will be more difficult, however, to explain how Ilythea spilota, Dichala caudata and D. brevicauda, Ochthera Mantis, etc., should have crossed the sea. The importation of some species, as, for instance, of the beautiful Psilopus albinotatus, discovered by me in Rhodus, seems almost inexplicable, and still this species is perfectly identical with the North American P. pallens. That all the species, now occurring on both continents, should have been gradually carried over from one to the other is utterly improbable. If we admitted this supposition, then, considering the large increase of the intercourse within more recent time, and the shortness of the passages now attained, we should also admit that most cases of importation have taken place, if not within the last ten or twenty years, at least during the last half a century, and secondly, we would have to infer that this importation of species was a reciprocal one. But if the latter was the case, the study of the European Diptera would have long ago detected the existence of these large importations from America; the Dipterous fauna of England especially, owing to the most frequent intercourse of this country with America, would have shown evident traces of such exchanges of species; in our sea-ports likewise, the appearance of single species of recent importation would have been noticed and their spreading from these centres, observed. Although I readily admit that the knowledge of the European Diptera is still very imperfect, nevertheless occurrences of this kind, owing to the large number of cases, would not have escaped attention. We have to conclude then, for the present, that the importation of species through the agency of frequent intercourse, does not afford a sufficient explanation of the large number of species common to Europe and North America.

As to those North American species, which are distinguished from European ones merely by a difference in the coloring, the question arises, whether they can be considered as descending from the same stock. It is an undoubted fact that species with a wide area of distribution show, in very remote parts of this area, a perceptible difference in coloring, sometimes even a very decided aberration in the picture. Such is, for instance, the case with Anthrax bifasciata, which shows toward the east a much more pronounced contrast between the white and the black coloring of the body, and acquires besides some slight, but very definite, peculiarities in the picture of the wings, so that an eastern specimen can be immediately recognized among a number of German ones. Still better known is the influence which certain regions exercise on the coloring of all the species occurring there; this is, for instance, in a very striking degree, the case with Iceland. A collection from that country, at a cursory view, seems to con. tain many new species, but upon closer examination, these spe

cies prove to be merely varieties of well known European species; they owe their existence to the propensity of all colors to merge into black and to the greater extent and intensity of the black itself, so that a light-colored picture upon black ground becomes much narrower or even disappears altogether. It can be likewise shown, that more confined localities exercise a similar, although less pronounced, influence on the coloring of the species. Under these circumstances, the question whether the North American species, above alluded to, are of the same descent with the corresponding European species, must be answered by an affirmation.

The same question may be proposed about those North American species which deviate from European species only by slight plastic differences, often merely a small variation in the size of some organ, or in the length and density of the hairs and bristles. Similar modifications are sometimes observed among specimens of European species. Thus the specimens of Gymnopternus Sahlbergi, caught in the southern parts of Switzerland differ so much from the Swedish specimens by the hairs and bristles on the first joint of the middle tarsi of the male, that they might be taken for different species, if all the intermediate forms did not occur in the countries lying between those two extremes. Still more striking is the difference between the male specimens of Empis maculata Fab., (not the Empis variegata confounded with it by Meigen,) caught in southern Germany and Sicilian specimens, the latter having on the fore tarsi hairs of unusual length and stoutness; the specimens from Lombardy are still a good deal like those from southern Germany; in those from Florence the hairs on the fore tarsi are already quite conspicuous, and they are still longer in the specimens from Rome, so that in this case the specific identity is proved by a gradual transition. Under such circumstances, the question, whether species showing but slight differences of the indicated kind should be considered as derived from the same stock, cannot be answered negatively. I readily acknowledge that it is rather difficult to state modifications of what parts in the Diptera have to be considered as essential and which as unessential, as different rules prevail in this respect in every family, in many families even in every genus, rules which a special research alone can determine. The only tolerably reliable general law prevailing in this case is, that all modifications in the structure of the mouth or of the genital organs are of the highest importance, whilst, on the contrary, all the other differences, observable even in the two sexes of the same species, are the least important.

We have now reached the category of those North American species which show a great resemblance to European species, but possess at the same time very definite plastic distinctive

characters; for brevity's sake I will call them analogous species. If we put now the question, whether it is to be assumed, that such analogous species may possibly have a genetic connection, we will find that all observations hitherto made on living Diptera warrant a negative answer. There is not a single instance on record which would justify the conclusion that under the now prevailing natural conditions, any species could be modified in that way, either through climatic influences, or in consequence of a compulsory change of food or through the contact with some other species. I do not deny that every time I compare such analogous species, the question forces itself upon my mind whether that, which seems impossible now, was not possible at some former period, as the impression left by such a comparison is most decidedly that of a common origin,

The European and the American Dipterous fauna always appear to me like two branches of the same stock, each having had a development of its own, very similar however to the development of the other. But if there really was such a common stock for both, it is to be sought among the Diptera of a former geological period, and if the European and the North American Dipterous fauna are to be considered as branches of this stock, the necessary inference would be that at a former period Europe and America had a continental connection.

Are the amber Diptera preserved fragments of this common stock?-Did a continental connection between Europe and America really exist at the time when they lived?-Did the submersion of an Atlantis tear asunder the branches of this stock? Was this catastrophe accompanied by changes which modified the general laws of development of the common stock in such a manner as to produce a difference between the further development of the stronger American branch and of the weaker European one, a difference not excluding at the same time a great deal of analogy?

