The Meaning of Adaptation
The world is inhabited by vast numbers of living creatures which occupy the most varying types of environment and which, on the whole, live sucoessfully under the particular conditions of life that characterize these environments. This means that the organism must possess features of structure and function that enable it to make the best of the conditions under which it lives. A character which renders an animal or plant well suited to its surroundings is called an adaptation. The meaning of the word adaptation can best be understood by considering how the structure of certain animals and plants makes for efficiency in their particular mode of life.
General Adaptations shown by Animals in Connection with Rapid Movement
All rapidly moving animals, whether they travel through air or water or on land, have a body shaped so as to reduce the resistance to the air or water as much as possible. The smooth, streamlined shape of a fast-swimming fish is an obvious adaptation to aid swift movement. The body of a bird is also streamlined, and the head, neck and body of a horse or antelope are joined by smooth, even curves, so that the body as a whole narrows towards the head and is roughly spindle-shaped. When a horse is running, its head and neck are stretched out and its ears are laid flat in order to present as little surface as possible to the resistance of the air. The eyes of swiftly moving animals do not project from the head, and the ear trumpets are either absent, as in fishes and birds, or, if present, are at the hind end of the head and can be laid flat against the neck. The tail is the main organ of a fish for causing movement, but in fast-moving mammals the tail is a rudder and may act as a fly switch, and is either short and furnished with long coarse hairs, as in the horse, or is fairly long and ends in a tuft of hairs, as in some antelopes.
Adaptations shown by Mammals in Connection with Rapid
Many mammals depend upon speed for catching their food or for escaping from their enemies, and show various adaptations 1 which enable them to travel faster. One of the first features to show modification in this direction is the posture of the foot. When we walk we place the whole of the foot on the ground, but when we run we lift the heel and move on our toes. The bear, however, does not rise on its toes in this way when running, but always shuffles along on the flat of its foot. Animals which move in this way are said to be plantigrade, and are usually slow movers. The effect of lifting the heel in running is to increase the speed of movement by lengthening the limb. The rabbit rests with the sole of the foot on the ground, but when moving raises the sole so that it progresses on the end joints of the fingers and toes. The rabbit and animals which have a similar foot structure are termed subplantigradc. Cats and dogs both stand and walk on the end joints of the fingers and toes and have the heel and wrist lifted permanently off the ground. They are said to be digitigrade.
In the horse the number of digits has become reduced so that only one, the third, is left in each leg. The horse walks on the very tip of this digit, and the hoof represents the nail of the digit. For this reason the horse is termed unguligrade. The joint of the horse ’s fore leg, which appears to correspond to the elbow, is really the wrist, while the elbow itself is embedded in the body of the animal. In a like manner the apparent knee of the hind leg is the ankle, and the knee is not visible. Traces of the second and third digits remain in the form of splint bones on either side of the metacarpal and metatarsal bones. The home of the wild horse is upon grassy plains, and its hoofed limbs are ideally suited for swift movement on a hard surface. It owes its speed largely to the great length of the leg, which results from the elongation of the digits, but other adaptations for rapid movement lie in the arrangement of the leg muscles and the nature of the joints. The muscles are arranged chiefly in the upper part of the leg, so that the end of the leg is light and capable of a rapid swing. Although the ball and socket joints at the shoulder and hip allow movement in all directions, the lower parts of the limbs can only be moved backward and forward in a pendulum-like manner. This movement is enforced by the joints of the elbow, wrist, knee and ankle, and by those between the bones of the digits ; at these joint surfaces lateral movement is prevented by the fact that one bone is ridged so as to fit into a groove on the other. Owing to this structure the horse ’s leg is stronger and more suited for running speedily than one which can be moved in all directions. The reduction in the number of digits and in the size of the tibia and fibula further strengthen the limb by reducing its power to twisting.
Sheep and cows are also unguligradc animals, but the reduction in the number of digits has not proceeded quite so far as in the horse, for these animals have two functional digits in each leg. The cloven hoof is formed by the nails of these two digits. The second and fifth digits are present as vestiges, the ends of which appear as a pair of small projections behind the hoof, and which do not touch the ground. In some ruminants these vestiges are not visible externally. In the wild state the cow frequented open park land, I.e. open grass land dotted with woods and thickets. After, the wild cow retired to a place of safety among the trees, where it could chew the cud undisturbed. For this reason, and also because it was able to protect itself with its horns, the running powers of the cow have not been developed to the extent seen in the horse. Moreover, its feet are adapted for movement over ground softer and more marshy than the hard, open plains inhabited by the wild horses. Thus the toes of the cloven hoof can be spread out so that a larger surface is presented to the ground, and consequently the foot docs not sink so deeply into soft ground. The feet of the reindeer show this adaptive feature to an even more marked extent than the cow, for not only do the ends of the two central digits spread out considerably when pressed on the ground, but the second and fifth digits are long enough to touch the ground. As a result the feet have a very broad surface in contact with the ground, which prevents them from sinking into the snow.
Several kinds of mammals are able to make gliding flights, but only bats are capable of active flight by means of flapping wings. The gliding mammals have a fold of skin along the sides of the body between the legs, and sometimes there is also a fold between the hind legs which includes the tail. Examples of such gliding mammals are. able to plane long distances from tree to tree. The bat resembles a bird in having the fore limb modified to form a wing, but differs in that the hind leg also is included in the wing structure. The wing is a fold of skin supported by the forearm and by the four fingers, which are spread out and much elongated. The short thumb has a hooked claw and projects forward from the edge of the wing. The wing membrane is joined to the ankle of the foot, and extends between the hind legs. The connection of the hind legs with the wing has resulted in the peculiarof the knees, which point backwards instead of forwards. Because of the adaptation of both fore and hind legs in connection with flight, the bat ’s movement on the ground is very awkward, and it can only crawl in an ungainly way. As in the case of the bird, a large surface is required for the attachment of the flight muscles of the bat; this is provided by a well-developed sternum, which bears a crest.
In hopping mammals, such as the kangaroo and jerboa, the hind legs are strong and well developed, and the feet are very long in order to help the animal to maintain its balance. The tail is also used for balancing, and in the kangaroo it is thick and strong, and is used as a prop when the animal rests without coming down on to all four legs. The jerboa has a long slender tail which is used for balancing only.