What was its prime function? It is an axiom of biology that an animal in its individual development recapitulates, albeit with many omissions and abbreviations, the ancestral history of its race. The thyroid gland appears in the embryo as a diverticulum of the anterior wall of the pharynx. It is remarkable in being a single, median, unpaired diverticulum, whereas almost all other organs are bilaterally symmetrical. The parathyroids are formed on the two sides in connection with certain of the branchial pouches. In its earliest development the thyroid gland resembles any other gland—a salivary gland, for example. Until a late stage it retains its connection with the back of the mouth. Occasionally indications of this primitive connection persist throughout life. In most cases the place where the duct of the thyroid gland used to open is clearly marked. At the back of the tongue—too far back to be seen without the aid of a dentist’s mirror—there is a =V=-shaped row of large papillæ (papillæ circumvallatæ). Just behind the meeting-point of the two limbs of the =V= a pit is to be seen—foramen cæcum. This pit is the vestige of the mouth of the duct of the thyroid gland which opened into the pharynx in the ancestors of fishes. It is an inconceivably long time since fishes diverged from other races of animals. We do not know which of the various orders of invertebrate animals now existent most nearly resembles our prepiscine ancestor. The organ which has developed into the thyroid body of mammals may possibly have disappeared from all the other descendants of the common stock from which vertebrates and their nearest relatives in the invertebrate sub-kingdom were evolved; but it is much more likely that it has been preserved, and is still performing its prime function in the higher invertebrate animals. Probably it is a functional organ in a cuttle-fish or a scorpion or a worm, but so unlike the thyroid gland of vertebrates that we fail to recognize its homology. There are other instances in the body of the persistence of an organ long after it has fallen into such ruin that not even archæologically-disposed biologists can guess what it was like, or what purpose it served in the days when it was at the height of its efficiency; but perhaps there is none other which so pregnantly illustrates the physiological doctrine of functional interdependence. Nature shows herself amazingly conservative in retaining primal organs—the pituitary body, the thymus gland, the thyroid gland, the suprarenal capsules—organs which millions of years ago forgot the very rudiments of their craft; but her conservatism is not mere force of habit. Although she no longer has any use for the wares which she created these pieces of apparatus to make, she cannot do without their refuse. Even the vermiform appendix may have its use. Dr. Gaskell’s theory of the thyroid gland involves a transformation so fantastic that it would provoke a smile