Allow me to conclude my discourse with these unanswered questions. All those problems to which the study of the living and fossil Diptera affords a solution, or at least seems to afford one, I have done my best to answer. In doing this, I purposely remained within the exclusive limits of Dipterology, partly ow ing to my conviction that the interest of truth is best promoted when one confines himself to the investigation and discussion of a question from the point of view of his own speciality, partly because condemned as I am to a total literary isolation and absorbed now for years with utterly unscientific occupations, I am but very imperfectly acquainted with the researches made in a similar direction, especially with those of later years.

ART. XXVIII.-Abstract of Prof. Meissner's Researches on Oxygen, Ozone, and Antozone;' by S. W. JOHNSON.

DR. MEISSNER has submitted the ozone and antozone question to an extended and masterly investigation; at least such is our impression from a careful perusal of his treatise, an octavo volume of 370 pages, the preface of which bears the date of Feb., 1863. This book is appropriately dedicated to SCHÖNBEIN, whose name will stand in imperishable connection with the remarkable discovery of the triple nature of oxygen-a discovery which must, ere long, give us a new insight into the relations of matter to force, and modify, in a radical manner, some of the doctrines now current in science.

In the preface it is distinctly announced of ozone and antozone that one of them can not be formed without the other simultaneously appearing. This is a discovery of the utmost importance, and we shall endeavor to present briefly the author's arguments in proof of its reality.

In the Introduction is presented a concise but comprehensive sketch of the history of the ozone question up to date of publication. Section I. bears the heading: THE RELATIONS OF ELECTRICITY TO OXYGEN, and is divided into two chapters, of which the 1st, of 200 pages, relates to Electrized Oxygen, and the 2d to Ozone and Antozone. These headings are made appropriate by the history and progress of the investigation rather than by its results. The second section, of 72 pages, is entitled: THE POLARIZATION OF OXYGEN IN THE ACT OF COMBUSTION; and the concluding section, of 84 pages, OZONE AND ANTOZONE IN THE ATMOSPHERE. In section I, Meissner states that the object of the first part of his investigation is to ascertain whether, as all previous experiments would appear to show, the effect of electricity on oxygen is simply to convert it, or a part of it, into ozone, or whether, as Schönbein in 1861 had assumed from theoretical grounds, the ordinary inactive oxygen is polarized into the two opposite oxygens, the negative-active ozone and the positive-active antozone.

To electrize oxygen the apparatus of von Babo (Verhandl. der Naturforsch. Gesellschaft zu Freiburg, ii, p. 331), imitated from an instrument of W. Siemens (Pogg. Ann., 1857, B. xii, p. 66, p. 120) was employed, in which ozonization takes place in a thin stratum of air, and is determined by the silent discharge from poor conductors. This apparatus is made as follows: twelve very fine copper wires, such as are used in covering violin strings, and about five decimetres long, are inserted each into a very

1 Untersuchungen über den Sauerstoff von Dr. G. MEISSNER, Professor in Göttingen. Mit einer Lithographirten Tafel. Hannover, 1863.

thin glass tube somewhat longer than itself and about 0.3 mm. in width. Each of these tubes is sealed at one end. Into the other end is fused a wire of platinum which, within the tube, is twisted with the copper wire, and without the tube projects an inch or so. The twelve tubes thus made, are arranged within a glass tube 7 mm. wide and 6 decimetres long, so that the projecting platinum wires of six of them are at one end and those of the other six are at the other end of this wide tube. These two sets of wires are each twisted about a larger platinum wire which passes through and is fused into the wall of the wide tube. The tubes of the one bundle are distributed among those of the other as equally as may be; they are, moreover, in close contact, and the spaces surrounding them are as narrow as possible. On connecting the extremes of these two series of inclosed wires with the electrodes of the secondary coil of a powerful induction apparatus, the electrical discharge takes place through the walls of the narrow tubes and through the air that surrounds them. The discharge is unattended with sparks, and on approaching the ear only a faint crackling sound is perceptible. In the dark the bundle of fine tubes shines throughout its whole length with a reddish-violet light. During the electrical action the air bathing the small tubes is powerfully ozonized. By adapting suitable apparatus to the large tube the ozonized air may be removed and submitted to examination, and its place supplied with fresh air, at pleasure. In Meissner's resarches the air of this ozoniser was renewed by the pressure of a gasometer. It was found necessary that the air should be perfectly dried before being subjected to electrization. In endeavoring to effect this object the author had no little difficulty. He finally accomplished it by the use, first, of a wide tube, more than a meter long, filled with chlorid of calcium, and secondly, of two or three wide tubes, 1-1 metres long, filled with coarse glass beads drenched in oil of vitriol.

The perfectly dry air, after traversing the ozonizer, was submitted to the action of reagents in receivers of glass connected with the ozonizer by means of a mercury joint, this metal being unaffected by dry ozone.

The first point Meissner sought to investigate was whether dry electrized air, after being deprived of ozone, possessed properties other than those of common oxygen and nitrogen. He found that by transmitting it through a strong solution of iodid of potassium it was readily and totally deprived of ozone; the stream of air thus deozonized exhibited nothing remarkable until it had been passed through pure water, but, as it emerged from the water, it appeared in the form of a thick white mist, perfectly similar to that formed by the cooling of steam, which was sometimes so dense as to render the part of the small vessel