simple external conditions. It is influenced by variations of heat and cold, is increased in volume by heat, is contracted or condensed by cold; it is influenced by atmospheric pressure; it is influenced by electrical conditions of the air; the inductive effects of electricity on the muscles of living animals are due, as it seems to me, to the disturbance excited by the electrical action upon the animal atmosphere; nay, I conceive, we ourselves are rendered conscious of the changes of external conditions of heat, of cold, of variations of the barometrical pressure, of electrical storms-by the sensible fluctuations of the atmosphere within us.

Through the nervous ether, itself a gas or vapour, other gases or vapours may readily and quickly diffuse, and by such diffusion may so modify the physical characters of the natural ether as to lead to modifications of nervous function. Thus those vapours which, being diffused into the body, produce benumbing influence as the vapours of alcohol, chloroform, bichloride of methylene, ethylic ether, and the like-produce their benumbing effects because they are not capable of taking the place of the natural ether into which they diffuse: they interfere, that is to say, with the physical conduction of impressions through what should be the pure atmosphere between the outer and the inner world. A dense cloud in the outer atmosphere shall shut out my view of the sun; a cloud in the inner atmosphere of my optic tract shall produce precisely the same obscurity.

Pain is the result of rapid vibration of the nervous ether; and pain, whether it be called physical or mental, is the same event. The so-called physical pain, that which comes from a blow or a cut, is excessive vibration, more than the brain can receive. The so-called mental pain is excessive vibration carried through the senses to the centres, or excited in the centres and carried to the outlets of the body for relief.

It is, I think, no figure of speech to say that nerves bleed -no figure of speech to affirm the phenomena of nervous exhaustion, of nervous collapse, of nervous strain and of nervous overstrain. Under mental labour or emotion nerves bleed as vessels do-bleed not blood in mass, but the richest product of blood. Under violent shock the whole nervous atmosphere is thrown into vehement vibration, the heart is held fixed by the commotion, and the failure of animal force is followed by sudden and overwhelming prostration. These are all clear physical phenomena. A feeble animal chemistry yields a feeble nervous tension, a powerful chemistry or action produces over-tension.

The infliction of physical pain is followed by the shriek, the sob, the moan, or the hard setting of muscle: the shriek, the

sob, the moan, or the muscular rigor is the echo of the pain; it is more, it is the outlet of the evil, the excess of vibration reflected, diverted, given forth. The infliction of mental pain is followed by tears, sighs, and other varied forms of grief; these are, again, the echoes and the outlets of the evil.

The tension of the nervous ether generally may be too high or too low; it may be so locally, owing to local changes in the nervous matter it invests and charges. Under undue tension of the brain or cord, both closed firmly in by bony walls, the ether, under sharp excitation, may vibrate as if in a storm, and plunge every muscle under cerebral or spinal control into uncontrolled motion-unconscious convulsion.

Lastly, the nervous ether may be poisoned; it may, I mean, have diffused through it, by simple gaseous diffusion, other gases or vapours derived from without; it may derive from within products of substances swallowed and ingested, or gases of decomposition produced, during disease, in the body itself. But here a field of observation opens relative to the production of some forms of acute and chronic diseases on which I must not enter, were even space at command.

I have tried, and I hope with success, to offer a simple and practical view of a very difficult subject. The philosopher may think the subject void, the public may think it obscure. There are many, I am aware, who will say that although the theory is reasonable it is comparatively worthless until more is known-until, in short, the physical character of the assumed nervous ether is demonstrated and certain definite phenomena are made manifest by its mediation. This criticism, which I should be the first to suggest, I am the last to ignore. I profess only at the present moment to submit a theory; I look to experiment for the trial of the theory, its truth, its falsity; and as it is a theory which experiment can slowly, but in the most striking and solemn manner, truly and faithfully try, I abide the result with leisure and contentment.

388

ON PLEISTOCENE CLIMATE AND THE RELATION THE PLEISTOCENE MAMMALIA TO THE

OF

GLACIAL PERIOD.

BY W. BOYD DAWKINS, M.A., F.R.S., F.G.S.

[PLATE LXXVIII.]

THE

HE animals which inhabited northern France, Germany, and Britain, during the Pleistocene* age, have been defined with the greatest accuracy by M. Lartet, Dr. Falconer, Professor Busk, and others, and probably will not be largely increased by any future discoveries. There are, however, certain inferences to be drawn from the comparison of the present with the former range of the Pleistocene animals, both as to climate and geography, which have not been brought prominently forward. The delicate question also of their relation to the glacial period, which has been sub judice for the last twenty years, demands a careful attention. In the following essay I shall attempt to show to what extent it may be solved by an appeal to the localities in Great Britain and Ireland, in which they have been discovered. I have already proved in the "Quarterly Geological Journal" for 1869, p. 192, that the contents of the bone caves and of the river deposits of Great Britain are, zoologically speaking, of the same age, and that the absence of one or two animals in the one or the other does not necessarily imply a difference in point of time. The cave bear is probably absent from the river beds because it was an inhabitant of caves, and was not liable to be swept down by the floods and buried in the river gravels and alluvia, just as the squirrel is absent from both, because of its arboreal habits: it has left the gnawed nut-shells in the Cromer forest bed as

The term Pleistocene is used in this essay as the precise equivalent of the term Quaternary, and as the natural name of the stage succeeding the Pleiocene. The Forest-hed mammalia, which I have formerly included in the Preglacial division of the Pleistocene, are relegated to the passage beds of the Pleiocene, or the Upper Pleiocene. Evidence for this change of nomenclature will shortly be published.