superiority in early times of a slowly changing physical condition of the whole globe over the partial and irregularly varying local conditions, which were continually augmenting, and are still augmenting in influence with the lapse of time." It may be objected to Professor Phillips's figures that he includes only the species belonging to the eight classes of animals which have been represented in each of the three great periods. I therefore give in the following Table the approximate total number of species (excluding plants only) that have been discovered in British stratified rocks up to the present time, with the result per 1000 feet, and the average number of feet per species in each great period: The evidence yielded by the analysis of these figures is even more in favour of the conclusion that the rate of geological change, according to the evidence of animal life, has progressively increased from the earliest to the latest times. With one other numerical illustration I shall close the argument drawn from the organic phase of the question. The late Professor Bronn, in his prize essay, and in the third edition of his 'Lethæa Geognostica,' gives the following as the total numbers of known species in 1850, just twenty years ago:-Palæozoic 6681, Mesozoic 10,879, and Cainozoic 15,138. If we assume the maximum thickness of the deposits of these periods to be 60,000, 25,000, and 10,000 respectively, we get the following numbers of species as occurring per 1000 feet of strata :-Palæozoic 111, Mesozoic 435, and Cainozoic 1513; thus showing exactly the same result almost in the same proportions. * The foregoing calculations are based on the assumption that the time required for the deposition of 1000 feet of strata was approximately the same in the Paleozoic, Mesozoic, and Cainozoic periods, and so far the argument is Uniformitarian; but the received interpretation of the physical conditions which prevailed in those periods renders it probable that the strata were deposited more slowly in the earlier than in the later periods, which, a fortiori, adds considerable strength to my conclusion. The inorganic aspect of the subject has been discussed more * I purposely make this number very large, so as not to run the risk of being charged with making too much of my argument from this point of view. frequently than that phase which we have just reviewed; but with this striking difference, that no geologist has treated it from the point of view of pure geology in the same direction as Professor Phillips has from a palaeontological standpoint, the geological arguments are all either purely Uniformitarian, or as purely Catastrophic. In the second of two essays "On the Measurement of Geological Time," which were published in Nature' only a few weeks ago, Mr. Wallace touches on this question. He remarks† that for the last 60,000 years the eccentricity of the earth's orbit has been very small, and that therefore the opposite phases of precession, each lasting 10,500 years, have during that time produced scarcely any effect on climate. This state of things, however, is regarded by him, as by Mr. Croll, as quite exceptional, for during nearly the whole of the last three million years the opposite state of things has existed, namely, a high eccentricity coupled with a change (in the extra-tropical regions) every 10,500 years, from a very cold to a very mild climate. Mr. Wallace therefore argues as follows:"This will necessarily have caused much migration both of plants and animals, which would inevitably result in much extinction and comparatively rapid modification. Allied races would be continually brought into competition, altered physical conditions would induce variation, and thus we should have all the elements for natural selection and the struggle for life to work upon and develop new races. High eccentricity would therefore lead to a rapid change of species, low eccentricity to a persistence of the same forms; and, as we are now, and have been for 60,000 years, in a period of low eccentricity, the rate of change of species during that time may be no measure of the rate that has generally obtained in past geological epochs." I shall not stop to criticize Mr. Wallace's attempt to measure geological time, as Mr. Dawkins has already pointed out the fallacy involved in the major premiss of this argument, viz. that all climatal change has depended solely on the eccentricity of the earth's orbit.‡ It is sufficient for my present purpose to point out that Mr. Wallace recognizes the principle that the rate of change of species may have varied in different geological epochs. To refer the cause of its variation to differences in the eccentricity of the earth's orbit is bably erroneous, but that error of ultimate explanation by no means diminishes the importance or the stability of the fact which is thus sought to be explained. pro Both Sir Charles Lyell and Mr. Wallace have attempted to estimate the duration of the several geological epochs, basing their calculations on a supposed rate of change in species of marine mollusca. In this way Sir Charles Lyell concludes that "we may Loc. cit., March 3rd, pp. 453 and 451. See 'Nature,' March 17, p. 505. *February 17th and March 3rd. consider a million years to represent the twentieth part of a complete revolution in species, and we might thus estimate the number of years required for the elaboration of the successive Tertiary formations." Proceeding thus, Sir Charles Lyell calculates that two hundred and forty millions of years have elapsed since the beginning of the Cambrian period. Mr. Wallace, however, by taking the same facts and figures, manipulates them differently, and comes to the conclusion that the lapse of time is exactly one-tenth of that estimated by Sir Charles Lyell. This difference of result is of very little consequence, as both calculations are equally speculative, and it is chiefly to a point of resemblance that I wish to draw attention. Sir Charles Lyell and Mr. Wallace, however much they may differ in other matters, whether they regard the revolution of species as having been accomplished in no less a period than twenty millions, or in one no greater than two millions, are agreed in using the same rate of change for the Palæozoic as for the more recent periods. Taking the figures of the latter author, and calculating the time required for the deposition of strata on his hypothetical rate of change in species, we get at the following results: : It may be urged that the Tertiary rocks are comparatively thin in England, owing to the absence of Miocene deposits; but the same argument may be applied to each of the three periods