of a forest of tree mosses to its discoverer. The absolute size and the degree of development attained by organic forms of the same family (whether plants or animals), depend on laws which are still unknown to us. In each of the great divisions of the animal kingdom, insects, crustacea, reptiles, birds, fishes, or mammalia, the size of the body oscillates between certain extreme limits. But these limits, which have been established by observation as far as it has yet gone, may be corrected by the discovery of species with which we are still unacquainted.
In land animals the higher temperatures of the low latitudes appear to have favoured organic development. The small and slender form of our lizards is exchanged in the south for the gigantic, heavy, and cuirassed bodies of crocodiles. In the formidable tiger, lion, and jaguar, we see repeated, on a larger scale, the form of the common cat, one of the smallest of our domestic animals. If we penetrate into the interior of the earth, and search the cemeteries in which the plants and animals of the ancient world lie entombed, the fossil remains which we discover not only announce a distribution inconsistent with our present climates,—they also disclose to us gigantic forms that contrast no less with those which now surround us, than does the simple heroism of the Greeks with the character of human greatness in modern times. Has the temperature of our planet undergone considerable changes,—possibly of periodical recurrence? If the proportion between land and sea, and even the height of the aerial ocean and its pressure,[14] have not always been the same, the physiog-
