we have seen the path of progressive evolution being retraced, at least in the case of these reptiles. The present state of knowledge does not permit an extension of the enquiry to the conditions found in higher animals. However, according to Flesch, traces of a sensory epithelium have been found in the pineal body of man, the horse, the sheep, and the bat: that is to say, that in these creatures too the oldest parts of the structure have resisted degeneration longest. We cannot refrain from the conclusion that in this series degeneration retraces to a large extent the steps of original advance. 2. Degeneration of the organs of sight in deepsea Crustacea.- -We cannot however establish the conclusion of the last paragraph · as a general principle. The fauna of the deep sea includes a large number of Crustacea, and in these the eyes, which are relatively useless, are often degenerate. The course of the degeneration is generally definite, and of all the structural parts the most long-lived are the eye-stalks, although we know that these are a recent formation. A number of examples chosen from Decapod Crustaceans, which are specially abundant, will illustrate this point.1 Nephropsis, which lives in the Atlantic and Indian Oceans at moderate depths, is a relative of the Lobster. The optic stalk is short and carries 1 See Pelseneer, l'Exploration zoologique des mers profondes (Conférences Universitaire de Bruxelles). 2 Année, 1890, a rudimentary eye which has neither pigment nor cornea and is coloured like the general surface of the body. Eryonicus (fig. 63) belongs to the same group, and comes from the region of Saint-Thomas in the Antilles, where it lives at a depth of about 825 yards. This animal has a reduced optic stalk, but at the extremity of this, where in littoral forms the eye is borne, there is only a depression as if the eye had been carefully scooped out. Willemæsia (fig. 64), a relative of the marine cray FIG. 63. Eryonicus cæcus. Bate? (After W. Faxon, fish and an in habitant of the The Stalk-eyed Crustacea, Mem. of Mus. of Comp., Atlantic at a depth of about 3500 yards, is completely devoid of eyes in the adult condition, although it possesses them in the larval stage. Scolophthalmus (fig. 65), which lives down to 4000 yards, is quite devoid of eyes, but possesses eye-stalks which terminate in spines. It seems, then, that different species of deep-sea Crustacea may present different degrees of degeneration of the eye. One species in itself exhibits all grades of degradation according to the depth at which it lives. This creature--Cymonomuswhich, when near the surface, has fully formed eyes upon movable stalks, at a depth of a few hundred yards exhibits movable stalks without eyes; and at 1500 yards the stalks are fixed and end in spines. Isopod Crustacea, which live in the deep sea, present similarly degenerate eyes. Many are blind FIG. 65.-Scolophthalmus lucifugus, FAX. a, optic peduncle transformed to a pine. (After W. Faxon, The Stalk-eyed Crustacea, Mem. of Mus. of Comp. Zool. Harvard College, vol. xviii., 1895.) and display all kinds of optic degeneration. Noesa, for instance, simply has eyes devoid of pigment. Thus, in abysmal Crustacea, the degeneration of the eyes is in no sense a retracing of developmental stages. Another instance chosen from examples of the atrophy of organs in individuals, shows that the supposed law of retracing cannot be made universal. 4. Atrophy of the branchial vessels in man.— Examination of a human embryo of about three weeks old shows the presence of a series of slits on the sides of the neck, the slits not being parallel, but converging towards the ventral surface. Between these slits are swellings, or pads, which pass up towards the dorsal surface and appear like the beginnings of hoops or ribs enclosing the visceral cavity; the elevations are the branchial arches, the slits are the gill-slits. с C C In the human embryo (fig. 66, A), as in fish, these slits appear from above downwards, and as they they are p formed, the corresponding bloodvessels arise. These vessels, or aortic arches, arise from a ventral aorta (a.) which gives off six lateral ap. -AD A FIG. 66. Diagram of branchial arches in mammals. branches (c.) at each side. These lateral branches run up between the gill-slits and form two main trunks on the dorsal side which converge to form the descending aorta (ad.). In man the branchial arches are transformed, parts of them entering into the structure of the face, and during the transformation parts of the aortic arches atrophy (fig. 66, B). But the order |