touched by an insect convey the excitement of the touch to the anther chambers, which instantly discharge the adhesive pollen-clubs at the head of the intruder. Thus, and thus alone, at least three species of the genus Catasetum are fertilized.

Mr. Darwin has manifestly spared no labour in collecting facts illustrative of his subject. He gives a list of twenty-three species of Lepidoptera captured with the pollen-clubs of O. pyramidalis attached to their probosces. He has himself made observations on nearly all the British species of orchids, and upon a large number of exotic species, and he arrives at the conclusion that, "throughout the vast Orchidean order--including, according to Lindley, 433 genera, and probably about 6,000 species-the act of fertilization is almost invariably left to insects."

The chapter on the homologies of Orchids is one of the most interesting portions of the book.

At Torquay, Mr. Darwin watched a number of plants of Spiranthes autumnalis, and saw them visited by humble-bees. "The bees always alighted at the bottom of the spike, and, crawling spirally up it, sucked one flower after another." "I believe humble-bees generally act thus when visiting a dense spike of flowers, as it is most convenient for them; in the same manner as a woodpecker always climbs up a tree in search of insects." This seems a most insignificant observation; but see the result. In the early morning, when the bee starts on her rounds, let us suppose that she alighted on the summit of a spike, she would surely extract pollinia (pollen-clubs) from the uppermost and last opened flowers, but when visiting the next succeeding flower, of which the labellum in all probability would not as yet have moved from the column, for this is slowly and very gradually effected, the pollen masses would often be brushed off her proboscis and be wasted. But nature suffers no such waste. The bee goes first to the lowest flower, and crawling spirally up the spike, effects nothing on the first which she visits till she reaches the upper flowers, then she withdraws the pollinia; she soon flies to another plant, and alighting on the lowest and oldest flower, into which there will be a wide passage from the greater reflection of the labellum, the pollinia will strike the protuberant stigma. If the stigma of the lowest flower has already been fully fertilized, little or no pollen will be left on its dried surface; but on the next succeeding flower, of which the stigma is viscid, large sheets of pollen will be left. Then, as soon as the bee arrives near the summit of the spike, she will again withdraw fresh pollinia, will fly to the lower flowers on another plant, and fertilize them; and thus as she goes her rounds and adds to her store of honey, she will continually fertilize fresh flowers, and perpetuate the race of our autumnal Spiranthes, which will yield honey to future generations of bees.

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SCIENTIFIC SUMMARY.

A

QUARTERLY RETROSPECT.

ASTRONOMY.

SMALL telescopic comet was discovered by Schwabe, at Athens, on July 2, and, a few hours later, at Marseilles, by Mr. Tempel. Subsequent observations and calculations proved that it passed its perihelion on June 22. It continued all along very faint, and was only visible in good telescopes. Its least distance from the sun was about ninety-five millions of miles-almost identical with the earth's mean distance from the central body. We have not seen it noticed that the orbit of this comet bears some resemblance to one discovered at Paris, on September 2, 1698. The direction of motion, the inclination to the ecliptic and the longitudes of the perihelion and node agree pretty well, and the least distance from the sun is not very dissimilar. The comet of 1698 is, however, stated to have been as bright as a star of the second or third magnitude.

A second and much brighter comet was discovered on July 18, at midnight, at the Cambridge (United States) Observatory, in the constellation of Camelopardalus. It was independently detected at Florence, on July 24, and, on the following night, at Rome, by Professor Rosa. On the latter day it is described as being round, with a nebulosity of from three to four minutes in diameter and a distinct nucleus. Although almost circular, it was slightly dilated in the direction of the sun. No tail was visible in the comet-seeker, although with the naked eye one was faintly perceptible. On August 3, when first seen by the writer, the comet was plainly visible to unassisted vision, and a slender tail was distinctly seen in the finder of the telescope. The nucleus was bright, and a well-defined fan-light was seen proceeding from one of the sides, which was almost at angles with the tail. On August 19 the comet had become a very conspicuous object in the Northern hemisphere and the tail could be traced with the naked eye for a distance of eight or ten degrees from the head. The luminous sector had, however, disappeared, and an exceedingly bright jet of cometic matter lay in the contrary direction to the tail. At midmight of August 24, the luminous sector again made its appearance, and a faint envelope was suspected as surrounding that part-a bright jet passed across the fantail from the nucleus, lying almost at right angles to the tail, which latter was as slender as on former occasions, and was separated from the head by an almost dark space. On August 25, at

9.30 p.m., the fantail was still faintly visible, but the bright jet of light was now turned in the contrary direction to the tail and appeared like a miniature comet. The aspect of the comet had considerably changed since the preceding evening, and the tail appeared to have become broader and brighter, and to be connected with the head in a more solid manner. No opportunity occurred of viewing the comet before August 31, when a great change was observed to have taken place. The head and tail were now similar in appearance to those of Donati's and the great comet of 1861 -the fantail was brighter than on former occasions: the edges being the most brilliant portions; and the tail was now almost as broad as the head, although considerably fainter. Instead of the appearance of a bulbous root attached to a slight stem, it had assumed the ordinary shape of large comets. It will be seen from the foregoing observations and accompanying plate that the position of the fantail, but more particularly that of the luminous jet within it, was constantly varying. The same phenomenon has been noticed in every great comet observed since the invention of the telescope, although it is only of late years that the matter has been thoroughly investigated. Bessel considered that it was a real oscillation to and fro, dependent on the action of the sun. The observations made by M. Chacornac on the present comet seem, however, to show that no oscillation takes place, and that the jets of light are separate and distinct. Having narrowly watched the different phases (on one occasion for upwards of sixteen hours without interruption), he comes to the conclusion that, as the comet approaches near the sun, the nucleus emits a vaporous jet (something like that of steam) in the direction of the central luminary. This preserves a rectilinear form for some time, showing that considerable force is exerted at the time; but this latter subsequently becomes comparatively weakened, and the jet takes the form of a cornucopia. It finally becomes foggy and diffused, and the active emission appears to have altogether ceased, when another ray and fresh jet make their appearance about thirty degrees from the first, which, passing through all the changes previously undergone by that, finally vanishes away in the lapse of about sixteen hours, whilst the original jet again comes into view. It remains to be seen whether this curious discovery will be verified by the whole series of observations; at all events, it will put observers on the watch to follow more narrowly the changes occurring in those (in every respect) erratic and mysterious objects. It may be added that the path described by this comet bears no resemblance to any previously observed. It passed at its shortest distance from the sun on August 23, when its distance from that luminary was about ninety-two millions of miles. It moved in an orbit inclined sixty-six degrees to the ecliptic, and the head, it was reckoned, was about 100,000 miles in diameter.

Two well-certified disappearances of nebula have been recorded within the last three months. In both cases the original observations have been of recent date, so that there can be no suspicion of mistake. The first is situate at 3h. 21m. of R.A., and +30° 54′ of Decl. A few years since it was visible in a two-foot comet-seeker. In 1859, February, it was barely visible in a telescope of three-inch aperture; and, in August of the present year, it was scarcely perceptible in the great Copenhagen refractor.