Foliage from the coal-formation, showing some of the forms of leaves instrumental in accumulating the carbon of our coal-beds, by their action on the atmosphere under the influence of sunlight.
From the boundless variety of illustrations which the animal kingdom presents I may select one—the contrivances by means of which marine animals are enabled to float or balance themselves in the waters. The Pearly Nautilus (see Fig. 15) is one of the most familiar, and also one of the most curious. Its coiled shell is divided by partitions into air-chambers so proportioned that the buoyancy of the air is sufficient to counterpoise in sea-water the weight of the animal. There are also contrivances by which the density of the contained air and of the body of the animal can be so modified as slightly to disturb this equilibrium, and to enable the creature to rise or sink in the waters. It would be tedious to describe, without adequate illustrations, all the machinery connected with these adjustments. It is sufficient for our purpose to know that they are provided in such a manner that the animal is practically exempted from the operation of the force of gravity. In the modern seas these provisions are enjoyed by only a few species of the genera Nautilus and Spirula; but in former geological ages, more numerous, as well as larger and more complex, forms existed. Further, this contrivance is very old. We find in the Orthoceratites and their allies of the earliest Silurian formations these arrangements in their full perfection, and in some forms[12] even more complex than in later types.
Fig. 15.
Section of the Pearly Nautilus and its shell, showing that the animal occupies only the outer chamber, the others being filled with air and acting as a float whose buoyancy can be modified by the action of the tube, or siphuncle, passing through the chambers.
The peculiar contrivances observed in the nautilus and its allies are possessed by no other mollusks, but there is another group of somewhat lower grade, that of the Ianthinæ, or violet snails, in which flotation is provided for in another way (see Fig. 16). In these animals the shell is perfectly simple, though light, and the floating apparatus consists in a series of horny air-vesicles attached to what is termed the "foot" of the animal, and which are increased in number to suit its increasing weight as it grows in size. There are some reasons to believe that this entirely different contrivance is as old in geological time as the chambered shell of the nautiloid animals. It was, indeed, in all probability, more common and adapted to larger animals in the Silurian period than at present.
Fig. 16.
Ianthina, or Violet Snail, attached to a float composed of horny hollow vesicles, to the under side of which its eggs are attached. When hatched, each young animal develops a small float similar to that of the parent.
Another curious instance—not, so far as yet known, existing at all in the modern world—is that of the remarkable stalked star-fish described by Professor Hall under the name Camerocrinus, and whose remains are found in the Upper Silurian rocks. The Crinoids, or feather-stars, are well-known inhabitants of the seas, in both ancient and modern times; but previous to Professor Hall's discovery they were known only as animals attached by flexible stems to the sea-bottom or creeping slowly by means of their radiating arms. It was not suspected that any of them had committed themselves to the mercy of the currents, suspended from floats. It appears, however, that this was actually realized in the Upper Silurian period, when certain animals of this group developed a hollow calcareous vesicle forming a balloon-shaped float, from which they could hang suspended in the water and float freely (see Fig. 17). So far as known, this remarkable contrivance was temporary, and probably adapted to some peculiarities of the habits and food of these animals occurring only in the geological period in which they existed.