I must here indulge in a few remarks more, which, though they may seem of an incidental character, have a direct bearing on the general subject. The geologist infers, in all his reasonings founded on fossils, that a race or species has existed from some one certain point in the scale to some other certain point, if he find it occurring at both points together. He infers on this principle, for instance, that the boulder clay, which contains only _recent_ shells, belongs to the _recent_ or post-Tertiary period; and that the Oolite and Lias, which contain _no_ recent shells, represent a period whose existences have all become extinct. And all experience serves to show that his principle is a sound one. In creation there are many species linked together, from their degree of similarity, by the _generic_ tie; but no perfect verisimilitude obtains among them, unless hereditarily derived from the one, two, or more individuals, of contemporary origin, with which the race began. True, there are some races that have spread over very wide circles,—the circle of the human family has become identical with that of the globe; and there are certain plants and animals that, from peculiar powers of adaptation to the varieties of soil and climate,—mayhap also from the tenacious vitality of their seeds, and their facilities of transport by natural means,—are likewise diffused very widely. There are plants, too, such as the common nettle and some of the ordinary grasses, which accompany civilized man all over the globe, he scarce knows how, and spring up unbidden where-ever he fixes his habitation. He, besides, carries with him the common agricultural weeds: there are localities in the United States, says Sir Charles Lyell, where these _exotics_ outnumber the native plants; but these are exceptions to the prevailing economy of distribution; and the circles of species generally are comparatively limited and well defined. The mountains of the southern hemisphere have, like those of Switzerland and the Scotch Highlands, their forests of coniferous trees; but they furnish no Swiss pines or Scotch firs; nor do the coasts of New Zealand or Van Dieman’s Land supply the European shells or fish. True, there may be much to puzzle in the identity of what may be termed the exceptional plants, equally indigenous, apparently, in circles widely separated by space. It has been estimated that there exist about a hundred thousand vegetable species, and of these, thirty Antarctic forms have been recognized by Dr. Hooker as identical with European ones. Had Robinson Crusoe failed to remember that he had shaken the old corn-bag where he found the wheat and barley ears springing up on his island, he might have held that he had discovered a new centre of the European cerealia. And the process analogous to the shaking of the bag is frequently a process _not_ to be remembered. There are several minute lochans in the Hebrides and the west