We feel ourselves constrained by these and other such facts to believe (with Macartney and Todd) that in many, perhaps in all, lumi nous animals, both terrestrial and marine, the light emitted is the consequence of an evolu tion of an imponderable agent by the nervous systems of the animals, just as the electrical fishes give their shock without the interposition of any visible or ponderable secretion. In this view we may regard the luminous organs as playing the same part in relation to the evo lution of light as the electrical organs of the torpedo do to the production of the shock. The single fact of luminousness continuing in the organs or in their effused fluids, after they have been removed from the body of the ani mal, seems to point to a great difference ex isting between the two classes of phenomena which we have just compared, in certain ani mals; but it may be that the difference is more apparent than real; for this fact may be explained by supposing that a phosphoric sub stance really does enter into the composition of the light-giving organs; and yet we may with great probability conjecture that it is not the chief agent in causing the phenomena of luminousness. It remains, then, for future in quirers to determine the chemical composition of the luminous organs, and the fluids emitted by the animals, the phenomena of whose lu minousness seem to be irreconcileable with the idea of their being dependent on the nature of these fluids ; if they be found to contain phosphoric matter, it may be concluded that, as this does not appear to be essential in them to the production of the phenomena of lu minousness, so neither may it be in other ani mals, in which it is believed to be the chief agent in the manifestation of their light-giving function.
To this theory, (which is only a combination of the two most generally received in modern times,) we do not, in the facts which have come under our notice, see any serious objec tion. The only argument adduced against Macartney's theory by Tiedemann and other physiologists, who have carefully considered the facts of the case, is founded on the circum stance of the light continuing, in a certain degree, for some time after the death of the animals, which, of course, cannot be supposed to be owing to the continued operations of the nervous system. This posthumous light, how ever, may depend on the phosphorescence of the luminous organs or their effused fluids in virtue of their composition, while the full evo lution of light during life may be produced chiefly by the play of imponderable agents in and from the nervous system, independently, in some eases, of the chemistry of the fluids ; in other uses, aided and modified by the nature of these and by the structure of peculiar organs.
It is scarcely necessary to take particular notice of the various other theories that have been suggested, as the facts stated in the pre ceding part of the article are sufficient to set them aside.
VIII. Uses of animal luminousness.—We Viii. Uses of animal luminousness.—We know nothing certainly with regard to the uses of the light-giving function ; but as almost all observers have remarked that male insects seem to be attracted towards their mates by the brilliancy of the light emitted by the latter, it has been generally supposed that the luminous ness is subservient to the generative function. Although it may be so to a certain extent, it is obviously not essentially connected with it, even in the glow-worm ; for the light endures long after the season of love is past. Some
have conjectured that the light may sometimes be the means of preserving its possessors from the destructive attacks of enemies. Thus Shep pard observed a large beetle running round a shining scolopendra, as if wishing to attack it, but seeming to be scared by the light. We may imagine, also, that the light enables its possessors to see surrounding objects at night, and so to thread their way in safety through the darkest places.
Considering that, in the ocean, there is abso lute darkness at the depth of 800 or 1000 feet, at least that, at such depths, the light of the sun ceases to be transmitted, Illacculloch has suggested* that, in marine animals, their lumi nousness may be "a substitute for the light of the sun," and may be the means of enabling them to discover one another, as well as their prey. He remarks, "It seems to be partieu cularly brilliant in those inferior animals which, from their astonishingpowers of reproduction, and from a state of feeling apparently little superiorlo that of vegetables, appear to have been in a great measure created for the supply and food of the more perfect kinds." IX. Luminousness of animals not innate, and other allied phenomena.—We have ac counts of the surface of the human body ap pearing luminous in consequence of phos phoric matter being largely mixed with the sweat in the course of various diseases. The urine also both of men and several of the lower animals is occasionally luminous under similar circumstances. It is said that the urine of Viverra mephitis and V. putorius is al ways so.t The eyes of human albinoes, almost all the mammalia which possess a tapetum lucidum, as also those of some birds of prey, serpents, and insects, seem to shine in a feeble light from the refiexion and concentration of the rays falling upon them from external objects. Pallas thought that this light was developed in the retina, and regarded it as an electrical phe nomenon. But it has been plainly proved by Prevost, Gruithuisen, and Esser,: that the shining of the eye depends, in most cases, on reflexion of light. They found that there was no appearance of luminousness in absolute darkness ; and that the eyes of dead animals gave the same effect as those of the living, when placed in similar circu mstances.§ It would appear, however, from some obser vations made by Rengger on the eyes of a certain South American ape, ( Nyctipithecus trivirgatus,) that there is reason to believe in the emission of light from the eyes of some animals, independently of the reflexion of in cident light. The ape in question is nocturnal. The luminousness of the eye was seen by Rengger only when there was total darkness ; and then the light was so brilliant that objects at the distance of a foot and a half from the eye were distinctly seen.* In commenting on this statement by Rengger, Treviranns remarks,} "that the intensity of light may be increased by the brilliant tapetum of the eye, while it is concentrated as in a concave mirror, cannot be doubted. But it is impossible that a feeble light so concentrated should illuminate objects placed at the distance of a foot and a half from the eye of this ape, in a dark place." The same physiologist seems to be satisfied that some dogs also have a similar power of gene rating light within their eyes. In these, be states, the light is seen only when an impres sion on the sight or hearing arouses the ani mal's attention, or when he is excited by the operation pf some instinct or passion.