_Evidences of a slowly declining Temperature, and, therefore, of a long Time.--The Process of Events by Catastrophe and by Law.--a.n.a.logy of Individual and Race Development.--Both are determined by unchangeable Law._

_Conclusion that the Plan of the Universe indicates a Multiplicity of Worlds of infinite s.p.a.ce, and a Succession of Worlds in infinite Time._

[Sidenote: Age of the earth.] A victory could not be more complete nor a triumph more brilliant than that which had been gained by science in the contest concerning the position of the earth. Though there followed closely thereupon an investigation of scarcely inferior moment--that respecting the age of the earth--so thoroughly was the ancient authority intellectually crushed that it found itself incapable of a.s.serting by force the Patristic idea that our planet is less than six thousand years old.

[Sidenote: The question is impersonally solved.] Not but that a resistance was made. It was, however, of an indirect kind. The contest might be likened rather to a partisan warfare than to the deliberate movement of regular armies under recognized commanders. In its history there is no central figure like Galileo, no representative man, no brilliant and opportune event like the invention of the telescope. The question moves on to its solution impersonally. A little advance is made here by one, there by another. The war was finished, though no great battle was fought. In the chapter we are entering upon there is, therefore, none of that dramatic interest connected with the last.

Impersonally the question was decided, and, therefore, impersonally I must describe it.

[Sidenote: Oriental and Western doctrines of the age of the earth.] In Oriental countries, where the popular belief a.s.signs to the creation of man a very ancient date, and even a.s.serts for some empires a duration of hundreds of thousands of years, no difficulty as respects the age of the earth was felt, there seeming to have been time enough for every event that human researches have detected to transpire. But in the West, where the doctrine that not only the earth, but the universe itself, was intended for man, has been carried to its consequences with exacting rigour, circ.u.mstances forbid us to admit that there was any needless delay between the preparation of the habitation and the introduction of the tenant. They also force upon us the conclusion that a few centuries const.i.tute a very large portion of the time of human existence, since, if we adopt the doctrine of an almost limitless period, we should fall into a difficulty in explaining what has become of the countless myriads of generations in the long time so past, and, considering that we are taught that the end of the world is at hand, and must be expected in a few years at the most, we might seem to arraign the goodness of G.o.d in this, that He has left to their fate immeasurably the larger proportion of our race, and has restricted His mercy to us alone, who are living in the departing twilight of the evening of the world.

[Sidenote: Correction of the European doctrine.] But in this, as in the former case, a closer examination of the facts brings us to the indisputable conclusion that we have decided unworthily and untruly; that our guiding doctrine of the universe being intended for us is a miserable delusion; that the scale on which the world is constructed as to time answers to that on which it is constructed as to s.p.a.ce; that, as respects our planet, its origin dates from an epoch too remote for our mental apprehension; that myriads of centuries have been consumed in its coming to its present state; that, by a slow progression, it has pa.s.sed from stage to stage, uninhabited, and for a long time uninhabitable by any living thing; that in their proper order and in due lapse of time, the organic series have been its inhabitants, and of these a vast majority, whose numbers are so great that we cannot offer an intelligible estimate of them, have pa.s.sed away and become extinct, and that finally, for a brief period, we have been its possessors.

Of the intentions of G.o.d it becomes us, therefore, to speak with reverence and reserve. In those ages when there was not a man upon the earth, what was the object? Was the twilight only given that the wolf might follow his fleeing prey, and the stars made to shine that the royal tiger might pursue his midnight maraudings? Where was the use of so much that was beautiful and orderly, when there was not a solitary intellectual being to understand and enjoy? Even now, when we are so much disposed to judge of other worlds from their apparent adaptedness to be the abodes of a thinking and responsible order like ourselves, it may be of service to remember that this earth itself was for countless ages a dungeon of pestiferous exhalations and a den of wild beasts.

[Sidenote: It elevates rather than degrades the position of man.] It might moreover appear that the conclusions to which we come, both as respects the position and age of the world, must necessarily have for their consequences the diminution and degradation of man, the rendering him too worthless an object for G.o.d's regard. But here again we fall into an error. True, we have debased his animal value, and taught him how little he is--how insignificant are the evils, how vain the pleasures of his life. But, as respects his intellectual principle, how does the matter stand? What is it that has thus been measuring the terrestrial world, and weighing it in a balance? What is it that has been standing on the sun, and marking out the orbits and boundaries of the solar system? What is it that has descended into the infinite abysses of s.p.a.ce, examined the countless worlds that they contain, and compared and contrasted them together? What is it that has shown itself capable of dealing with magnitudes that are infinite, even of comparing infinites together! What is it that has not hesitated to trace things in their history through a past eternity, and been found capable of regarding equally the transitory moment and endless duration? That which is competent to do all this, so far from being degraded, rises before us with an air of surpa.s.sing grandeur and inappreciable worth. It is the soul of man.

