Yet, strange to say, a mysterious connection continues to exist between the thyroid gland and the generative organs, even up to the highest vertebrate. That the thyroid gland, situated as it is in the neck, should have any sympathy with sexual functions if it was originally a gland concerned with digestion is, to say the least of it, extremely unlikely, but, on the contrary, likely enough if it originated from a glandular organ in connection with the sexual organs of the palæostracan ancestor of the vertebrate. Freund has shown, and shown conclusively, that there is an intimate connection between the condition of the thyroid gland and the state of the sexual organs, not only in human beings, but also in numerous animals, such as dogs, sheep, goats, pigs, and deer. He points out that the swelling of the gland, which occurs in consequence of sexual excitement (a fact mentioned both in folk-lore tales and in poetical literature), and also the swelling at the time of puberty, may both lead to a true goitrous enlargement; that most of the permanent goitres commence during a menstrual period; that during pregnancy swelling of the thyroid is almost universal, and may become so extreme as to threaten suffocation, or even cause death; that the period of puberty and the climacteric period are the two maximal periods for the onset of goitre, and that exophthalmic goitre especially is associated with a special disease connected with the uterus. {216}SUMMARY. Step by step in the preceding chapters the evidence is accumulating in favour of the origin of vertebrates from a member of the palæostracan group. In a continuously complete and harmonious manner the evidence has throughout been most convincing when the vertebrate chosen for the purpose of my arguments has been Ammocoetes. So many fixed points have been firmly established as to enable us to proceed further with very great confidence, in the full expectation of being able ultimately to homologize the Vertebrata with the Palæostraca even to minute details. Perhaps the most striking and unexpected result of such a comparison is the discovery that the thyroid gland is derived from the uterus of the palæostracan ancestor. Yet so clear is the evidence that it is difficult to see how the homology can be denied. In the one animal (Palæostraca) the foremost pair of mesosomatic appendages forms the operculum, which always bears the terminal generative organs and is fused in the middle line. In many forms, essentially in Eurypterus and the ancient sea-scorpions, the operculum was composed of two segments fused together: an anterior one which carried the uterus, and a posterior one which carried the first pair of branchiæ.