of all sorts of animals and plants have come into existence, flourished for a time, propagated their kind, and then vanished for ever. Glimpses of these evanescent life-panoramas show streams of varied beings arranged into groups or species, and each group possessing characters admirably adapted for making the best use of certain conditions in their respective environments. Some were provided with special organs for attack and defence. Birds and insects had their bodies transformed into flying machines, not, however, always on the same plan. For example, the Pterodactyle had its fore limbs modified so as to be used partly as prehensile organs and partly as wings. It has three of the fingers on each limb free and furnished with claws, while the fourth was enormously elongated, in order to support the outer edge of a sail-like membrane, which, on the inner side, was attached to the arm, body, hind limb, and tail. On the other hand, the Archæopterix though differing very much from the birds of the present day, had large feathers on its wings and tail, and its feet were similar to those of modern perching birds. Again, some air-breathers, such as whales and seals, reverted to an aquatic life; and their limbs, which were originally adapted for locomotion on land, were converted into paddle-like flappers for propulsion in water. But, with respect to the air-breathing organ a remarkable fact is to be noted. Although these animals were actually descendants of aquatic ancestors possessed of gills, it was entirely beyond the power of nature to retrace her own evolutionary steps and restore them their lost gills. And hence, these animals have had ever since to come to the surface at stated intervals to breathe. Animals when they become stereotyped on defined lines have not the power of reconsidering their position by way of retracing their steps. They are impelled, as it were, by a vis a tergo, to pursue their course on the selected path, subject only to modifications possible within the lines adopted. For example, no conceivable morphological change in the structure of the foot of a horse could improve it as a means of locomotion, on the plan by which the animal has achieved its special position in the organic world, i.e., by gradually lengthening the bones of one of the original five toes, and dispensing with the others. If hard pressed and beaten in swiftness, extinction would be its fate,