[Sidenote: Relations of the earth in time.] From the facts given in the last chapter respecting the relations of the earth in s.p.a.ce, we are next led to her relations in time.

So long as science was oppressed with the doctrine of the human destiny of the universe, which, as its consequence, made this earth the great central body, and elevated man to supreme importance, there was much difficulty in treating the problem of the age of the world. The history of the earth was at first a wild and fict.i.tious cosmogony. Scientific cosmogony arose, not from any theological considerations, but from the telescopic ascertainment of the polar compression of the planet Jupiter, and the consequent determination by Newton that the earth is a spheroid of revolution. With a true cosmogony came a better chronology.

[Sidenote: Anthropocentric ideas of the beginning and end of the world.]

The patristic doctrine had been that the earth came into existence but little more than five thousand years ago, and to this a popular opinion long current was added, that its end might be very shortly expected.

From time to time periods were set by various authorities determining the latter event, and, as true knowledge was extinguished, the year 1000 came to be the universally appointed date. In view of this, it was not an uncommon thing for persons to commence their testamentary bequests with the words, "In expectation of the approaching end of the world."

But the tremendous moment pa.s.sed by, and still the sun rose and set, still the seasons were punctual in their courses, and Nature wore her accustomed aspect. A later day was then predicted, and again and again disappointment ensued, until sober-minded men began to perceive that the Scriptures were never intended to give information on such subjects, and predictions of the end of the world fell into discredit, abandoned to the illiterate, whose morbid antic.i.p.ations they still amuse.

As it was thus with the end of our planet, so it was as regards her origin. By degrees evidence began to acc.u.mulate casting a doubt on her recent date, evidence continually becoming more and more cogent.

[Sidenote: Rise of the doctrine of illimitable age.] In no insignificant manner did the establishment of the heliocentric theory, aided by the discoveries of the telescope, a.s.sist in this result. As I have said, it utterly ruined past restoration the doctrine of the human destiny of the universe. With that went down all arguments which had depended on making man the measure of things. Ideas of unexpected sublimity as to the scale of magnitude on which the world is constructed soon enforced themselves, and proved to be the precursors of similar ideas as to time. At length it was perceived by those who were in the van of the movement that the Bible was never intended to deliver a chronological doctrine respecting the beginning any more than the end of things, and that those well-meaning men who were occupied in wresting it from its true purposes were engaged in an unhappy employment, for its tendency could be no other than to injure the cause they designed to promote. Nevertheless, so strong were the ancient persuasions, that it was not without a struggle that the doctrine of a long period forced its way--a struggle for the age of the earth, which, in its arguments, in its tendencies, and in its results, forcibly recalls the preceding one respecting the position of the earth; but, in the end, truth overrode all authority and all opposition, and the doctrine of an extremely remote origin of our planet ceased to be open to dispute.

In a scientific conception of the universe, illimitable s.p.a.ces are of necessity connected with limitless time.

[Sidenote: Indications depending on the progressive motion of light.]

The discovery of the progressive motion of light offered the means of an absolute demonstration of this connexion. Rays emitted by an object, and making us sensible of its presence by impinging on the eye, do not reach us instantaneously, but consume a certain period in their pa.s.sage.

If any sudden visible effect took place in the sun, we should not see it at the absolute moment of its occurrence, but about eight minutes and thirteen seconds later, this being the time required for light to cross the intervening distance. All phenomena take place in reality anterior to the moment at which we observe them by a time longer in proportion as the distance to be travelled is greater.

There are objects in the heavens so distant that it would take many hundreds of thousands of years for their light to reach us. Then it necessarily follows, since we can see them, that they must have been created and must have been shining so long.

The velocity with which light moves was first determined by the Danish astronomer Romer from the eclipses of Jupiter's satellites, November, 1675. It was, therefore, a determination of the rate for reflected solar light in a vacuum, and gave 198,000 miles in a second. In 1727, Bradley determined it for direct stellar light by his great discovery of the aberration of the fixed stars. More recently, the experiments of M.

Foucault and those of M. Fizeau, by the aid of rotating mirrors or wheels, have confirmed these astronomical observations, Fizeau's determination of the velocity approaching that of Romer. Probably, however, the most correct is that of Struve, 191,515 miles per second.

[Sidenote: Investigation of the age of the earth through the phenomena of heat.] This astronomical argument, which serves as a general introduction, is strengthened by numerous physical and physiological facts. But of the different methods by which the age of the earth may be elucidated, I shall prefer that which approaches it through the phenomena of heat. Such a manner of viewing the problem has led to its determination in the minds of many thinking men.

