These general considerations led us to contemplate more in detail the correlation between structural affinity and barriers to free migration. Such barriers, of course, differ in the cases of different organisms. Marine organisms are stopped by land, unsuitable temperature, or unsuitable depths; fresh-water organisms by sea and by mountain-chains; terrestrial organisms chiefly by water. Now it is a matter of fact which admits of no dispute, that in each of these cases we meet with a direct correlation between the kind of barrier and the kind of organisms whose structural affinities are affected thereby. Where we have to do with marine organisms, barriers such as the Isthmus of Panama and the varying depth of the Western Pacific determine three very distinct faunas, ranging north and south in closely parallel lines, and under corresponding climates. Where we have to do with fresh-water organisms, we find that a mountain-chain only a few miles wide has more influence in determining differences of organic type on either side of it than is exercised by even thousands of miles of a continuous land-area, if this be uninterrupted by any mountains high enough to prevent water-fowl, whirlwinds, &c., from dispersing the ova. Again, where we have to do with terrestrial organisms, the most effectual barriers are wide reaches of ocean; and, accordingly, we find that these exercise an enormous influence on the modification of terrestrial types. Moreover, we find that the _more_ terrestrial an organism, or the _greater_ the difficulty it has in traversing a wide reach of ocean, the _greater_ is the modifying influence of such a barrier upon that type. In oceanic islands, for example, many of the plants and aquatic birds usually belong to the same species as those which occur on the nearest mainlands, and where there are any specific differences, these but rarely run up to generic differences. But the land-birds, insects, and reptiles which are found on such islands are nearly always specifically, and very often generically, distinct from those on the nearest mainland--although invariably allied with sufficient closeness to leave no manner of doubt as to their affinities with the fauna of that mainland. Lastly, no amphibians and no mammals (except bats) are ever found on any oceanic islands. Yet, as we have seen, on the theory of special creation, these islands must all be taken to have been the theatres of the most extraordinary creative activity, so that on only three of them we found no less than 1258 unique species, whereof 657 were unique species of land animals, to be set against one single species known to occur elsewhere. Nevertheless, notwithstanding this prodigious expenditure of creative energy in the case of land-birds, land-shells, insects, and reptiles, no single new amphibian, or no single new mammal, has been created on any single oceanic island, if we except the only kind of mammal that is able to fly, and the ancestors of