delightful "Malay Archipelago," first published in 1860, cannot know all the treasures given to science by Mr. Wallace's eight years' expatriation, for before writing his travels he had contributed no fewer than eighteen papers to the transactions or journals of the Linnean, Zoological, and Entomological Societies, and twelve articles to various scientific periodicals, while in his subsequent volumes on "Natural Selection," 187 1, his monumental work on the "Geographical Distribution of Animals," 1876, on "Tropical Nature," 1878, and on "Island Life," 1880, he laid open still more fully his accumulations of travel and thought in both hemispheres. One of the most valuable results of his travels in Malaysia was the establishment of a line dividing the archipelago into two main groups, Indo-Malaysia and Austro-Malaysia, marked by peculiar species and groups of animals. This line, now everywhere known as Wallace's line, is marked by a deep sea belt between Celebes and Borneo, and Lombok and Bali respectively; and it is curious that a similar line, but somewhat further east, divides on the whole the Malay from the Papuan races of man. The new facts on butterflies, on birds of paradise, on mimicry between various animals and plants, and on the Malay and Papuan races are only a few of the subjects of intense interest illuminated by Mr. Wallace as the result of his travels in Malaysia.
In a paper in the Annals and Magazine of Natural History for September, 1855, "On the Law that has regulated the Introduction of New Species," Mr. Wallace had already drawn the conclusion that every species has come into existence coincident both in space and time with a pre-existing closely allied species. In the same paper is a brief expression of the idea which Mr. Darwin expanded into one of his fine passages comparing all members of the same class of beings to a great tree. The varied facts of the distribution of animal and plant life, set forth and explained in this paper, foreshadow the author's future great work on the subject. Mr. Darwin, already an observer and student of long standing on the question of the origin of species, had noted this paper and agreed to the truth of almost every word of it. In October 1856, Mr. Wallace wrote to Mr. Darwin from Celebes, and in replying to his letter Mr. Darwin, on May Ist, 1857, said he could plainly see that they had thought much alike, and had to a certain extent come to similar conclusions; and later