[Sidenote: Astronomical heat alone on the earth's surface.] As correct astronomical ideas began to prevail, it was perceived that all the heat now on the surface of our planet is derived from the sun. Through the circ.u.mstance of the inclination of her axis of rotation to the plane of her annual motion, or through the fact of her globular form occasioning the presentation of different parts of her surface, according to their lat.i.tudes, with more or less obliquity, and hence the reception of less or more of the rays, there may be local and temporary variations. But these do not affect the general principle that the quant.i.ty of heat thus received must be the same from year to year.

[Sidenote: The equilibrium of interior heat.] This thermometric equilibrium not only holds good for the surface, it may also be demonstrated for the whole ma.s.s of the planet. The day has not shortened by the 1/200 of a second since the time of Hipparchus, and therefore the decrease of heat can not have been so much as the 1/300 of a Fahrenheit degree, on the hypothesis that the mean dilatation of all terrestrial substances is equal to that of gla.s.s, 1/180000 for one degree. If a decline had taken place in the intrinsic heat of the earth, there must have been a diminution in her size, and, as a necessary consequence, the length of the day must have become less. The earth has therefore reached a condition of equilibrium as respects temperature.

[Sidenote: Its ancient decline.] A vast body of evidence has, however, come into prominence, establishing with equal certainty that there was in ancient times a far higher temperature in the planet; not a temperature concerned with a fraction of a degree, but ranging beyond the limits of our thermometric scale. The mathematical figure of the earth offers a resistless argument for its ancient liquefied condition--that is, for its originally high temperature. But how is this to be co-ordinated with the conclusion just mentioned? Simply by the admission that there have elapsed prodigious, it might almost be said limitless, periods. [Sidenote: Necessity for a long time.] As thus the true state of affairs began to take on shape, it was perceived that the age of the earth is not a question of authority, not a question of tradition, but a mathematical problem sharply defined: to determine the time of cooling of a globe of known diameter and of given conductibility by radiation in a vacuum.

In such a state of things, what could be more unwise than to attempt to force opinion by the exercise of authority? How unspeakably mischievous had proved to be a like course as respects the globular form of the earth, which did not long remain a mere mathematical abstraction, but was abruptly brought to a practical issue by the voyage of Magellan's ship. And on this question of the age of the earth it would have been equally unwise to become entangled with or committed to the errors of patristicism--errors arising from well-meant moral considerations, but which can never exert any influence on the solution of a scientific problem.

[Sidenote: Indications of the interior heat of the earth.] One fact after another bearing upon the question gradually emerged into view. It was shown that the diurnal variations of temperature--that is, those connected with night and day--extend but a few inches beneath the surface, the seasonal ones, connected with winter and summer, to many feet; but beyond this was discovered a stratum of invariable temperature, beneath which, if we descend, the heat increases at the rate of 1 Fahr. for every fifty or seventy feet. The uniformity of this rate seemed to imply that, at depths quite insignificant, a very high temperature must exist. This was ill.u.s.trated by such facts that the water which rushes up from a depth of 1794 feet in the Artesian well of Grenelle has a temperature of 82 Fahr. The mean temperature of Paris being about 51 Fahr., these numbers give a rate of 1 for every fifty-eight feet. If, then, the increase of heat is only 100 per mile, at a depth of less than ten miles every thing must be red hot, and at thirty or forty in a melted state. It was by all admitted that the rise of temperature with the depth is not at all local, but occurs in whatever part of the earth the observation may be made. The general conclusion thus furnished was re-enforced by the evidence of volcanoes, which could no longer be regarded as merely local, depending on restricted areas for the supply of melted material, since they are found all over the land and under the sea, in the interior of continents and near the sh.o.r.es, beneath the equator and in the polar regions. It had been estimated that there are probably two thousand aerial or subaqueous eruptions every century. Some volcanoes, as aetna, have for thousands of years poured forth their lavas, and still there is an unexhausted supply. Everywhere a common source is indicated by the rudely uniform materials ejected. The fact that the lines of volcanic activity shift pointed to a deep source; the periodic increments and decrements of force bore the same interpretation. They far transcend the range of history. The volcanoes of central France date from the Eocene period; their power increased in the Miocene, and continued through the Pliocene; those of Catalonia belong to the Pliocene, probably. Coupled with volcanoes, earthquakes, with their vertical, horizontal, and rotary vibrations, having a linear velocity of from twenty to thirty miles per minute, indicated a profound focus of action. The great earthquake of Lisbon was felt from Norway to Morocco, from Algiers to the West Indies, from Thuringia to the Canadian lakes. It absolutely lifted the whole bed of the North Atlantic Ocean. Its origin was in no superficial point.

