Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/192

142 oceans,—there being no obstacle to their free dispersion. The relation of the living animals to those found fossil in the same countries is very significant in this respect, the Apteryx of New Zealand representing the gigantic Dinornis, the Armadillo and Sloth the extinct Glyptodon and Megatherium. Further, in reference to Geographical Distribution, the fact of the plants and animals of islands being like those of the nearest island or mainland is as important to the Geologist as to the Botanist and Zoologist. Thus, Mr. Wallace explains the similarity of the plants and animals of Sumatra, Java, Borneo, etc. to those of southern Asia by supposing that these islands once formed part of that continent, being connected with it by Malacca; while the Celebes, Moluccas, New Guinea, resembling in their flora and fauna Australia, are regarded as forming with it another continent, the islands of Bali and Lombok indicating the limits of these ancient continents. Mr. Wallace says, in crossing over the straits separating these islands, "we may pass in two hours from one great division of the earth to another, differing as essentially in their animal life as Europe does from America." The study of South America and the Malay Archipelago suggested to Messrs. Darwin and Wallace their explanation of the Geographical Distribution of plants and animals through what Mr. Darwin calls Natural Selection, which may be expressed as follows:

Plants and animals struggle for existence. The immediate descendants are never absolutely like their parents; while remote posterity often differs considerably from them.

Those plants and animals whose modified organization gives them an advantage over those not so favored survive, or are naturally selected, and transmit their modifications to posterity.

Let us examine these statements, and try to explain how the conclusion follows.