DISTRIBUTION OF LIFE ANIMAL AND VEGETABLE IN SPACE AND TIME the subject specially discussed under this heading is the Distribution of Life, Animal and Vegetable, in Space, and Time.

So long as each species of organism was supposed to have had an independent origin, the place it occupied on the earth's surface or the epoch where it first appeared had little significance. It was, indeed, perceived that the organization and constitution of eaeli animal or plant must, be adapted to the physical conditions in which it was placed ; but this consideration only accounted for a few of the broader features of distribution, while the great body of the facts, their countless anomalies and curious details, remained wholly inexplicable. But the theory of evolution and gradual development of organic forms by descent and variation (some form of which is now universally accepted by men of science) completely chauges the aspect of the question and invests the facts of distribu-tion with special importance. The time when a. group or a species first appeared, the place of its origin, and the (11T1 it now occupies upon the ea.rth, become essential portions of the history of the universe. The course of study initiated and so largely developed by Mr Darwin has 110W shown us the marvellous interdependence of every part of nature. Not only is each organism necessarily related to and affected by all things, living and.dead, that surround it, but eve.ry detail of form and structure, of colour, food, and habits, must - it is now held - have been developed in harmony with, and to a great extent as a. result of, the organic and inorganic environinent3. Dis-tribution becomes, therefore, as essential a part of the science of life as anatomy or physiology. It shows us, as it were, the form and structure of the life of the world considered as one vast organism, and it enables us to comprehend, however imperfectly, the processes of development and variation during past ages which have resulted in the actual_ state of things. It thus affords one of the best tests of the truth of our theories of development ; because, the count-less facts presented by the distribution of living things in present and past time must be explicable in accordance with any true theory, or at least must never directly contradict it.

From these indications of the scope and bearing of the subject, it will be seen that its full and adequate treatment would require volumes, and would necessarily involve an amount of details only suited to specialists in the various branches of natural history. All that can be attempted here is to give such a general sketch of the whole sub-ject as to place the reader in possession of the main re-sults arrived at, and enable him to comprehend the bear-ing of the more detailed information he may meet with elsewhere.

A rrangement of the Subject. - The three great heads under which the various matters connected with distribution may be classed are - lst, the geographical distribution of living organisms ; 2d, the geographical distribution of extinct organisms ; and 3d, the geological succession of the chief forms of life. Owing, however, to the fact that the study of animals and of plants form very distinct sciences, and that there are special peculiarities in the phenomena pre-sented by each which require to be carefully discriminated, it is found to be necessary to inake a primary division of the subject into the distribution of animals and of plants respectively.

The distribution of living animals in space naturally forms the first division of our subject, both because the phenomena are simpler and better known, and because it puts before us the main problems and difficulties to the solution of which the other divisions furnish the key. Animals may be ronghly divided into two great series, broa,dly distingnished as regards their mode of life - the terrestrial and the aquatic ; and for the purpose of our present study these divisions are of primary importance, because that element which limits the range of the one class offers a free passage to the migrations of the other, and vice versa. The first series is by far the most important. It is the best known, and includes ahr_ost all the higher animals ; while the variety and interest of the various land divisions of the globe are far greater than in the case of that portion of its surface covered by water. We shall therefore consider first, and with a greater amount of detail, the distribution of laud animals, including among them the fresh-water forms whose range is limited by the same general conditions.