[Sidenote: The pre-organic time.] To Steno, a Dane, is due the recognition of pre-organic in contradistinction to organic rocks, a distinction the terms of which necessarily involve the idea of time.

Soon it became generally recognized that the strata in which organic remains occur are of a later date than those devoid of them, the pre-organic rocks demonstrating a pre-organic time. Moreover, as facts were developed, it was plain that there are essential differences in the relations of fossils, and that, though in Italy the same species of sh.e.l.ls may occur in the mountains that occur in the adjacent seas, this was very far from being the case uniformly elsewhere. At length the truth began to emerge, that in proportion as the strata under examination are of an older date, so are the differences between their organic remains and existing species more marked. It was also discovered that the same species often extends superficially over immense districts, but that in a vertical examination one species after another rapidly appears in a descending order--an order which could be verified in spite of the contortions, fractures, and displacements of the strata.

A very important theoretical conclusion was here presented: for the rapid succession of essentially different organic forms, as the rocks were older, was clearly altogether inconsistent with one catastrophe, as the universal deluge, to which it had been generally referred. It was plain that the thickness of the strata in which they were enveloped, and the prodigious numbers in which they occurred, answered in some degree to the period of life of those fossils, since every one of them, large or small, must have had its time of birth, of maturity, and of death.

[Sidenote: Insufficiency of a single catastrophe.] When, therefore, it could be no longer doubted that strata many hundreds of feet in thickness were crowded with such remains, it became altogether out of the question to refer their entombment to the confusion of a single catastrophe, for every thing indicated an orderly and deliberate proceeding. Still more cogent did this evidence become when, in a more critical manner, the fossils were studied, and some strata were demonstrated to be of a fresh-water and others of a marine origin, the one intercalated with the other like leaves in a book. To this fact may be imputed the final overthrow of the doctrine of a single catastrophe, and its replacement by a doctrine of periodical changes.

[Sidenote: The orderly progression of organization.] From these statements it will therefore be understood that, commencing with the first appearance of organization, an orderly process was demonstrated from forms altogether unlike those with which we are familiar, up to those at present existing, a procedure conducted so slowly that it was impossible to a.s.sign for it a shorter duration than thousands of centuries. Moreover, it seemed that the guiding condition which had controlled this secular march of organization was the same which still determines the possibility of existence and the distribution of life.

The succession of organic forms indicates a clear relation to a descending temperature. The plants of the earliest times are plants of an ultratropical climate, and that primitive vegetation seemed to demonstrate that there had been a uniform climate--a climate of high temperature--all over the globe. The coal-beds of Nova Scotia exhibited the same genera and species as those of Europe, and so well marked was the botanical connexion with the declining temperature in successive ages that attempts were made to express eras by their prevailing organisms; thus Brongniart's division is, for the Primary strata, the Age of Acrogens; the Secondary, exclusive of the Cretaceous, the Age of Gymnogens; the third, including the Cretaceous and Tertiary, the Age of Angiosperms. It is to be particularly remarked that the Cretaceous flora, in the aggregate, combines the antecedent and succeeding periods, proving that the change was not by crisis or sudden catastrophe, but that the new forms rose gently among the old ones. After the Eocene period, dicotyledonous angiosperms became the prevalent form, and from that date to the Pleistocene the evidences of a continued refrigeration are absolute.

[Sidenote: Climates in time and in place.] As thus an examination was made from the most ancient to the later ages, indications were found of a climate arrangement more and more distinct--in the high lat.i.tudes, from the ultratropical through the tropical, the temperate, down to the present frigid state; in lower lat.i.tudes the declining process stopping short at an earlier point. It therefore appeared that there has been a production of climates both in an order of time and, in an order of locality, the greatest change having occurred in the frigid zone, which has pa.s.sed through all mean temperatures, an intermediate change in the temperate, and a minimum in the torrid zone. The general effect has thus been to present a succession of surfaces on the same planet adapted to a varied organization, and offering a more magnificent spectacle than if we were permitted to inspect many different planets; for in them there might be no necessary connexion of their forms of life, but in this there is, so that, were our knowledge of Comparative Physiology more perfect, we might amuse ourselves with intercalating among the plant and animal organisms familiar to us hypothetical forms that would make the series complete, and verify our principles by their subsequent discovery in the deep strata of the earth.

