I must now bring to a close my review of the ancient plant-bearing beds of the Arctic regions. We may conclude that in the greater number of cases it is evident that the plants really grew in the regions in question. Although we know of fossil plants in some marine deposits, as, for instance, in the Senonian of Greenland and perhaps also in the Trias of Spitzbergen, these are exceptions which lack importance, since other deposits of a closely corresponding age are of fresh-water origin. While it may be admitted that even in Spitsbergen part of the Tertiary flora may have been transported from a more or less distant country by a river, yet other deposits on approximately the same horizon indicate that the greater number of the species, and among them the most important types, have actually flourished in the region itself. Taking into account the facts which I have enumerated, it is evident that the fossil floras of the Arctic should be still regarded as the foundation of every discussion of the former climates of that region. How are these favorable climates to be explained? That is a question to which we are not able to reply at the present moment and of which the solution belongs to the future. RECENT ADVANCES IN OUR KNOWLEDGE OF THE PRODUCTION OF LIGHT BY LIVING ORGANISMS. By F. ALEX. MCDERMOTT, Washington, D. C. INTRODUCTION. The original paper (48)1 upon which the following is based was read under the title "The Chemistry of Biophotogenesis," before the Chemical Society of Washington, D. C., on October 13, 1910, and subsequently published in the Scientific American Supplement, No. 1842. In revising this paper for publication here it has been found that so much new work of importance has been published or has come to the author's attention in the interim, that it has been decided to reconstruct the paper along the line indicated by the above title, taking up the more significant recent work, and adding a few heretofore unpublished observations of the author. The term "Biophotogenesis," in its complete sense of the production of light by living organisms, covers a group of phenomena accompanying the vital processes in a wide range of animal and vegetable forms. The fireflies (Lampyridae) are to most of us the commonest and most brilliant of these forms, and the following will therefore have special reference to these insects. An excellent monograph (5) of the subject of the emission of light by living forms, including an extensive bibliography of the important papers and a general review of the literature, has appeared under the title "Die Produktion von Licht," by Prof. E. Mangold, as the second half of the third volume of Winterstein's Handbuch der vergleichende Physiologie. The whole literature of the subject is very extensive, although much of the older work is without significance to-day. A considerable number of scientific men, including workers in the fields of chemistry, physiology, biology, entomology, and physics, are working on the problems presented by the phenomena of luminous organisms, and important developments in our knowledge of this subject are to be expected. 1 Numbers refer to literature references at the end of paper. For the sake of convenience the subject will be considered in the following sections in this paper: 1. Physical properties of physiologic light. 2. The chemistry of the photogenic process. 3. The effect of chemical reagents, etc., on the luminous tissue. 4. The photogenic organs. 5. Fluorescent substances in luminous insects. 6. Biologic relations of the phenomena. 1. PHYSICAL PROPERTIES OF PHYSIOLOGIC LIGHT. It is an interesting and significant fact that the luminous radiations of the majority of luminescent organisms produce upon the human retina the sensations of yellowish green, green, or bluish green. That this color is the result of the actual composition of the emitted light and not a subjective phenomenon, has been demonstrated by a number of investigators. The light of one of the common fireflies of the eastern United States, Photinus pyralis, has recently been made the subject of a very interesting spectrophotographic study by Drs. Ives and Coblentz, (31) at the Bureau of Standards. These observers found the light of this insect to resemble very closely the light of the Cuban cucuyo (Pyrophorus noctilucus Linn.), as studied and described by the late Prof. S. P. Langley and Prof. F. W. Very (39) 20 years ago. Briefly, the spectrum of the light of Photinus pyralis consists of a structureless, unsymmetrical band in the red, yellow, and lower blue portions of the visible spectrum, with a maximum at about that portion having the greatest illuminating effect with the minimum of actinic and thermal effects. It gives no hint of continuation in the infra-red or ultraviolet portions of spectrum. More recently, Ives (32) has shown that there is no infra-red radiation between 0.7065 micron and 1.5 microns in the light of the firefly, nor is any ultra-violet radiation present. Coblentz (5) has shown that the chitin covering the luminous organs of the firefly has a very low transmissivity for radiations of greater wave length than 2.5 microns; so low, in fact, that it would be difficult, in these wave lengths, to distinguish radiations from the photogenic organs from those due to the ordinary animal heat. However, the same author (5, 6) has found that the luminous segments of the firefly do give evidence of being at a slightly higher temperature than the rest of the insect's body, and that the ventral side of the luminous segments is at a higher temperature than the dorsal. He did not observe an increase of temperature during light emission. While it seems probable that the light of most other luminous organisms is essentially similar to that of Photinus pyralis, slight differences may be noted even between closely related species. A few H marine forms, e. g., the Cephalopoda and the Pennatulidæ, give lights of several colors. Among the insects the only forms known definitely to present wide differences from this general similarity are the tropical species of Phengodes as observed by Barber (2) and others, which possess a photogenic organ, located back of the head, that gives a distinctly reddish light. No spectroscopic studies of this red light of Phengodes have been made. Coblentz (5a, 6) has given spectrographic proof of the differences in the color of the lights of Photinus pyralis, P. consanguineus, and Photuris pennsylvanica, attention having been called to the physiologic differences by the author (4) and others, [Knab (3), Turner (8)]. The author has recently had an opportunity to examine the light of Phengodes laticollis (female) with the pocket spectroscope referred to in his paper in the Canadian Entomologist, 1910 (4), and found it to consist mainly of a narrow band in the yellow-green and green, with very much fainter ends stretching toward the red and blue; the definite ends of the band could not be made out on account of the feebleness of the light, and the predominance of the greenish band may, of course, be mainly due to the greater retinal sensitiveness to these tones. Forsyth (21) has claimed that cultures of certain photobacteria give spectrophotographic evidence of the existence of ultra-violet rays in their emitted light. It seems to the author that this observation is in need of confirmation, not that it is impossible, of course, but that it is at variance with previous work and with present ideas of the properties of physiologic light. McDermott (10) failed to find evidence of ultra-violet radiation in the light from cultures of Pseudomonas lucifera Molisch, and, as would be expected, also failed to find any indication of appreciable radioactivity. It need scarcely be said that the light of the firefly affects the photographic plate; obviously spectrophotographic studies could not otherwise have been made upon it. Photographs have been taken by means of the light of the photogenic bacteria and of the cucuyo. In 1896 Muraoka (54) announced that he had proved the penetration of metal films by means of the light of the firefly in a manner similar to that of the X-rays. The author has failed to find any evidence of the penetration of thin sheet copper, aluminum foil, or the black paper with which X-ray plates are wrapped by the light of Photinus pyralis. It seems that under certain circumstances substances which do not actually emit penetrating radiations may affect the photographic plate, and an explanation of Muraoka's results has been offered by Molisch (52, 58) based upon bacterial or vapor influence; but when we consider that his results were published only a little while after the discovery of the X-ray it seems possible that Muraoka was just a little over-enthusiastic. However, Singh and Maulik (1) have recently published some results similar to those of Judged from the fact that the light produced by many chemiluminescent reactions appears greenish to the human eye, it would seem that these should give spectra approximating that of the firefly [see Radziszewski (60), Trautz (67)], but in view of the facts that the light FIG. 1.-Spectral ranges of light from different organisms. emitted in most of these cases is very feeble and that the human eye is decidedly more sensitive to the greenish tones than to others, it may be simply that the amount of radiation, other than that giving the sensation of green, produced by these reactions is insufficient to cause the human retina to respond. However, it is difficult to class the production of light by living forms as other than a vital expression of |