[Sidenote: Proof from the mean density.] A still more universal proof of a high temperature affecting the whole ma.s.s of the interior of the globe was believed to be presented in the small mean density of the earth, a density not more than 566 times that of water, the mean density of the solid surface being 27, and that of the solid and sea-surface together 16. But this is not a density answering to that which the earth should have in virtue of the attraction of her own parts. It implied some agent capable of rarefying and dilating, and the only such agent is heat.

Although the law of the increase of density from the upper surface to the centre is unknown, yet a comparison of the earth's compression with her velocity of rotation demonstrated that there is an increasing density in the strata as we descend. The great fact, however, which stands prominently forth is the interior heat.

Not only were evidences thus offered of the existence of a high temperature, and, therefore, of the lapse of a long time by the present circ.u.mstances of the globe; every trace of its former state, duly considered, yielded similar indications, the old evidence corroborating the new. And soon it appeared that this would hold good whether considered in the inorganic or organic aspect.

[Sidenote: Inorganic proofs of a former high temperature.] In the inorganic, what other interpretation could be put on the universal occurrence of igneous rocks, some in enormous mountain ranges, some ejected from beneath, forcing their tortuous way through thus resisting superinc.u.mbent strata; veins of various mineral const.i.tution, and, as their relations with one another showed, veins of very different dates?

What other interpretation of layers of lava in succession, one under another, and often with old disintegrated material between? What of those numerous volcanoes which have never been known to show any signs of activity in the period of history, though they sometimes occur in countries like France, eminently historic? What meaning could be a.s.signed to all those dislocations, subsidences, and elevations which the crust of the earth in every country presents, indications of a loss of heat, of a contraction in diameter, and its necessary consequence, fracture of the exterior consolidated sh.e.l.l along lines of least resistance? And though it was a.s.serted by some that the catastrophes of which these are the evidences were occasioned by forces of unparalleled energy and incessant operation--unparalleled when compared with such terrestrial forces as we are familiar with--that did not, in any respect, change the interpretation, for there could have been no abrupt diminution in the intensity of those forces, which, if they had lessened in power, must have pa.s.sed through a long, a gradual decline. [Sidenote: These necessarily imply long time.] In that very decline there thus spontaneously came forth evidences of a long lapse of time. The whole course of Nature satisfies us how gradual and deliberate are her proceedings; that there is no abrupt boundary between the past and the present, but that the one insensibly shades off into the other, the present springing gently and imperceptibly out of the past. If volcanic phenomena and all kinds of igneous manifestations--if dislocations, injections, the intrusion of melted material into strata were at one time more frequent, more violent--if, in the old times, mundane forces possessed an energy which they have now lost, their present diminished and deteriorated condition, coupled with the fact that for thousands of years, throughout the range of history, they have been invariably such as we find them now, should be to us a proof how long, how very long ago those old times must have been.

[Sidenote: Support from astronomical facts.] Thus, therefore, was perceived the necessity of co-ordinating the scale of time with the scale of s.p.a.ce, and such views of the physical history of the earth were extended to celestial bodies which were considered as having pa.s.sed through a similar course. In one, at least, this a.s.sertion was no mere matter of speculation, but of actual observation. The broken surface of the moon, its volcanic cones and craters, its mountains, with their lava-clad sides and ejected blocks glistening in the sun, proved a succession of events like those of the earth, and demonstrated that there is a planetary as well as a terrestrial geology, and that in our satellite there is evidence of a primitive high temperature, of a gradual decline, and, therefore, of a long process of time. Perhaps also, considering the rate of heat-exchange in Venus by reason of her proximity to the sun, the pale light which it is said has been observed on her non-illuminated part is the declining trace of her own intrinsic temperature, her heat lasting until now.

[Sidenote: Astronomical facts imply slow secular changes.] If astronomers sought in systematic causes an explanation of these facts if, for instance, they were disposed to examine how far changes in the obliquity of the ecliptic are connected therewith--it was necessary at the outset to concede that the scale of time on which the event proceeds is of prodigious duration, this secular variation observing a slow process of only 457'' in a century; and hence, since the time of Hipparchus, two thousand years ago, the plane of the ecliptic has approached that of the equator by only a quarter of a degree. Or if, again, they looked to a diminishing of the eccentricity of the earth's...o...b..t, they were compelled to admit the same postulate, and deal with thousands of centuries. Under whatever aspect, then, the theory was regarded, if once a former high temperature were admitted, and the fact coupled therewith that there has been no sensible decline within the observation of man, whether the explanation was purely geological or purely astronomical, the motion of heat in the ma.s.s of the earth is so slow, yet the change that has taken place is so great, the variations of the contemplated relations of the solar system so gradual--under whatever aspect and in whatever way the fact was dealt with, there arose the indispensable concession of countless centuries.

