ably suited to the function. Every animal is a bundle of adaptations, which have been wrought out through ages and have often attained a high degree of perfection. Disharmonies are sometimes to be detected, especially when the organism is changing its habitat or habits, but they are few and far between. When we consider organs such as the eye, the heart, the kidneys, the placenta binding mother and young together in their intimate ante-natal symbiosis, we discover a multitude of subtle adaptations; and "the narrowest hinge in my hand puts to scorn all machinery "-for the perfection of mammalian joints is extraordinary. When the modern zoologist speaks of the adaptation of an organ he means not only that it is fit, effective, and well-adjusted, but that it is a product of a long process of evolution-the theoretical interpretation of which is still a very difficult problem. Some well-shaped structures, like the heart, are for the most part concerned with the internal economy of the body; others, like limbs, are mainly significant in relation to the environment. But there is no hard and-fast line here, for the ptarmigan's heart is specially adapted to high altitudes and the antelope's to the necessity of rapid escape on the plains. Some adaptations are mainly structural, as we see in the strong arch of a tortoise's carapace, and others mainly functional, as in the arrangements for regulating the temperature of the blood in birds and mammals; but the two aspects are inseparable. Some of the most exquisite adaptations are those that secure the harmonious co-operation of different parts of the body, as when the mother-mammal is prepared for her offspring before its development begins, during the ante-natal life, and after it is born.

1

From amid innumerable adaptations 1 we select one other instance, that of an African egg-eating snake, Dasypeltis scabra, a weak-bodied creature less than a yard in length, which is able to swallow birds' eggs three times the diameter of the thickest part of the body. The jaws are almost toothless, but a few posterior teeth are present which serve to grip the egg. There is the usual 1 See the author's Wonder of Life, p. 523 (Melrose; London, 1914).

Among the many adaptations the following may be noted.

(I) The skeleton is lightly built, many of the bones are hollow girders-light for their size. In many bones of
many birds the marrow disappears very early and air-sacs continuous with the lungs pass into the bones.
air-sac system effects economy in the respiratory function and by internal perspiration helps to regulate the body
temperature. Underneath the dense cortex of the bones there is often a spongy texture.

(II) At various parts of the skeleton fusion of bones occurs. This is very marked in the skull, and may make
it a better pecking instrument. It is characteristic of the dorsal or thoracic vertebræ (5), thus forming a firm
fulcrum against which the wings can work. It is very marked in the sacral region, where the true sacrals form
along with one or two thoracic, all the lumbars, and half of the caudals a composite syn-sacrum, to which the ilia
(6 to 7) are firmly fused. The significance of this is that as the bird walks as a biped, with much of its body in front
of a perpendicular dropped from the acetabulum (where the femur or thigh-bone, 17, works on the hip-girdle), there
is need for the hip-girdle having a long and strong grip of the backbone. The figure 8 is on the ischium, and
10 on the pubis or post-pubis. After a number of free caudal vertebræ comes the ploughshare bone or pygostyle
(9), a terminal fusion which affords a support for the tail-feathers or rectrices. There is a remarkable fusion in the
hand, where half of the wrist-bones (carpals) and all the (three) palm-bones (metacarpals) are fused in one bone,
the carpo-metacarpus (13 and 24), to which and to the two fingers (15 and 16) the long primary feathers of the
wing are attached. It is obviously adaptive that this region should be stiff. A small tuft, the ala spuria, is carried
by the thumb (14). The secondary feathers are borne by the ulna (11), which does not move much on its narrower
companion bone, the radius (12). Two carpal bones (radiale and ulnare) remain free (23). The humerus is marked
26. There is also a curious fusion of half of the ankle (tarsal) bones. The proximal ones fuse on to the lower
end of the tibia, forming the tibio-tarsus (18). The distal ones fuse on to three fused metatarsals, forming the
tarso-metatarsus (19). If there be four toes, the first (20) is turned backwards and has a separate metatarsal.
The second last joint or phalanx of the fourth toe is marked 25.

(III) As the bird has surrendered its arm to making a wing, the skull has to serve as a hand.
exaggeration of the premaxillæ to form a beak. Adaptive also is the large number (sometimes over a score) of
cervical vertebræ (28), which have great freedom of movement. In mammals, with four exceptions, which do not
include whale or giraffe, there are but seven cervical vertebræ. The bird's neck has to let the mouth reach the
ground, to reach the preen gland at the pygostyle (9), to catch flying insects, and so on. A power of swallowing
large hard objects is allowed by the articulation of the lower jaw with the mobile quadrate (27).

(IV) The powerful muscles of flight are chiefly attached to the keel or carina (3) of the breast-bone or sternum.
A backward extension of the sternum is marked 22. It will be seen that the breast-bone extends far back, forming
a floor to a great part of the abdominal cavity, which is important in a flying creature. The breast-bone is
elastically linked to the back-bone by the ribs, which are linked to one another by uncinate processes (21).
sabre-like scapula is marked 5. The merrythought (clavicles and interclavicles) is marked 2. The strong coracoids
(4) abutting against the sternum resist the inward crushing action of the down-stroke.

The

alternate gripping and muscular engulfing, and the intact egg slips into the gullet. It is then met by the sharp points of the inferior spines of a number of the vertebræ, which project into the gullet, and cut the egg-shells. It is said that they are actually tipped with enamel. The result of the structural adaptation is that none of the precious egg is wasted. Mr. Ditmars, the Curator of Reptiles at the Zoological Park in New York, who has a wide experience of living snakes, says that the empty egg-shells are always returned, and that this habit is quite unique.

The student is strongly advised to make a practical study of some structures, such as a sheep's heart, a bullock's eye, a dog's skull, a bird's skeleton, the appendages of a crayfish, the lantern of a sea-urchin, in order to become convinced of the intricacy and finish of adaptations. We have taken the bird's skeleton as an illustration (Fig. 59).

CHAPTER XII

BACKBONELESS ANIMALS

1. Protozoa-2. Sponges-3. Stinging-animals or Coelentera-4. "Worms”—5. Echinoderms-6. Arthropods-7. Molluscs— 8. Other Types

1. Protozoa.—It is likely that the first breath of life was in the water, for there most of the simplest animals and plants have their haunts. We call them simple, but there was much truth in Ehrenberg's view, who described some of them in 1838 as "perfect organisms." He was wrong in thinking he saw stomach, heart, and similar organs in them, but right in recognising that they often have a very intricate structure.

There is a widespread erroneous idea that these animalcules are to be found swarming in any drop of water. The clear water of daily use will generally disappoint, or rather please us by showing little trace of living things. But take a test-tube of water from a stagnant pool, hold it between your eyes and the light, and it is likely that you will see many forms of life. Simple plants and simple animals are there, the former represented by threads, ovals, and spheres in green, the latter by more mobile almost colourless specks or whitish motes which dance in the water. But besides these there are jerky swimmers whose appearance almost suggests their popular name of " water-fleas," and wriggling worms," thinner than thread and lither than eels: both of these may be very small, but closer examination shows that they have parts and organs, that they are many-celled not singlecelled animals.

66

Vary the observations by taking water in which hay