Page:Popular Science Monthly Volume 33.djvu/804

784 the pineal gland is not covered. It lies just beneath the parietal suture, that portion of the top of the skull where the bones are still ununited in new-born children. In many reptiles, notably Hatteria (Fig. 1) and Aurelia, the pineal gland is found to be an optic lobe, united to the nerve-stalk of a true eye, richly supplied with a branched blood-vessel and nerve. Although this eye still possesses every essential part of a visual organ, yet degenerative changes have set in, which show that it has been long useless. In Varanus giganteus (Fig. 2), where a scale on the top of the head is fitted by its transparency and whiteness to act as a cornea, a large mass of pigment has accumulated just beneath, effectually preventing the possibility of any rays of light reaching the retina. In Hatteria, the eye appears fitted in all respects for vision, but a thick band of connective tissue has formed above it, and there is no modified scale. In both animals, Hatteria and Varanus, the rods and cones of the retina are strangely elongated in certain parts, as though from straining to catch the last rays of vanishing light. The rods of this portion are at least three times the length of the ordinary ones, and are in connection with a special group of nucleated cells.

In modern amphibians the greatly degenerated eye is separated entirely from the pineal stalk, though a connection still exists during embryonic life. But there is reason to think that among ancient amphibians—more especially among the labyrinthodonts—the pineal eye reached its very highest development, since it is found outside the skull. A large parietal opening, with roughnesses of the skull-bones serving as attachments for powerful muscles, is found in the great extinct amphibians and reptiles. The pineal eye was pre-eminently a sense-organ of pre-tertiary periods; it has probably never been functional since these remote ages, and yet its rudiments persist in every human brain. Moreover, these eyes are of the invertebrate type, pointing back to that conjectural molluscoid ancestor which was "transparent and had a median eye."

The records of pathology teem with instances of rudimentary organs which have lost their use and have become sources of danger and disease. I venture to think these facts are far too little known to those outside of the medical profession who are interested in evolution. It is not necessary to do more than allude to the "appendix vermiformis," since every reader of the "Descent of Man" will remember it as the typical instance of a mischievous rudimentary organ, given by Darwin.

All mammals possess, during their embryonic life, three sets of kidneys. The first set of tubules cease very early in fetal life to act as kidneys; they take on a new function of supreme importance, their ducts becoming the oviducts in most fishes, amphibians,