with greater or less force; and at any rate the relative rapidity of deposit would not be very much disturbed if we took the maximum thickness all over the world instead of those occurring in the British Islands. Leaving denudation entirely out of the question, it does not seem at all probable that the Paleozoic rocks should have been deposited at the swift rate of one foot in 175 years; and if we deduct from the calculation the period represented by "breaks," which Professor Ramsay regards as longer even than that represented by strata, we shall have the conclusion drawn that the Palæozoic rocks were deposited at the rate of one foot in less than a century! At the present day there are many localities peculiarly favourable to the rapid accumulation of deposits over limited areas, as, for instance, the deltas of the great rivers; but even in such cases the rate of deposition is probably not more than what, according to Mr. Wallace's estimate, must have been general all over the aquiferous surface of the earth during Palæozoic times. Whether we measure the relative lapse of time occupied by the successive events of Geological History by the known facts of the accumulation of deposits, or by the comparative changes which have occurred in the life of successive periods, we are led equally to infer that the rate of geological change has been more rapid in the later than in the earlier geological periods, and that that rate has increased progressively from the earliest to the latest times. Such an inference, though it may at first sight seem heretical, is in reality but the natural result of those conditions of the earth's surface which the most orthodox geologists regard as characteristic of successive periods. A greater uniformity of climate and of surface in the earlier Palæozoic periods than at the present day has long been considered the legitimate inference to be drawn from the thick masses of uniform deposits spread over large areas, and containing species of fossils possessing an enormous geographical distribution. In ascending the geological scale the deposits gradually become more differentiated, and the fossils belong to species which had a more restricted geographical range; these differences are usually and properly regarded as the result of greater diversity of climate and surface-configuration during the later periods, and these more diversified conditions must have been accompanied by a greater rapidity in the rate of geological change, if for no other reason than that there were a greater number of centres of change, acting and reacting on each other. IV. AIR-POLLUTION BY CHEMICAL WORKS. A MANUFACTURER, having realized his primary object of making what he can out of the materials which pass under his hands, and having utilized all that he deems valuable in them, finds there is yet another need to be fulfilled; he must get rid of his refuse, and that as speedily as possible. Our present object is to watch this latter operation, and, losing sight of the beautiful or useful results of his work, to direct our attention to what is waste or refuse, and inquire how he disposes of it. When this is solid and bulky, it must be removed at the cost of much labour, and a place must be provided where it can be deposited. When the refuse is a liquid, the process of getting rid of it is generally less expensive; it will flow away in the water-courses if only proper drains and passages are provided. When the refuse is gaseous, this process of removal is easier still; no passages need be cut, no culverts nor bridges built, the vapour can be allowed to pass into the air, and is blown away. In each of these cases the manufacturer's object is attained; he is rid of the refuse, and has room for renewed work. Unfortunately, however, although he is rid of it, his neighbours are not so; they find, on the one hand, that the water of their brook is no longer fit for use, nor pleasant to look at, and the air they breathe is polluted with unsavoury and noxious vapours. Where a manufactory of this kind stands alone, or where only those who are dependent on it for their subsistence dwell in its vicinity, this state of things goes on for a long time without calling forth much complaint. Sooner or later, however, complaints must come; we cannot all live at arm's length. Population increases, we are pressed together, and valuable though the various products of manufacturing industry may be, pure air and pure water are more valuable still. Yet we cannot do without the manufactory, unless we return to barbarism. A naked savage eating uncooked roots erects no chimney to pour its black or acrid vapour into the air; he discharges the liquor from no dye back into the clear brook of the glen-but he remains a savage. We must keep our manufactories; by their products we are warmly clothed, our houses are firmly built and are decorated with colours; the wind is shut out by panes of transparent glass; the paper on which we write is white and fair. These and a thousand other things are the results of many a mechanical or intricate chemical process, the waste products from which, solid, liquid, and gaseous, are unpleasant enough. If, then, we will not go back to barbarism to get rid of our smoke and our dirty water, can we go forward and by greater skill diminish or suppress them? The answer must be "Yes." Yet those only who have to work out the problem know with what difficulty this answer has in many cases been given, whilst in many it is not given yet. The materials which the manufacturer throws away, we have already classed in correct school-room fashion as solid, liquid, and gaseous. With the first of these the manufacturer alone is concerned, and it may be safely left in his charge. The more of it he produces, the more must he expend in its removal, the more land must he purchase on which to deposit it; and if he throws away that which is valuable, he is the chief loser. We may, therefore, safely leave him, with certain reservations, to look after his solid refuse, knowing that no sharper impulse can be applied to induce him to diminish its amount, or to save what is valuable in it, than the spur of self-interest which already exists. We say it may be safely left in his charge; but if, through some process of fermentation or change, a portion of it shall slip out of his custody, and yield, after rain, a noxious liquid to drain into the nearest brook, or a gaseous escape to contaminate the air around, it falls back into the two other classes. For the present we propose to direct attention to the latter of these two classes only, the gaseous. In doing so, we would first dwell VOL. VII. 2 A |