whereas similar
fishes are found in as remote waters as the Pacific and Indian
