Another example of so-called vicarious or "analogous" species, affording an easier basis for induction, is provided by the comparison of the snails of Southern Europe, and especially of Spain, with those of North Africa, on which we are indebted to Bourguignat for some excellent observations. In accordance with other botanical and zoological facts, he has established that the shell fauna of Spain and North Africa forms a whole, so that the Algierian snails appear a mere appendage to those of Southern Europe, nothwithstanding the separation by the Straits of Gibraltar. Now it is proved that, in geologically recent times, this region of North Africa was in fact a peninsula of Spain, and that its union with Africa was effected on the north by the rupture of the Straits of Gibraltar, and on the south by an upheaval to which the Sahara owes its existence. The shores of the former Sea of Sahara are still marked by the shells of the same snails that live on the shores of the Mediterranean. But all North African species are not identical with those of Spain; of many African sorts, only "analogous" species are found on our side. Now if certain Spanish species do not themselves occur in Africa, but are yet replaced by very similar forms, our standpoint at once connects with the otherwise unmeaning word "analogous" species the idea of the common derivation of the forms replacing one another, and of the local variations superinduced by isolation. and altered conditions.

A severe test is applied to those who believe that species were separately created, by the air-breathing land snails (pulmo-gasteropoda), when it is seen that in isolated islands and island groups these earth-bound

animals, migrating with so much difficulty, have attained an extraordinary diversity. In the Madeira Islands, 134 species of pulmo-gasteropoda were reckoned about ten years ago, of which only 21 were to be found in the Africo-European fauna. These and the 113 other species are mostly confined to narrow districts and single valleys. Are we to suppose that the 113 species for Madeira, and the 21 species for Madeira and Africa. with Europe, were each separately created? Must we not much rather infer that a connection at one time existed between Europe and the present island group of Madeira, and that these 21 species remained what they were before the separation; while from unknown species still appearing in analogous forms upon the continent emanated the remarkable profusion of new species ? They, and their comrades on other isolated islands, were spared a conflict many sided, and they doubtless afford a favourable example of Wagner's law of migration, as with the difficulties of locomotion, and the improbability of a large subsequent arrival, the secluded individuals, under even slightly different influences, had had a prospect of diverging from the parent species.

The unscientific opinion, that under like, or nearly like, external conditions, like or similar organisms were created in great numbers, receives a severe blow by the perception that the direct reverse has frequently occurred. Why has America no horses in the present era, although it is proved that the horses introduced, thrive capitally? It is not necessary for us to explain why the fossil horses which existed in America, as well as in the Eastern hemisphere, became extinct without

leaving any progeny-we do not know the cause, though we may yet be able to fathom it; but in this and all similar cases the adherents of the doctrine of Creation must confess the inadequacy of their theory of belief.

Our exposition has shown that the species now extant are the progeny of organisms previously existing; the present apportionment on the earth is therefore a consequence of the distribution of the progenitors of the present organisms, and of the manifold displacements of land and water by which they were indirectly or directly affected. We cannot hope ever to picture to ourselves a faithful representation of the perpetual transformations of the surface of the earth. Only, if this could be accomplished, and if we, moreover, had an accurate register of the animals at each period inhabiting the former islands, continents, and oceans-only then could the distribution of the present organisms be thoroughly fathomed and established. But in thus acknowledging the incompleteness of our statistical means, we are at least able to lay down with certainty the course of inquiry. We must, in the first place, proceed in the method of the older vegetal and animal geography to ascertain the natural limits and regions of distribution; and, secondly, to collate these facts with the facts of the distribution of the former progenitors of the present animate world as it was determined by the geological conditions of those times. It is needless to say that Darwin has furnished the outlines for this work also. But among his followers two are specially worthy of distinction: Wallace, with his researches on the Malay Archipelago," abounding in subtle observation;

and Rütimeyer, in his treatise already cited. In what follows we may essentially adhere to the latter.

Our knowledge of the regions of distribution of the animal world is still extraordinarily deficient. What do we know, for instance, of the occurrence of marine animals? Few years only have elapsed since the depths of the sea were rendered accessible to research, and the result has almost entirely upset our earlier notions of the geological significance of the sea-bottom and its habitability. After the strong impulse given by Maury to the investigation of the physical condition of the sea, we are now occupied in ascertaining the submarine temperatures and currents, the constitution of the sea-bottom, the occurrence of deep-sea organisms, and the conditions of their existence. We are therefore just beginning to collect the material for a future geography of marine organisms. Among terrestrial animals, certain groups of which the actual distribution can be defined, are useless for our general purpose.

Butterflies, for instance, which are an easy prey to currents of air, defy geological barriers, and, above all, that important partition which from the tertiary era has been erected, or rather excavated in the bottom of the sea, between Australia and India." It is the same with bats, and also with migratory, predatory, and aquatic birds; while, as Wallace shows, the other orders of this class are in tropical regions very reliable and stable inhabitants of their often limited districts, seemingly suggestive of migration. Exclusive of these, there remains therefore little more than the Mammalia, whose extraction may be inferred with certainty from a comparison of their present cantonments (Cantonirung),

-an expression which we borrow from Rütimeyer,— with the encampments of their former kindred, whence are derived general points of view as to the causes of the present geographical apportionment of organisms.

If in the preliminary establishment of facts we therefore confine ourselves to the Mammalia, exclusive of whales and bats, a superficial survey is enough to show that not only single species, but families also, have each a certain region of greatest density of occurrence, a focus of distribution, and that from thence radiations have taken place according to the convenience and fitness of the territory. Lion and tiger, elephant and camel, range over a definite area; the monkeys of the New World differ from those of the Old World not only geographically, but also in family characteristics. Marsupials are chiefly concentrated in Australia; sloths and armadilloes in South America. And these examples, easy to multiply, indicate how individuals of widely dispersed species, and the species themselves, emanated from single points of the earth's surface and flowed over the territory of distribution now occupied. When to this observation is added the other, that in past eras also the same groups had the same centres of distribution, for instance, Brazil not only harbours sloths and armadilloes now, but was once peopled by more numerous and partly colossal species of these families, and Australia has furnished the most numerous and important fossil remains of Marsupials, the cognizance of this persistent localization becomes very significant, and we account for the "repetition" of these forms by derivation.

Now if the centres of distribution, at the first glance