To the astronomer such a concession is nothing extraordinary. It is not because of the time required that he entertains any doubt that the sun and his system accomplish a revolution round a distant centre of gravity in nineteen millions of years, or that the year of epsilon Lyrae is half a million of ours. He looks forward to that distant day when Sirius will disappear from our skies, and the Southern Cross be visible, and Vega the polar star. He looks back to the time when gamma Draconis occupied that conspicuous position, and the builders of the great pyramid, B.C. 3970, gave to its subterranean pa.s.sage an inclination of 26 15', corresponding to the inferior culmination of that star. He tells us that the Southern Cross began to be invisible in 52 30' N., 2900 years before our era, and that it had previously attained an alt.i.tude of more than 10. When it disappeared from the horizon of the countries on the Baltic, the pyramid of Cheops had been erected more than a thousand years.

[Sidenote: Proofs of time from aqueous effects,] We must pa.s.s by a copious ma.s.s of evidence furnished by aqueous causes of change operating on the earth's surface, though these add very weighty proof to the doctrine of a long period. The filling up of lakes, the formation of deltas, the cutting power of running water, the deposit of travertines, the denudation of immense tracts of country, the carrying of their detritus into the sea, the changes of sh.o.r.es by tides and waves, the formation of strata hundreds of miles in length, and the imbedding therein of fossil remains in numbers almost beyond belief, furnished many interesting and important facts. Of these not a few presented means of computation. It would not be difficult to a.s.sign a date for such geographical events as the production of the Caspian and Dead Seas from an examination of the sum of saline material contained in their waters and deposited in their bed, with the annual amount brought into them by their supplying rivers. Such computations were executed as respects the growth of Lower Egypt and the backward cutting of Niagara Falls, and, though they might be individually open to criticism, their mutual accordance and tendency furnished an evidence that could not be gainsaid. The continual acc.u.mulation of such evidence ought not to be without its weight on those who are still disposed to treat slightingly the power of geological facts in developing truth.

[Sidenote: and from the movements of the earth's crust.] To such facts were added all those, with which volumes might be filled, proving the universality of the movements of the solid crust of the earth--strata once necessarily horizontal now inclined at all angles, strata unconformable to one another--a body of evidence most copious and most satisfactory, yet demonstrating from the immensity of the results how slowly the work had gone on.

How was it possible to conceive that beds many hundred feet in thickness should have been precipitated suddenly from water? Their mechanical condition implied slow disintegration and denudation in other localities to furnish material; their contents showed no trace of violence; they rather proved the deposition to have occurred in a tranquil and quiet way. What interpretation could be put upon facts continually increasing in number like those observed in the south-east of England, where fresh-water beds a thousand feet thick are covered by other beds a thousand feet thick, but of marine origin? What upon those in the north of England, where ma.s.ses once uplifted a thousand feet above the level, and, at the time of their elevation, presenting abrupt precipices and cliffs of that height, as is proved by the fractures and faults of the existing strata, have been altogether removed, and the surface left plain? In South Wales there are localities where 11,000 feet in thickness have been bodily carried away. Whether, therefore, the strata that have been formed, and which remain to strike us with astonishment at their prodigious ma.s.s, were considered; or those that have been destroyed, not, however, without leaving unmistakable traces of themselves; the processes of wearing away to furnish material as well as the acc.u.mulation, of necessity required the lapse of long periods of time. The undermining of cliffs by the beating of the sea, the redistribution of sands and mud at the bottom of the ocean, the washing of material from hills into the lowlands by showers of rain, its transport by river courses, the disintegration of soils by the influence of frost, the weathering of rocks by carbonic acid, and the solution of limestone by its aid in water--these are effects which, even at the quickest, seem not to amount to much in the course of the life of a man.

A thousand years could yield but a trifling result.

