Even the _vermiform appendix_ of the cæcum, which since Darwin’s time has served as a classic example of a rudimentary organ in man, is, in reality, not a functionless organ. Darwin, however, was of opinion that it was not only useless, but positively harmful. “With respect to the alimentary canal,” he says, “I have met with an account of only a single rudiment, namely, the vermiform appendage of the cæcum. ... Not only is it useless, but it is sometimes the cause of death, of which fact I have lately heard two instances. This is due to small hard bodies, such as seeds, entering the passage and causing inflammation.” (“Descent of Man,” 2nd ed., ch. I, pp. 39, 40.) The idea that seeds cause appendicitis is, of course, an exploded superstition, the hard bodies sometimes found in the appendix being fecal concretions and not seeds—“The old idea,” says Dr. John B. Deaver, “that foreign bodies, such as grape seeds, are the cause of the disease, has been disproved.” (Encycl. Americana, vol. 2, p. 76.) What is more germane to the point at issue, however, is that Darwin erred in denying the utility of the vermiform appendix. For, although this organ does not discharge in man the important function which its homologue discharges in grain-eating birds and also in herbivorous mammals, it subserves the secondary function of lubricating the intestines by means of a secretion from its muciparous glands. Darwin gives the _semilunar fold_ as another instance of a vestigial organ, claiming that it is a persistent rudiment of a former third eyelid or _membrana nictitans_, such as we find in birds. “The nictitating membrane, or third eyelid,” he says, “with its accessory muscles and other structures, is especially well developed in birds, and is of much functional importance to them, as it can be rapidly drawn across the whole eyeball. It is found in some reptiles and amphibians, and in certain fishes as in sharks. It is fairly well developed in the two lower divisions of the mammalian series, namely, in the monotremata and marsupials, and in some higher mammals, as in the walrus. But in man, the quadrumana, and most other mammals, it exists, as is admitted by all anatomists, as a mere rudiment, called the semilunar fold.” (_Op. cit._, ch. I, pp. 35, 36.) Here Darwin is certainly wrong about his facts; for the so-called third eyelid is not well developed in the two lower divisions of the mammalian series (_i.e._ the monotremes and the marsupials) nor in any other mammalian type. “With but few exceptions,” says Remy Perrier, “the third eyelid is not so complete as among the birds; (in the mammals) it never covers the entire eye. For the rest, it is not really perceptible except in certain types, like the dog, the ruminants, and, still more so, the horse. In the rest (of the mammals) it is less developed.” (“Elements d’anatomie comparée,” Paris, 1893, p. 1137.) Moreover, Darwin’s suggestion leaves us at sea as to the ancestor, from whom