homology, and that it is the representation of the current of heredity. The resemblances arise from common origin, the

FIG. 106.-Ears of various anthropoid apes and of man, showing human vestigial characters: 1, hairy human ear; 2, Barbary ape; 3, chimpanzee; 4 and 5, human ears; 6, ear of human foetus; 7, orang-outang.

variations from the demand of differing external conditions. It may be said that the inside of an animal tells what it is, the outside where it has been. In the internal structure, ancestral

FIG. 107.-Head of a fivemonths human embryo showing embryonic haircovering. (After Ecker.)

in the body of any of of the higher plants.

traits are perpetuated with little change. through geologic ages. The external characters affected by every feature of the surroundings may be rapidly altered through response to demands of environment and through the destruction of individuals whose life fails of adjustment.

It is in the persistence of heredity that we find the explanation of vestigial organs. An organ well developed in one group of animals or plants may in some other be reduced to an imperfect organ or rudiment so incomplete as to serve no purpose whatever. Such rudimentary or functionless structures may be found the higher animals and in most or all As a rule such structures are more fully

developed in the embryo than in
the adult, becoming atrophied
with age.
Familiar examples
are the appendix vermiformis
and the unused muscles of the
ears in man, the atrophied lung,
pelvis, and limbs of the snake,
the air bladder of the fish, the
"thumb" (or rather index fin-
ger), of the bird, the splint bone
of the horse, and the like.

The anatomist Wiedersheim has recorded 180 vestigial organs in man. These structures occur in all the systems of organs, integument, skeleton, muscles, nervous system, sense organs, digestive, respiratory, circulatory, and urino-genital systems. Most of these remnants of structures are to be found completely developed in other vertebrate groups. Eleven of them are characteristic as functional organs of fishes only, four of amphibians and reptiles. The fact that structures are vestigial is shown often by cases of atavistic de

velopment.

Within the brain of man, near the optic lobes, is a little spheroid structure. scarcely larger than a pea, known as the "pineal gland" or conarium. It has no evident function, and Descartes once suggested that it might be the seat of the soul. It is larger in the embryo and still larger in the brains of some of the lower vertebrates. Recent investigations have shown that it is especially developed in

certain lizards, notably in a very primitive New Zealand lizard of the genus Sphenodon (Hatteria) (Fig. 109), and that, in these lizards, the pineal body ends in a more or less perfect eye-like structure placed between the true eyes in the center of the forehead. A trace of this eye is shown in the limbless lizard called slow worm (Anguis), of Europe, and in several American species. In the horned toad (Phrynosoma) (Fig. 110) its place

is covered by a translucent pearly scale. These lizards have in fact three eyes, and the pineal body is the nervous ganglion from which the third eye arises. The natural conclusion from this that all vertebrates originally had three eyes, is probably a too-hasty one. Perhaps the pineal body was an organ of sense, which developed into an eye in the lizards and their ancestors only, not in any of the Amphibians or fishes, and not in any mammals or birds, although these are descended from reptilian stock. Whatever the origin or primitive function of the pineal ganglion, its existence in man as a vestigial organ is due to the persistence of heredity.

In the living species of horse, Equus, there is but a single toe, with its basal bones. On each side of the base bone of this toe is a small bone known as a splint bone. The splint bones are apparently useless to the horse, but in extinct species of horse these bones are developed as digits, bearing small hoofs. Occasionally even now colts are born in which these splint bones bear rudimentary hoofs. In the museum of Stanford University is the leg of a high-bred colt from Milpitas, California, bearing a small hoof on each of the two splint bones.

The remains (Fig. 111) of over thirty different ancient horse-like animals have been found in the rocks of the

Tertiary era. The Eohippus,

а

***

FIG. 111.-Foot changes in evolution of

the horse: a, Equus, Quaternary (recent); b, Pliohippus, Pliocene; c, Protohippus, Lower Pliocene; d, Miohippus, Miocene; e. Mesohippus, Lower Miocene; f, Orohippus, Eocene. (After FIG. 254 of Animal Studies.")

the earliest of these horselike animals, found in the oldest Tertiary rocks, was little larger than a fox, and its forefeet had four hoofed toes, with the rudiment of a fifth, while the hind feet had three hoofed toes. In the later rocks is found the Orohippus, also small, but with the rudimentary fifth toe of the forefoot gone. Still later appeared the Mesohippus and Miohippus, horses about the size of sheep, with three hoofed toes only, on both forcfeet and hind feet, but with the rudiment of the fourth toe in the forefeet, of the same size in Mesohippus, smaller in Miohippus. Also, the middle toe and hoof of the three toes in each foot was distinctly larger than the others in both Mesohippus and Miohippus. Next came the Protohippus, a horse about the size of a donkey, with three toes, but with the two side toes on each foot reduced in size, and probably no longer of use in walking. The middle toe and hoof carried all the weight. Still later in the Tertiary era lived the Pliohippus, an "almost complete horse." The side toes of Pliohippus are reduced to mere rudi

ments or splints. This animal differs from the present horse somewhat in skull, shape of hoof, length of teeth, and other minor details. Lastly came the present horse, Equus, with the splint bones or concealed rudiments of the side toes very small and the hoof of the middle toe rounder. In spite of the great difference between the one-toed foot of the living horse and the dog's five-toed foot there was once a kind of horse which had a five-toed foot, and there is after all a close relationship between the foot of the horse and the foot of the dog.

FIG. 112.-Homology of digits of four odd-toed mammals, showing gradual reduction in number and consolidation of bones above. (After Romanes.)

In man there is developed at the proximal end of the cæcum or blind sac of the large intestine a small structure as shown in Fig. 113. This appendage has no function, and it is subject to inflammation or suppuration, known as appendicitis. In the embryo the appendix vermiformis is notably larger than in the adult man; and in the lower animals, as in the dog or the kangaroo (see Fig. 113), it may be recognizable as a prolongation of the cæcum, scarcely less in diameter than the intestine itself. The appendix vermiformis is therefore a vestige of a long cæcum which had its part in the process of digestion.

In the embryo of all chordate animals, without exception, respiratory or gill slits are developed, homologous with those seen in the embryo of the fish. The presence of these slits or their vestiges is one of the most important secondary distinctive characters of the great group of Chordata, which includes the vertebrates. The human embryo is, in this regard, at certain