deep-sea animals than those dwelling on the land; but the investigations which I have cited, show that while the seafauna has undergone slight modifications since the dawn of the Cretaceous Epoch, the land-fauna has been subjected to the most marked deviations.

May not, then, these fluctuations of temperature be due to causes which operate from the exterior? It is necessary to assume that, throughout the lapse of all time, our planet has occupied its present relation to the sun, or the solar system? Is not the recession of Sirius, which is now going on, an argument against the fixity of the siderial heavens?

We are assured that ours is not a central sun, but one in the great possession of stars which is sweeping towards the constellation Hercules; and that in the region of either there are spaces of densely-clustered stars, and other spaces which are comparatively barren. Now every star is a sun, emitting light and heat, a portion of which is transmitted to us. Our planet at this time is moving through one of those starless spaces, and therefore is not in a position to receive the full influence of such a cause. The distinguished Swiss botanist, Heer, to whom we are so largely indebted for our knowledge of the Miocene flora, has suggested that it is to this source rather than to telluric causes we are to resort to explain the varying distribution of temperature as manifested in past geological times.

Again: Have we the right to assume that, throughout all past ages, the poles of our planet have pointed in the same direction? We can conceive that, if its axis were to form with the plane of the ecliptic, the same angle which it now forms with the equatorial plane, there would ensue an entire change of climate, and consequently of organic forms. Why should the astronomer insist on the immutability of the siderial system, when to the geologist is unfolded a record of seas displaced and continents elevated; of great cycles of heat and cold; of the disappearance of old, and the appearance of new forms of organic life? Change, not constancy, is inscribed on every leaf in the volume of Nature.

I am not a believer in the doctrine of multiplied shocks. I would not, in the explanation of natural phenomena, resort to blind catastrophes. But is there not behind all, and over all, and pervading all, a great governing principle to whose operation we can refer these changes? Does it not exist in the celestial mechanism itself? To the solution of this problem the attention of several physicists has been directed.

The speculations of the French savant, Adhemar, are not altogether to be overlooked, based as they are on the precession of the equinoxes and the movement of the apsides; a movement which, I believe, was unknown to the elder astronomers. If we compare the movement of the earth with the stars, it requires the lapse of 25,000 years to bring the equinox to correspond with the same point in space it now occupies; but the orbit itself being movable, this period is reduced to about 21,000 years. This is called the Great Year, being the measure of time before the winter solstice will again exactly coincide with the perihelion, and the summer solstice with the aphelion, and before the seasons will again harmonize with the same points of the terrestrial orbit.

The earth, at this time, approaches nearest the sun in the northern hemisphere during autumn and winter, and it is only when it recedes the farthest from the source of heat that the northern hemisphere receives the full effect of its vivifying warmth. As the earth between the vernal and autumnal equinox traverses a longer circuit than during the other half of the year, and also experiences an accelerated movement as it draws near the sun, the result is, that the northern summer is longer than the southern by about eight days; but after the lapse of ten thousand five hundred years these conditions will be reversed. It was in the year 1248, according to Adhemar, that the Great Northern Summer culminated, since which time it has continued to decrease, and that decrease will go on until the year 11,748, when it will have attained its maximum.

This compound movement, the precession of the equinoxes and the shifting of the line of apsides, it is claimed, exerts a marked influence in the distribution of the earth's temperature. While the Great Winter prevails at the north pole, the refrigeration is so excessive that the heats of summer are insufficient to melt the snow and ice precipitated during the winter, and hence, year after year and century after century, they go on accumulating, until the circumpolar region is in a state of glaciation, and the added weight becomes sufficient to displace the centre of gravity, which would be equivalent to a subsidence at one pole and an elevation at the other. M. Adhemar has even calculated the extent of this movement, and states that it would amount to about 5,500 feet. Now, let it be borne in mind that Professor Ramsey has shown that in Wales the submergence of the land during the Drift Epoch amounted to 2,300 feet, and our own observations show that in the northern portions of this country the glacial action proper may be traced to the height of 2,000 feet; although there were mountains which served as radiating centres, on whose flanks the Drift action may be traced much higher. These geographical points, roughly estimated, are about midway between the equator and the pole, and the extent of the subsidence would correspond very well with the calculations before referred to.

