The nictitating membrane, which in birds and many reptiles consists of a half-transparent curtain acting as a lid to sweep the eye, is in the human eye dwindled to a small membranous remnant, draped at the inner corner. The growth of hair over the human body surface may be regarded, in view of the sartorial habits of man, as a vestigial inheritance from hairy ancestors. One of the most notorious of the vestigial organs of man is the vermiform appendix, a small slender sac opening from the large intestine near where the large intestine is joined by the small intestine. In some animals this organ is large and performs an important part in the process of digestion. But in man it is a mere rudiment, not only of no possible aid in digestion, but the source of frequent disease, and even of death. There are in all, according to Darwin, about eighty vestigial organs in the human body. But these organs occur everywhere throughout the animal kingdom. There is not an order of animals, nor of plants either, without them. They are necessary facts growing out of evolution. Organic structures are the result of adjustment to surrounding conditions. The continual changes in environment to which all organisms are exposed necessitate corresponding changes in structure. And the vestiges found in the bodies of all animals represent parts which in the previous existence were useful and necessary to a complete adjustment of the organism, but which, owing to a change of emphasis in surroundings, have become useless, and consequently shrunken. They are the obsolete or obsolescent parts of animal structure—parts which have been outgrown and superseded—the ‘silent letters’ of morphology. They sustain the same relation to the individual organism as dead or dwindling species sustain to a fauna. They furnish indisputable proof of the kinship and unity of the animal world.