With these three organs—the thyroid gland, the suprarenal capsules, and the pituitary body—we must leave the subject of internal secretions. Each of these organs is a ductless gland. Each has a history which the zoologist is unable to transcribe. The document is a palimpsest, the earlier script so faint as to be illegible beneath the dark letters which a new era has written over it. Even the modern script is smudged and blotted. The laws which it sets forth seem, as a rule, to be destitute of sense, but a sinister meaning is evident at times. We are tempted to regard these codes as obsolete, until the mischief which follows their suppression calls our startled attention to the fact that they are, in the most lively sense, extant. Myxœdema, Addison’s disease, acromegaly, are ominous warnings that the three ductless glands are no mere monuments of a past epoch, which owe their survival to Nature’s indolence. They teach us that we must not attribute the persistence of such organs to a conservatism which resists innovation, or suppose that they would long ago have been wiped off the statute-book if her inertia could have been overcome. Undoubtedly Nature gives us many excuses for adopting this attitude of mind. The “chestnuts” on a horse’s legs, the “dew-claws” of a dog’s foot, are vestiges which would have disappeared if every part of the body had to establish its claim to be regarded as useful before it became entitled to share in the common supply of food; so, at least, we are disposed to think. But, tempting though it be to attribute to sheer conservatism the retention of an organ which has been superseded in its original functions, and for which we cannot recognize any new use, it is a temptation which must be severely checked. It is safer to suppose that the fact that it has been retained is _prima-facie_ evidence that the body has need of it.