of reptiles and amphibians; and its basal portion, the pineal gland, is found in practically all vertebrates. The interesting New Zealand reptile, Sphenodon, which is the only surviving representative of an otherwise extinct reptilian order, has an exceptionally well developed pineal eye with a retina, lens, and nerve, although it is so deeply buried beneath the skin that it is probably no longer functional. The larger aperture on the upper part of the skull of several extinct reptiles suggests that the pineal eye may have been more highly developed in these forms. Our external ear, while perhaps not altogether useless, is considerably reduced in both size and functional efficiency, and it is nearly as much reduced in the higher apes as it is in man. There are three muscles which extend from the cartilage of the ear to the bones of the skull, but in most individuals they are practically functionless. In addition to these, there MALE CHIMPANZEE. R&Holding are smaller muscles confined to the cartilage of the ear which appear to be quite functionless (Fig. 186). If you inspect the ears of your fellow creatures you will observe in a large percentage of cases a small point, now known as Darwin's tubercle, which is situated on the outer rim near the upper side. This represents the tip of the ear of lower mammals. In many of the apes it is also absent or very inconspicuous, but it is well developed in the more primitive monkeys. Man is rather exceptional among mammals on account of the isch FIG. 189-Skeleton of the whalebone whale. The upper figure represents the rudimentary pelvis (p, isch) and femur, f. (After Romanes.) scanty development of hair on the body. This characteristic is a very variable one both in individuals and in races. There are very short, fine hairs over a large part of the body, and it FIG. 190-Rudimentary pelvis and hind limbs of a python. (After Romanes.) is an interesting fact that the arrangement of these hairs is very similar in man and in the anthropoid apes. In the human embryo of about the fifth month the whole face and body are covered with a short, fine, woolly hair, the so-called lanugo. This coat normally disappears before birth, but occasionally anomalous individuals occur in which this embryonic hair continues to grow, forming in the adult a long, fleecy covering of the face and body. This character is hereditary and is correlated with deficient teeth. Besides the rudiments usually present in the human body, there are many other structures occasionally present which are homologous with structures normally found in the lower animals. The gluteus quartus, levator claviculæ, and several other muscles that only occasionally occur in man are regularly present in the apes. The supra-condyloid foramen, an aperture through the outer end of the humerus in several of the monkeys and other mammals, is found in only about one per cent of human subjects. Professor Wiedersheim in his book The Structure of Man: An Index of His Past History, has described more than one hundred atavistic structures and peculiarities which occur constantly or occasionally in various parts of our body. Haeckel has designated these structures as "useless primitive heirlooms," but some of them, the vermiform appendix, for example, are occasionally worse than useless. si ν li Some very remarkable structural peculiarities of especial significance in relation to the theory of descent are furnished by a family of crabs, the Lithodidæ, which are unusually abundant on the Pacific Coast of North America. In these crabs the abdomen is more or less asymmetrical, especially in the females, which have appendages for holding the eggs only on the left side. Correlated with the asymmetry of the abdomen there FIG. 191-Cæcum, c, and vermiform appendix, v, of man; li, large intestine; si, small intestine. FIG. 192-Crabs of the family Lithodidæ: 1, Dermaturus mandtii; 2, Acantholithodes hispidus. Note that the right pincer in both is the larger. (After Schmitt.) is an inequality in the size of the pincers, the right one being uniformly larger than the left. The species of this curious family are exceedingly diverse in form, size, and habits, and there is nothing in their mode of life which suggests any possible utility of their asymmetrical structure. In fact, their abdominal FIG. 193-Upper side of the carapace of a hermit crab Pagurus (left), and Hapalogaster cavicauda, one of the Lithodida (right). The small letters indicate homologous lines. asymmetry, and especially the absence of appendages on one side, would seem to be a distinct handicap. The key to the puzzle is revealed when we study the structure of the nearest relatives of the Lithodidæ, the hermit crabs. Most species of hermit crabs live in the empty coiled shells of B gastropod molluscs. The A FIG. 194-Abdomen of the female of Dermatu right pincer in the hermits is commonly larger than the left. The abdomen is soft, since it is adequately protected by the shell into which it is inserted. It also shows an asymmetrical twist in adaptation to the coil of the rus mandtii: A, from above; B, from below, showing appendages only on the left side. shell, and in most hermits it has appendages only on the left side. Since the Lithodidæ are more closely related to the hermit crabs |