Now, the zoological divisions of the earth, as described by Mr. Wallace in his latest work treating of this subject, are six in number, as follows: :

I. PALEARCTIC, Europe, with northern temperate Africa and Asia.

II. ETHIOPIAN, Africa south of the Sahara, with Mada

gascar.

III. ORIENTAL, Tropical Asia to the Philippines and Java. IV. AUSTRALIAN, Australia, with the Pacific Islands, Moluccas, etc.

V. NEARCTIC, North America to North Mexico.

VI. NEOTROPICAL, South America, with tropical North America and the West Indies.

By comparing this list of zoological regions with the regions of fracture already described, it will be seen that the lines of division between the zoological areas in three cases—namely, through the Malay Archipelago, Behring Strait, and the Gulf of Mexico-correspond roughly with the zone of fracture. The two zoological boundaries which are to be met with outside of the zone of fracture are determined, one by the Sahara Desert; and the other by the Soliman, the Hindoo Koosh, the Himalaya, and the Nanling Mountains. Now, the Sahara is as impassible a barrier for most quadrupeds as any

1 Island Life, p 52.

sea of equal size would be; the Himalayas are, as every schoolboy knows, the highest mountains in the world; and the Hindoo Koosh is a very lofty range. And so Africa is effectually cut in two by the Sahara; while tropical Asia is almost as completely shut off from the rest of the continent by the mountain ranges just mentioned, which entirely surround the bases of the two great peninsulas of British India and IndoChina.

If the animal population of the earth has spread gradually over its surface, each new species extending its limits over the whole adjacent territory, we should expect to find a certain degree of resemblance among the animals of all continents, or parts of continents, which are not separated by a natural barrier. On the other hand, when there is such a barrier, we should expect to find marked differences between the fauna on either side of it. And just as we should expect to find things in this regard, so they actually are, as, I trust, the sentences quoted from Mr. Wallace at the beginning of the chapter have illustrated. The old idea, that

1 Of course in every case allowance must be made for the possibility of obstructions or aids to the passage of animals back and forth, having arisen from changes in the earth's surface during comparatively recent geological time.

the types of animal life on the land are boundei by the zones of climate, has been finally abandoned. The boundaries of the zoological regions are rather determined by the presence of bodies of water, deserts, or lofty mountains, and are in great measure independent of latitude and longitude. In this connection it might very naturally be asked how it happens that Northern Africa is to be classed with the palæarctic region, although it is separated from Southern Europe by the Mediterranean. To this the answer is, first, that there is roundabout communication between Africa and other parts of the palæarctic region by way of the Isthmus of Suez, and second, that, in comparatively recent geological time, there has been land communication between the points of Africa and Europe now separated by the Strait of Gibraltar.1 On studying the fauna of each of the six great zoological divisions and of the minor subdivisions of each, we shall find, that, on the whole, such animals as land-shells (snails, etc.) and freshwater fishes have the least range, that is are most strictly confined, each species to its own area. Fresh-water crustacea (such as the crawfish), insects, toads and frogs, and warm-blooded

1 Schmidt, Descent and Darwinism, p. 224.

quadrupeds may range more widely, and reptiles even farther. Birds of some families have an almost world-wide distribution: the fish-hawk and the barn-owl are to be found ranging over the greater portion of every continent. It has already been stated in previous chapters, that the native animals and plants of a country are often less perfectly adapted to hold their own in it than are species from abroad. Couple this fact with the other fact just stated, that it is those animals that are best fitted to make long journeys, — namely the birds, that are found most widely scattered over the earth's surface, while the proverbially sluggish snail is often restricted to a single valley,1- and but one explanation of these facts of distribution is possible. We cannot avoid the conclusion that the wide range of birds and the narrow one of freshwater fishes is due to the fact that the former can easily fly from region to region, while the latter can only traverse the fresh waters in which alone they can live.2

Experiments in fish-culture have demonstrated

1 "Each valley [in the Sandwich Islands], and often each side of a valley, and sometimes every ridge and peak, possesses its peculiar species." - ISLAND LIFE, p. 299.

2 This does not at all conflict with the statement made in a preceding chapter in relation to the occurrence of one spe

that many species of fresh-water fishes (the European carp, for instance) thrive in lakes and rivers of a foreign continent. And to assert, in the face of such facts, that the fish was created for its former home, and so was found in it, but not for the latter, and so was not found in it, would be merely playing with words.

Among the most interesting facts relating to distribution are those which connect it with geology. As a result of the investigations of Rütimeyer and others in Germany, and of Wallace in England, it may be laid down as fully ascertained: —

1. That antiquated forms of life are found in abundance only in regions which have long been shut off from communication with the great northern land-masses.

2. That in two portions of land or water which were once united, but have become separated, the amount of difference between the animals inhabit ing them will bear some proportion to the length of time that has elapsed since their separation.

3. That oceanic islands1 (aside from the intro

cies of hunming-bird in a single extinct volcanic crater in South America; for humming-birds frequent certain flowers only, and are not to be found except where these grow.

1 Those which never have been connected with any conti

nent.