We have already alluded to another point of view from which these mechanical effects were considered. The level of the land and sea has unmistakably changed. There are mountain eminences ten or fifteen thousand feet in alt.i.tude in the interior of continents over which, or through which sh.e.l.ls and other products of the sea are profusely scattered. And though, considering the proverbial immobility of the solid land and the proverbial instability of the water, it might at first be supposed much more likely that the sea had subsided than that the land had risen, a more critical examination soon led to a change of opinion. Before our eyes, in some countries, elevations and depressions are taking place, sometimes in a slow secular manner, as in Norway and Sweden, that peninsula on the north rising, and on the south sinking, at such a rate that, to accomplish the whole seven hundred feet of movement, more than twenty-seven thousand years would be required if it had always been uniform as now. Elsewhere, as on the south-western coast of South America, the movement is paroxysmal, the sh.o.r.e line lifting for hundreds of miles instantaneously, and then pausing for many years. In the Morea also, range after range of old sea cliffs exist, some of them more than a thousand feet high, with terraces at the base of each; but the Morea has been well known for the last twenty-five centuries, and in that time has undergone no material change. Again, in Sicily, similar interior sea-cliffs are seen, the rubbish at their bases containing the bones of the hippopotamus and mammoth, proofs of the great change the climate has undergone since the sea washed those ancient beaches. Italy, pre-eminently the historic country, in which, within the memory of man, no material change of configuration has taken place since the Pleistocene period, very late geologically speaking has experienced elevations of fifteen hundred feet. The seven hills of Rome are of the Pliocene, with fluviatile deposits and recent terrestrial sh.e.l.ls two hundred feet above the Tiber. There intervened between the older Pliocene and the newer a period of enormous length, as is demonstrated by the acc.u.mulated effects taking place in it, and, indeed, the same may be said of every juxtaposed pair of distinctly marked strata. It demanded an inconceivable time for beds once horizontal at the bottom of the sea to be tilted to great inclinations; it required also the enduring exertion of a prodigious force. Ascent and descent may be detected in strata of every age: movements sometimes paroxysmal, but more often of tranquil and secular kind. The coal-bearing strata, by gradual submergence, attained in South Wales a thickness of 12,000 feet, and in Nova Scotia, a total thickness of 14,570 feet; the uniformity of the process of submergence and its slow steadiness is indicated by the occurrence of erect trees at different levels: seventeen such repet.i.tions may be counted in a thickness of 4515 feet. The age of the trees is proved by their size, some being four feet in diameter. Round them, as they gradually went down with the subsiding soil, calamites grew at one level after another. In the Sidney coal-field fifty-nine fossil forests thus occur in superposition.

[Sidenote: Organic proofs of a former high temperature.] Such was the conclusion forcing itself from considerations connected with inorganic nature. It received a most emphatic endors.e.m.e.nt from the organic world, for there is an intimate connexion between the existence and well-being both of plants and animals, and the heat to which they are exposed. Why is it that the orange and lemon do not grow in New York? What is it that would inevitably ensue if these exotics were exposed to a cold winter?

What must take place if, in Florida or other of the Southern states, a season of unusual rigor should occur? Does not heat thus confine within a fixed boundary the spread of these plants? And so, again, how many others there are which grow luxuriantly in a temperate climate, but are parched up and killed if fortuitously carried beneath a hot tropical sun. To every one there is a climate which best suits the condition of its life, and certain limits of heat and cold beyond which its existence is not possible.

If the mean annual heat of the earth's surface were slowly to rise, and, in the course of some centuries, the temperature now obtaining in Florida should obtain in New York, the orange and lemon would certainly be found here. [Sidenote: Boundary of organisms by heat.] With the increasing heat those plants would commence a northward march, steadily advancing as opportunity was given. Or, if the reverse took place, and for any reason the heat of the torrid zone declined until the winter's cold of New York should be at last reached under the equator, as the descent went on the orange and lemon would retreat within a narrow and narrower region, and end by becoming extinct, the conditions of their exposure being incompatible with the continuance of their life. From such considerations it is therefore obvious that not only does heat arrange the limits of the distribution of plants, erecting round them boundaries which, though invisible, are more insuperable than a wall of bra.s.s, it also regulates their march, if march there is to be--nay, even controls their very existence, and to genera, and species, and individuals appoints a period of duration.

[Sidenote: Animals localized as well as plants.] Such observations apply not alone to plants; the animal kingdom offers equally significant ill.u.s.trations. Why does the white bear enjoy the leaden sky of the pole and his native iceberg? Why does the tiger restrict himself to the jungles of India? Can it be doubted that, if the mean annual temperature should decline, the polar bear would come with his iceberg to corresponding southern lat.i.tudes, or, if the heat should rise, the tiger would commence a northward journey? Does he not, indeed, every summer penetrate northward in Asia as far as the lat.i.tude of Berlin, and retire again as winter comes on? Why is it that, at a given signal, the birds of pa.s.sage migrate, pressed forward in the spring by the heat, and pressed backward in the autumn by the cold? The annual migration of birds ill.u.s.trates the causes of geological appearances and extinctions.

Do we not herein recognize the agent that determines animal distribution? We must not deceive ourselves with any fancied terrestrial impediment or restraint. Let the heat rise but a few degrees, and the turkey-buzzard, to whose powerful wing distances are of no moment and the free air no impediment, would be seen hovering over New York; let it fall a few degrees, and he would vanish from the streets of Charleston; let it fall a little more, and he would vanish from the earth.

Sh.e.l.l-fish, once the inhabitants of the British seas, retired during the glacial period to the Mediterranean, and with the returning warmth have gone back northward again.