Does not this progression of life in our planet suggest a like progression for the solar system, which in its aggregate is pa.s.sing in myriads of years through all organic phases? May we not also, from our solar system, rise to a similar conception for the universe?

There are two very important considerations, on which we must dwell for the complete understanding of the consequences of these changes: 1st.

The mechanism of the declining temperature; 2d. Its effect in the organic world.

[Sidenote: The nature of terrestrial declining temperature.] 1st. A uniformly high temperature could never be manifested all over the surface of our planet through any heating influence of the sun. A high and uniform temperature unerringly points to an internal cause; and the gradual appearance of climates, manifesting a relatively increasing power of the sun, indicates the slow diminution of that internal heat.

But this is precisely the conclusion which was come to from a contemplation of the earth from a purely physical point of view. So long as its intrinsic heat overpowered that derived from the sun, it was not possible that any thing answering to climates could be established; and, until a certain degree of cooling by radiation had been accomplished, the heat must have been comparatively uniform in all lat.i.tudes; but, that point gained, there necessarily ensued an arrangement of zones of different temperatures, or, in other words, climates appeared, the process being essentially slow, and becoming slower as the loss of heat went on. Finally, when loss of heat from the earth ceased, an equilibrium was reached in the climate arrangement as we now find it.

Thus purely physical as well as geological considerations brought philosophers on this point to the same conclusion--that conclusion which has been so often repeated--very long periods of time.

[Sidenote: Consequent effect on the Flora and Fauna.] 2nd. As to the effect on the organic world. Nothing can live at a temperature higher than the boiling-point of water, for the condition of life implies that there shall circulate from part to part of a living mechanism a watery liquid, sap, or blood. From this it necessarily follows that a planet, the temperature of which is above a certain limit, must necessarily have a lifeless surface; and this seemed to be the interpretation of that pre-organic time to which we have referred. Moreover, when the temperature suitably descends so as to come within the limit at which life is possible, its uniformity over the surface of a planet will produce a sameness in the organization. It would be an ident.i.ty if heat were the only regulating condition of life. At this stage of things, the solar heat being overpowered, and a sensibly uniform temperature in all lat.i.tudes existing, still the only possible organic forms are those consistent with a high temperature, uniformity in the physical condition impressing a general uniformity in the aspect of life geographically.

[Sidenote: Production and distribution of new organisms.] But the moment that climate arrangement has become possible, variety of organic form becomes possible. Now also ensues another all-important result--geographical distribution. Both of plants and animals, those whose vital conditions are inconsistent with the occurring change must retire from the affected locality. In plants this retrocession is brought to pa.s.s by the gradual sickening and death of individuals, or the impossibility of reproduction; in animals there is added thereto, because of their power of locomotion, voluntary retirement, at least in the case of individuals, and immobility in the species is corrected by locomotion in the individual. The affected region has become unsuitable, cheerless, uncomfortable; they abandon it; and as the boundary they thus, in the one case, can not, and in the other will not overpa.s.s, advances, so do they recede before it. If the change were abrupt, or took place by a sudden crisis, there would seem to be no other possible event than an overcrowding of the unaffected region and a desolation of the part that had varied. But, since a developing cell under a new condition produces a new form, and since the physical change is taking place with extreme slowness, the appearance of modified structures ensues. And thus, by decline of temperature, two distinct results are accomplished--first the production of organic forms in an order of succession, new ones replacing the old, as if they were trans.m.u.tations of them, and, secondly, geographical distribution.