In the year 1248, the Great Winter terminated at the south pole, where for 10,500 years the accumulation of snow and ice had been going on, attended with the phenomena which we have described. "Here then," says M. Julien, an advocate of this theory, "is an irresistible force which, following the invariable law of the irregular precession of the equinoxes, must make the earth's centre of gravity periodically oscillate."

Mr. Croll, an English physicist, has elaborately discussed this question in a series of papers in the "Edinburgh New Philosophical Magazine," which have excited profound attention. With great labor he has prepared tables showing the

amount of the earth's eccentricity for the period of three millions of years, at intervals of 10,000 years for a greater portion of that time, and 50,000 years for the remainder. He infers that a glacial period occurs when the eccentricity of the earth's orbit is at a maximum, and the solstices fall when the earth is in perihelio and in aphelio; and that only one hemisphere has a glacial climate at the same time, which occurs when the winter is in aphelio.

In this connection I may mention the labors of our own countryman, Mr. Stockwell, who has prepared a paper, now on file in the Smithsonian Institution, embodying his own calculations as to the earth's eccentricity for the past two millions of years.

There is such an intimate connection between the several branches of science that the researches in one field often throw light upon the obscure points in another. In the solution of this difficult problem, the geologist may invoke, and I trust not unsuccessfully, the aid of the astronomer.

That a set of causes were active during the Drift Epoch, in one hemisphere, which remained dormant in the other, admits of little doubt; and the advocates of the astronomical theory, as evidences of the shifting of vast amounts of water from one pole to the other, point to the marked differences in the topographical features of the two hemispheres. In the Austral region we meet with projecting headlands and peninsula-like terminations of continents, and groups and chains of islands in the Pacific and Indian oceans extending over vast areas, which rise up like the peaks and crests of mountains. These are the evidences of a gradually engulfed hemisphere. In the Boreal region we have wide expanses of land diversified by mountains, prairies, and plains; elevated sea-beaches and river-terraces, most conspicuously displayed on the borders of the Arctic Sea; vast oceanic shoals; a marine fauna of a northern type preserved in beds of 1,400 feet, and stratified beds of gravel and sand 2,000 feet, above the ocean-level; clusters of lakes yet re

taining their bitter waters; shallow seas once salt, but each decade becoming more brackish; vast desert tracts which up to a recent time formed the ocean bed;-all these phenomena indicate a hemisphere gradually emerging from the waters. Perhaps the physicist can discern in these great periodic oscillations, the method by which Nature perpetually renews the youth of our planet, and maintains its fertility.

Gentlemen of the American Association: - The hour which, in your courtesy, had been assigned to me, has now lapsed, and I must bring these remarks to a close. The topics which have passed under review open up spheres of thought with regard to time and space too vast to be compressed within the limits of a mere oral discourse. Asserting no ability by reason of profound research to pass authoritatively on these results, may I not inquire: Have they not disclosed new paths in the great domain of Nature, which may be profitably explored jointly by the geologist and the astronomer; and is there not a probability that there will be found to exist an intimate relation between the periodic fluctuations of temperature on our planet, and the periodic pertubations to which it is subjected as a part of the solar system? Great as have been our achievements in science during the past, we profoundly believe that new triumphs await the patient observer.

VARIATIONS IN TRILLIUM AND WISTERIA.

BY THOMAS MEEHAN.

IN a recent number of the "Bulletin of the Torrey Botanical Club," of New York, Mr. J. H. Hall describes a plant of Trillium erectum, which he has had under his observation for several years, and which produced some years white, and other years the regular brown purple flowers. I have made