[Sidenote: Control of animals by food.] Animals are thus controlled by heat in an indirect as well as a direct way. Indirectly; for, if their food be diminished, they must seek a more ample supply; if it fails, they must perish. Doubtless it was insufficient food, as well as the setting in of a more rigorous climate, that occasioned the destruction of the mastodon giganteus, which abounded in the United States after the drift period. Such great elephantine forms could not possibly sustain themselves against the rigors of the present winters, nor could they find a sufficient supply of food for a considerable portion of the year.

The disappearance of animals from the face of the earth was, as Palaeontology advanced, ascertained to have been a determinate process, a condition of their existence, and either inherent in themselves or dependent on their environment. It was proved that the forms now existing are only an insignificant part of the countless tribes that have lived. [Sidenote: Nature of creations and extinctions.] The earth has been the theatre of a long succession of appearances and removals, of creations and extinctions, reaching to the latest times. In the Pleistocene of Sicily, 35/124 of the fossil sh.e.l.ls are extinct; in the bone caverns of England, out of thirty-seven mammals eighteen are extinct. But judging, from what may be observed of the duration of races contemporary with us, that their life is prolonged for thousands of years, successive generations of the same species in a long order replacing their predecessors before final removal occurs, this again resistlessly brought forward the same conclusion to which all the foregoing facts had pointed, that there have transpired since the introduction of animal life upon this globe very long periods of time.

Through the operation of this law of extinction and of creation, animated nature, both on the continents and in the seas, has undergone a marvellous change. In the lias and oolitic seas, the Enaliosauria, Cetiosauria, and Crocodilia dominated as the Delphinidae and Balaenidae do in ours; the former have been eliminated, the latter produced. Along with the cetaceans came the soft-scaled Cycloid and Ctenoid fishes, orders which took the place of the Ganoids and Placoids of the Mesozoic times. One after another successive species of air-breathing reptiles have emerged, continued for their appointed time to exist, and then died out. The development has been, not in the descending, but in the ascending order; the Amphitheria, Spalacotheria, Triconodon of the Mesozoic times were subst.i.tuted by higher tertiary forms. Nor have these mutations been abrupt. If mammals are the chief characteristic of the Tertiary ages, their first beginnings are seen far earlier; in the tria.s.sic and oolitic formations there are a few of the lower orders struggling, as it were, to emerge. The aspect of animated nature has altogether changed. No longer does the camelopard wander over Europe as he did in the Miocene and Pliocene times; no longer are great elephants seen in the American forests, the hippopotamus in England, the Rhinoceros in Siberia. The hand of man has introduced in the New the horse of the Old World; but the American horse, that ran on the great plains contemporary with the megatherium and megalonyx, has for tens of thousands of years been extinct. Even the ocean and the rivers are no exception to these changes.

[Sidenote: Creations and extinctions by law.] What, then, is the manner of origin of this infinite succession of forms? It is often sufficient to see clearly a portion of a plan to be able to determine with some degree of certainty the general arrangement of the whole; it is often sufficient to know with precision a part of the life of an individual to guess with probable accuracy his action in some forthcoming event, of to determine the share he has borne in affairs that are past. It is enough to appreciate thoroughly the style of a master to ascertain without doubt the authenticity of an imputed picture. And so, in the affairs of the universe, it is enough to ascertain the manner of operation of a part in order to settle the manner of operation of the whole. When, therefore, it was perceived how the disappearance of vanishing forms from the surface of the globe is accomplished--that it is not by a sudden and grand providential intervention--that there is no visible putting forth of the Omnipotent hand, but slowly and silently, yet surely, the ordinary laws of Nature are permitted to take their course--that heat, and cold, and want of food, and dryness, and moisture, in the end, as if by an irresistible destiny, accomplish the event, it seemed to indicate that, as regards the introduction of new-comers, a suitableness of external conditions had called them forth, as an unsuitableness could end them. Changes in the const.i.tution of the air or its pressure, in the composition of the sea or its depth, in the brilliancy of light or the amount of heat, in the inorganic material of a medium, will modify old forms into new ones, or compel their extinction. Birth and death go hand in hand; creation and extinction are inseparable. The variation of organic form is continuous; it depends upon an orderly succession of material events; appearances and eliminations are managed upon a common principle; they stand connected with the irresistible course of great mundane changes. It was impossible that geologists could reach any other conclusion than that such phenomena are not the issue of direct providential interventions, but of physical influences. The procession of organic life is not a motley march; it follows the procession of physical events; and, since it is impossible to re-establish a sameness of physical conditions that have once come to an end, or reproduce the order in which they have occurred, it of necessity follows that no organic form can reappear after it has once died out--once dead, it is clean gone for ever.