[Sidenote: Delusive nature of organic equilibrium.] In my "Physiology" I have endeavoured to explain in detail the principles here set forth. I have endeavoured to show that the aspect of sameness presented by an animal or plant is no proof of unchangeability. Those forms retain in our times their special aspect because the conditions of the theatre in which they live do not change; but let the mean temperature rise, let the sun-rays become brighter, change the composition of the air, and forthwith the world of organization would show how profoundly it was affected. Nor need such changes, in one sense, be more than insignificant to produce prodigious results. Thus the air contains only 1/2000 of its volume of carbonic acid gas. That apparently trifling quant.i.ty taken away, in an instant the whole surface of the earth would become a desolate waste, without the possibility of vegetable life.

[Sidenote: The Coal period.] As physical geology advanced, the Coal period was perceived to be the chief epoch in the history of our planet.

Through a slow decline of temperature, a possibility had gradually been attained, so far as the condition of heat was concerned, for a luxuriant vegetable growth. All that prodigious ma.s.s of carbon now found in the earth in the various forms of coal existed as carbonic acid in the atmosphere. The proportion of free oxygen was less than at present by a volume equal to the excess of carbonic acid. [Sidenote: Effects of light on the atmosphere,] A change in the const.i.tution of this primaeval atmosphere was occasioned by the action of the light; for, under the influence of the sun-rays, plants decompose carbonic acid, appropriating its carbon, and, for the most part, setting the oxygen free. The quant.i.ty of carbon which can thus be condensed for the use of a plant, and, indeed, every such decomposing action by light, is directly proportionate to the quant.i.ty of light consumed, as experiments which I have personally made have proved. For the production of so great a weight of combustible matter a very long period of time was necessarily required, that the sun might supply the necessary luminous influence.

Age after age the sunbeams continued their work, changing the mechanical relations and composition of the atmosphere, the const.i.tution of the sea, and the appearance of the surface of the earth. There was a prodigious growth of ferns, lepidodendra, equisetaceae, coniferae. The percentage of oxygen in the air continually increased, that of carbonic acid continually declined; the pressure of the air correspondingly diminished, partly because of the replacement of a heavy gas by a lighter one, and partly because of the general decline of temperature slowly taking place, which diminished the absolute volume of vapour.

[Sidenote: and also on the sea.] The sea, in its deepest abysses, was likewise affected by the sunlight; not directly, but in an indirect way; for, as the removal of carbonic acid from the atmosphere went on, portions of that gas were perpetually surrendered by the ocean in order to maintain a diffusion-equilibrium between its dissolved gas and the free gas of the air. And now no longer could be held in transparent solution by the water those great quant.i.ties of carbonate of lime which had once been concealed in it, the deposit of a given weight of coal in the earth being inevitably followed by the deposit of an equivalent weight of carbonate of lime in the sea. This might have taken place as an amorphous precipitate; but the probabilities were that it would occur, as in fact it did, under forms of organization in the great limestone strata coeval with and posterior to the coal. The air and the ocean were thus suffering an invisible change through the disturbing agency of the sun, and the surface of the solid earth was likewise undergoing a more manifest, and, it may be said, more glorious alteration. Plants, in wild luxuriance, were developing themselves in the hot and dank climate, and the possibility was now approaching for the appearance of animal types very much higher than any that had yet existed. [Sidenote: Cold-blooded animals succeeded by hot.] In the old heavy atmosphere, full of a noxious gas, none but slowly-respiring cold-blooded animals could maintain themselves; but after the great change in the const.i.tution of the air had been accomplished, the quickly-respiring and hot-blooded forms might exist. Hitherto the highest advancement that animal life could reach was in batrachian and lizard-like organisms; yet even these were destined to partic.i.p.ate in the change, increasing in magnitude and vital capacity. The pterodactyl of the chalk, a flying lizard, measures nearly seventeen feet from tip to tip of its wings. The air had now become suitable for mammals, both placental and implacental, and for birds. One after another, in their due order, appeared the highest vertebrates: marine, as the cetacean; aerial, as the bat; and in the terrestrial, reaching, in the Eocene, quadrumanous animals, but not, until after the Pliocene, man.

