same tawny colour, in these other tawny people, which seems to be of the same kind, but different in degree, must proceed from a like cause, that is, the thickness or density of their skins; and accordingly it will be found, that all such people have skins of a thickness or density proportional to the whiteness or darkness of their colours.

So that, from the whole, we may conclude, that the proximate cause of the colour of negroes is threefold, viz. the opacity of their skins, proceeding from the thickness and density of their texture, which obstructs the transmission of the rays of light, from the white and red parts below them ; together with their greater refractive power, which absorbs those rays, and the smallness of the particles of their skins, which hinder them from reflecting any light.

Hence we may justly infer, 1. That there is not so great, unnatural, and unaccountable a difference between negroes and white people, on account of their colours, as to make it impossible for both ever to have been descended from the same stock, as some people, unskilled in the doctrine of light and colours, are very apt, too positively, to affirm and believe. 2. That the epidermis, besides its other uses, tends to preserve the uniformity of the colours of people throughout the world. The Influence of the Sun, in hot Countries, and the Ways of Life of the Inhabitants in them, are the remote Causes of the Colour of Negroes, Indians, &c. And the Ways of Living, in use among most Nations of white People, make their Colours whiter than they were originally, or would be naturally. As for what relates to the remote causes of the colours of negroes, it has been generally supposed, though not universally believed, that the power of the sun in hot countries is the principal, if not the only agent, in producing this effect. The skin, then, is deprived of its white colour, by the force and influence of the sun, these four ways: 1. By being rendered opaque, from a dissipation of its more aqueous and pellucid juices: the known effect of the sun's heat, and which will render all bodies opaque.

2. By a concretion of its vessels and glandules, from this dissipation of their aqueous contents, which renders the skin both thicker and denser, or more callous or rigid.

3. By a new accretion of many new membranes, which render it thick and opaque. For the sun-beams act as a vibrating force, or external friction, on the skin, which derives fresh supplies of juices to it; by which new membranes, or lamellæ, are formed, in the same manner as the epidermis is renewed when abraded, which is very soon and easily done.

4. By increasing those parts or principles, in the composition of the epidermis, which have the greatest refractive powers. As the terrestrial, and fixed saline; but especially the tenacious sulphureous, which refract and absorb light more strongly than any other substances; while the more transparent and pellucid principles, as the aqueous, spiritous, and volatile saline, are evaporated by the heat, which causes the other more fixed principles to be accumulated in greater quantities, and combined in larger collections; and these particles, being likewise more comminuted by the sun, will on that account be black, as happens to oil when well boiled. Hence it will appear, that the power of the sun's heat in hot countries, and its more immediate application to the body, or the increase of its force, by the nature of the soil, or ways of life, is the remote cause of the blackness, and the different degrees of blackness, of the inhabitants of the torrid zone: whereas the luxurious customs, and the effeminate lives of the several nations of white people, in the northern climes, are the remote causes of their respective fair complexions.

Observations on several Species of Fresh-water Polypi. By M. ABRAHAM TREMBLEY, F.R. S.-[1744.]

WE find, in divers places, on water-plants, and other bodies in the water, a whitish substance, that looks at first only like a sort of mould: we sometimes see plants, sticks of wood, snail-shells, and the like, that are entirely covered over with this substance. But if we take any of these, put them into a glass of clear water, and then examine with a magnifying glass what is upon them, we soon discover in the little bodies, which by their assemblage form this whitish substance, such motions as give sufficient reason to consider them as living animals; and this will appear yet more sensible, when they come to be observed with a microscope.

In one case where the polypi are simple, they are not above the 240th part of an inch in length, and are of a shape nearly resembling that of a bell: one of these is represented exceedingly magnified. The anterior part, a c, generally appears open, when it properly presents itself; the posterior part, ib, is fixed to a stem or pedicle, be; and it is by the extremity, e, of this pedicle, that the polypus fastens itself to any other sort of body. The polypus of this sort generally appears to the microscope of a brownish colour, excepting at its smaller end b, where it is

When the anterior

transparent, as well as its pedicle be. part, a c, is open, we may perceive about its edges a very lively motion; and when the polypus presents itself in a certain manner, it discovers, on either side of these edges of its anterior part, somewhat very much resembling the wheels of a little mill, that move with great velocity. These polypi are able to contract themselves; and they do so often, and suddenly.

We should begin to observe a polypus soon after it has fixed itself singly, in order to see regularly in what manner the clusters form themselves, and in what way these small creatures multiply. The stem or pedicle of a polypus that is yet single, and which has but lately fixed itself, is at first very short, but lengthens itself in a little time. After that, the polypus multiplies; that is to say, it divides or splits itself into two, lengthwise. The lips are first drawn into the body, whose anterior part closes, and becomes round: the motion that was to be seen before the lips were drawn in no longer appears; yet we may see, by looking with attention, a slow motion within the body, during all the time that the polypus remains closed. The anterior part of the polypus becomes flattened afterwards by degrees, and spreads in proportion, becoming broader as it shortens; it then gradually splits down through the middle, that is, from the middle of the head to the place where the posterior end joins to the pedicle; so that, in a little while, there appear two separate round bodies joined to the extremity of the pedicle that just before supported but one.