[Sidenote: Interst.i.tial molecular creations.] In the course of the life of individual man, the parts that const.i.tute his system are undergoing momentary changes; those of to-day are not the same as those of yesterday, and they will be replaced by others to-morrow. There have been, and are every instant, interst.i.tial deaths of all the const.i.tuent particles, and an unceasing removal of those that have performed their duty. In the stead of departing portions, new ones have been introduced, interst.i.tial births and organizations perpetually taking place. In physiology it became no longer a question that all this proceeds in a determinate way under the operation of principles that are fixed, of laws that are invariable. The alchemists introduced no poetical fiction when they spoke of the microcosm, a.s.serting that the system of man is emblematical of the system of the world. The intercalation of a new organic molecule in a living being answers to the introduction of a new form in the universal organic series. It requires as much power to call into existence a living molecule as to produce a living being. Both are accomplished upon the same principle, and that principle is not an incessant intervention of a supernatural kind, but the operation of unvarying law. Physical agents, working through physical laws, remove in organisms such molecules as have accomplished their work and create new ones, and physical agents, working through physical laws, control the extinctions and creations of forms in the universe of life. The difference is only in the time. What is accomplished in the one case in the twinkling of an eye, in the other may demand the lapse of a thousand centuries.

[Sidenote: Defence of the process of all things by law.] The variation of organic forms, under the force of external circ.u.mstances, is thus necessary to be understood in connexion with that countless succession of living beings demonstrated by geology. It carries us, in common with so much other evidence, to the lapse of a long time. Nor are such views as those to which we are thus constrained inconsistent with the admission of a Providential guidance of the world. Man, however learned and pious he may be, is not always a trustworthy interpreter of the ways of G.o.d. In deciding whether any philosophical doctrine is consistent or inconsistent with the Divine attributes, we are too p.r.o.ne to judge of those attributes by our own finite and imperfect standard, forgetting that the only test to which we ought to resort is the ascertainment if the doctrine be true. If it be true, it is in unison with G.o.d. Perhaps some who have rejected the conception of the variation of organic forms, with its postulate--limitless duration, may have failed to remember the grandeur of the universe and its relations to s.p.a.ce and to time; perhaps they do not recall the system on which it is administered. Like the anthropomorphite monks of the Nile, they conceive of G.o.d as if he were only a very large man; else how could it for a moment have been doubted that it is far more--I use the expression reverently--in the style of the great Constructor to carry out his intentions by the summary operations of law? It might be consistent with the weakness and ignorance of man to be reduced to the necessity of personal intervention for the accomplishment of his plans, but would not that be the very result of such ignorance? Does not absolute knowledge actually imply procedure by preconceived and unvarying law? Is not momentary intervention altogether derogatory to the thorough and absolute sovereignty of G.o.d? The astronomical calculation of ancient events, as well as the prediction of those to come, is essentially founded on the principle that there has not in the times under consideration, and that there will never be in the future, any exercise of an arbitrary or overriding will. The corner-stone of astronomy is this, that the solar system--nay, even the universe, is ruled by necessity. To operate by expedients is for the creature, to operate by law for the Creator; and so far from the doctrine that creations and extinctions are carried on by a foreseen and predestined ordinance--a system which works of itself without need of any intermeddling--being an unworthy, an ign.o.ble conception, it is completely in unison with the resistless movements of the mechanism of the universe, with whatever is orderly, symmetrical, and beautiful upon earth, and with all the dread magnificence of the heavens.

[Sidenote: Historical sketch of early Palaeontology.] It was in Italy that particular attention was first given to organic remains. Leonardo da Vinci a.s.serts that they are real sh.e.l.ls, or the remains thereof, and hence that the land and sea must have changed their relative position.

At this time fossils were looked upon as rare curiosities, no one supposing that they were at all numerous, and many were the fantastic hypotheses proposed to account for their occurrence. Some referred them to the general deluge mentioned in Scripture; some to a certain plastic power obscurely attributed to the earth; some thought that they were engendered by the sunlight, heat, and rain. To Da Vinci is due the first clear a.s.sertion of their true nature, that they are actually the remains of organic beings. Soon the subject was taken up by other eminent Italians. Fracaster wrote on the petrifactions of Verona; Scilla, a Sicilian, on marine bodies turned into stone, ill.u.s.trating his work by engravings. Still later, Vallisneri, 1721, published letters on marine bodies found in rocks, attempting by their aid to determine the extent of the marine deposits of Italy. These early cultivators of geology soon perceived the advantage to be gained by the establishment of museums and the publication of catalogues. The first seems to have been that of John Kentman, an example that was followed by Calceolarius and Vallisneri.

Subsequently Fontanelle proposed the construction of charts in accordance with fossil remains; but the principle involved was not applied on the great scale as a true geological test until introduced by Smith in connexion with the English strata.