DISTRIBUTION OF LIFE, ANIMAL AND VEGETABLE. In the light of modern dis coveries, the interdependence of every part of nature is clearly revealed, and the life of the world is seen to be one symmetrical organism, the different parts of which are dis tributed in time and in space by the operation of laws as yet but imperfectly understood. Animals and plants, though sustaining very close relations to each other, form two dis tinct branches of study, whose phenomena require to be carefully discriminated. Ani mals are divided into terrestrial and aquatic, the first class being the most important and best understood. Their distribution is considered in two aspects—the climatical and the geographical—which present distinct and sometimes conflicting classes of facts. Of the two, the geographical conditions are the most important. The range of animals is deter mined in some degree by the altitude or depression of the land-surface on which they dwell. A very important element to be considered, in determining the causes of the distribution of animals, is found in their different powers of dispersal or migration, some having no means of passing over seas, or lofty mountains, or arid deserts, while others, especially the insect tribes, are not thus limited. But migrating animals can not always maintain themselves in a new region, the organisms in previous possession of the soil being too strong for them. The power of adaptation is generally inferior to the power of dispersion. The nature of the vegetation determines the range of some animals. Deserts, marshes, and forests have each their peculiar inhabitants, which do not often stray beyond their limits. Tropical forests especially supply the wants of a great num ber of peculiar form of life. Mountains of great height and in unbroken ranges form a barrier to the migration of many groups, but their geological age is limited, while oceans, owing to their great antiquity, have separated the faunas of different continents for countless ages. The zoological regions of the earth, according to the best authority, are six in number, each one having marked and distinct peculiarities. The last of these divisions is the Nearctic, which comprises all temperate North America, and is sub divided into the Californian, the Rocky mountain, the Alleghany, and the Canadian regions. The peculiar fauna of the Nearctic region is best represented in the United States, where many peculiar genera of rnammalia, birds, reptiles, and insects are found. The distribution of the higher animals during thepost-tertiary and tertairy periods is a sub ject of very deep interest. It is found that, during the post-tertiary period, the reindeer and the antelope inhabited France ; elephants and rhinoceroses roamed all over Europe; in North America there were lions, horses, camels, bisons, elephants, and mastodons. This period was characterized by great movements or migrations of the higher animals, and by the extinction of many huge creatures belonging to almost every order of mammalia, and several orders of birds. The tertiary fauna of North America, compared with that
of Europe, exhibits proof of a former communication between the two northern conti nests. From the knowledge now possessed of the extinct fauna of most of the great continents, scientists can approximately determine the original birthplace of some now widely distributed groups. The distribution of the marine animals also presents many interesting phenomena, but they cannot be noticed here. The geological record on which depends our knowledge of the distribution of animals in respect to time, though it reveals much important truth, is yet very imperfect. The evidence, so far as it goes, tends, it is thought, to confirm the doctrine of evolution.
The distribution of vegetable life is involved in much obscurity. For a long time the investigation of the subject was pursued under great disadvantages, and with very unsatisfactory results. The writings of Darwin, Hooker, Gray, and Bentham, however, have thrown much light on the subject. Bentham recognizes three ancient floras—the northern, the tropical, and the southern. The northern is divided into that of the old and new world by the severance of North America from Northern Asia, and by the barriers of the Rocky mountains. The divergences in the flora of these two regions originated in distance, but have been greatly increased by isolation. Lesquereux believes that the origin of the present American flora is American. There is a strong analogy, however, between it and the miocene flora of Central Europe, and the American element in the latter is supposed to be derivative, confirming the observation of Gray that plants tend to migrate from east to west, rather than from west to east. The boundaries of the northern flora, under the influence of climatic variations, have also undergone longi tudinal changes. The northern flora, by the combined influence of physical and genetic causes, has undergone a specialization into three distinct groups—the Arctic-Alpine, the temperate, and the Mediterraneo-Caucasian. The southern flora is still more complex in its relations, and is described in five types—the Antarctic-alpine, the Australian, the Andine, the Mexico-Californian, and the South African; the latter, though limited in extent, being the richest of all. The tropical flora has hardly as yet been investigated. It presents three subdivisions—the Indo-Malayan, the American, and African; the latter, especially, being very imperfectly known. This whole branch of science is, in many respects, an unexplored field and a very inviting one to naturalists. See GEOGRAPHICAL DISTRIBU TION OF ANIMALS, AND GEOGRAPHICAL DISTRIBUTION OF PLANTS, ante.