The anterior part of each of these bodies then opens by degrees; and, as they open, the lips of the new polypi show themselves more and more. Then is the time of observing these lips with attention, and of forming an idea of their true form, and of their motion. This motion is at the first very slow, it quickens as the polypi continue to open; and, as soon as they have done, it becomes as swift as that which appeared in the lips of the whole single polypus, before it began to divide; and then these new polypi may be considered as entirely formed. They are at first less than the polypus from which they were formed; but they grow to the same size in a very little time. A polypus is about an hour in dividing itself.

The engraving represents a cluster of eight polypi; and by this figure it may be apprehended in what manner the pedicles of the polypi become disposed, as their numbers increase. These several pedicles become so many branches

of the cluster or sprig. This figure particularly represents a cluster, whose progress M. T. followed in the month of September, 1744. It consisted, on the 9th day of that month, of only one single polypus, which was placed as at b; this polypus divided itself that evening, and at half an hour after eight o'clock, there were to be discovered at b two perfect polypi, whose pedicles. or branches, bd, bd, continued lengthening till the morning of the next day, being the 10th of the same month of September at about a quarter after nine that morning, these two polypi, which were then at d, d, began also

each to divide; so that at a quarter past 11, there were at d and d four complete polypi, whose several pedicles, di, di, di, di, formed themselves soon after. On the 11th of the same September, about half an hour after seven in the morning, these last four polypi had already again divided themselves; that is, that there were at i, i, i, i, eight distinct polypi; and this cluster, so consisting of eight polypi, is here represented as it appeared on the 12th of the same month, between 10 and 11 in the forenoon.

Mr. T. has seen four other species of polypi, that increase in the same manner as those above mentioned; that is, which split and divide themselves according to their length. Those which come the nearest to the first are somewhat more slender, and the branches of their clusters are transparent; yet they appear, when there is a number of them together, of a changeable violet-colour: the clusters of these bear a good resemblance to a sprig or aigrette of spun glass.

Mr. T. also observed, regularly, other small polypi, of a different sort from those that are found in clusters. These are nearly in shape like a tunnel, pretty long in proportion to the opening of their larger ends. For this reason, Mr. De Reaumur has thought proper to distinguish them by the name of tunnel-like polypi. He is acquainted with three species of these last polypi, which are, respectively, green, blue, and white. Their anterior end, particularly, is of a far more com pounded shape than may at first be imagined. There may be observed round the edges of this part a sensible motion, much resembling that of an indented wheel, or rather of an endless screw, turned very fast about. These tunnel-like polypi form no clusters, like the others. The little bodies,

that pass floating near the anterior parts of these insects, are in some manner drawn into the mouths of their tunnels; and sometimes a considerable number of very small round animalcula fall one after another into these openings. Some of these were indeed afterwards let out again, at another opening; but it could plainly be seen, that many of these little round bodies remained within the bodies of the polypi; and it is therefore apparent that these little bodies, so taken in, became their food.

These tunnel-like polypi also multiply by dividing into two, but they divide otherwise than the clustering polypi; they neither divide longitudinally nor transversely, but sloping and diagonal-wise. Of two tunnel-like polypi, just produced by the division of one, the first has the old head and a new posterior end; and the other the old posterior end, with a new head.

On Fossil Shells. By the Rev. ROGER PICKERING.

AT Woodbridge, in Suffolk, in a farmer's ground, there are some pits, in depth equal to the usual height of houses, consisting of several strata of shells, from the bottom to within about nine feet of the surface, where the natural soil of gravel and sand begins. The mass of shells here collected is prodigious; the sorts various: but the buccinum vulgare, or whilk, prevails the most. The depth to which these shells reach is not yet dug down to. Woodbridge is seated seven miles N. E. from Ipswich; and is about the same distance from Orford on the sea-coast, which bears from it due east. How such a mass of shells should get there at such a distance from the sea, when history has not informed us of any remarkable inundation in those parts, or that such a tract of land was ever recovered from the sea, appears difficult to determine. Indeed the river Deben, which rises at Debenham, some miles off, runs by Woodbridge, within half a mile of these pits, in its course to the German Ocean, where it empties itself: but such a collection of shells can hardly be supposed to have been thrown up by it, and a surface of earth to the depth of nine feet, settled over it, without allowing a space of time for such a circumstance almost equal to the interval between us and the deluge.

Concerning an extraordinary large Fossil Tooth of an Elephant. By Mr. HENRY BAKER, F. R. S.-[1745.] THE fossil tooth Mr. B. received from Norwich. It seems to be a grinder belonging to the left under jaw of a very