Relative to the body weight it is largest in the mammalia, and smallest in the fishes. It therefore grows larger as the vertebrate ascends in the scale. It has, in fact, developed in direct proportion to and side by side with the fundamental, differentiating vertebrate characteristics. Of these, the possession of a dry hairy skin instead of a moist or mucus bearing, chitinous skin, the ownership of an internal bony skeleton and a large skull, and a complicated development of brain, are the diagnostic signs. Thyroid internal secretion has a very definite controlling relation to all of them: to skin, its hairiness, moisture and amount of mucus, to the growth and size of the bones, especially the bones of the extremities and the skull, and to intelligence and the complexity of the convolutions of the brain. Injury to the thyroid, especially in growing animals, is followed by profound retrogression or arrest of development in skin, skeleton and brain. In the fishes and the cyclostomes the thyroid is represented only by some small scrubby patches, little larger than the heads of pins, scattered along the aorta, the great blood vessels from the heart, and out a little way along each gill. It becomes larger and more compact among the amphibians and reptiles, but still remains quite small. Large and prominent among the birds and mammalia, it is largest and most prominent among the primates and man. It is hence permissible to think of the thyroid as a dictator of evolution, to crown it as the vertebrate gland par excellence, and to call the typical vertebrate brand marks secondary _thyroid_ characteristics in precisely the sense of Darwin classing the horns of cattle as secondary _sexual_ characteristics. In such enthusiasm for the thyroid as a determinant of evolution, its pillar of cloud by day and column of fire by night, one should not forget the other glands of internal secretion. In them all, we may suppose, Life, tired of inventing merely prehensile, destructive and reproductive organs, hit upon the happy thought of contrivances which are in essence chemical factories to speed up the rate of variation and so of a higher evolution. CREATOR OF THE LAND ANIMAL