[Sidenote: The date of organisms may change, but the order not.]

Although the advance of geology may hereafter lead to a correction of some of the conclusions thus attained to respecting the first dates of different organic forms, and carry them back to more ancient times, it is scarcely likely that any material modification of their order of occurrence will ever be made. Birds, mammals, reptiles, fishes, and invertebrates may each be detected in earlier strata; even in some of those formations now regarded as non-fossiliferous, organisms may be found; but it is not at all probable that the preponderance of reptiles will ever cease to be the essential characteristic of the Secondary rocks, or that of mammals of the Tertiary, or that a preceding period of vast duration, in which the type of life had been the invertebrate, will ever be doubted. Nothing, probably, will ever be discovered to invalidate the physical conclusion that, while there was an excess of carbonic acid in the air, the Flora would tend to be Cryptogamic and Gymnospermic, and that there would be a scarcity of monocotyledons and dicotyledonous angiosperms in the coal; nothing to disprove the fact that the animals were slow-breathing and cold-blooded; and that it was not until after the oxygen of the air had increased and the mean temperature had declined that birds made their appearance. Though both placental and marsupial animals may hereafter be found earlier than in the Stonesfield slate; though wood and herb-eating beetles, gra.s.shoppers, dragon-flies, and May-flies may be found beneath the lias, and scorpions and c.o.c.kroaches beneath the coal, though, also beneath the coal, salamanders and Sauroid batrachians, of which the archegosaurus is an example, may occur; though reptiles, as the telerpeton, may be found deeper than the old red sandstone; yet the connexion between aerial const.i.tution and form of life will never be shaken. Still will remain the facts that the geographical distribution of types was anterior to the appearance of existing species, that organisms first appeared in a liquid medium, primitively marine, then fluviatile, and at last terrestrial; that Radiates, Molluscs, Articulates, Vertebrates, were all at first aquatic, and that the Radiates have ever remained so; that the plane of greatest vital activity has ever been the sea-level, where the earth and air touch each other; that the order of individual development is the order of mundane development. Still will remain the important conclusions that the mammalian Fauna has diverged more rapidly than the testaceous; that hot-blooded animals have not had that longevity of species which has been displayed by the cold, just as we observe in the individual the possibility of muscular contraction by a given galvanic force lasts much longer in the latter than in the former; that if the hot-blooded tribes have thus a briefer duration, they enjoy a compensation in the greater energy of their life--perhaps this being the cause and that the effect; that, notwithstanding the countless forms exhibited by species, their duration is so great that they outlive vast changes in the topographical configuration of countries--the Fauna of some countries having been in existence before those countries themselves; that the plan of individual development has ever been as it is now, and that sameness of external influence produces similarity of organization.

[Sidenote: The doctrine of catastrophes and uniformity.] In its early history theoretical geology presented two schools--one insisting on a doctrine of catastrophes, one on a doctrine of uniformity. The former regarded those changes which have manifestly taken place in the history of our planet as having occurred at epochs abruptly. To this doctrine the prevailing impression that there had been providential interventions lent much force. The other school, reposing on the great principle of the invariability of the laws of Nature, insisted that affairs had always gone on at the same rate and in the same way as they do now.

Hence it maintained an opposition to the catastrophists, and in this, it may be said, was actually not true to its own principles. Any doctrine of uniformity, rightly considered from its most general point of view, includes an admission of catastrophes. Numerous ill.u.s.trations of this truth spontaneously suggest themselves. A tower, the foundations of which are slowly yielding, may incline more and more for many centuries, but the day must come in which it will fall at last. In the uniformity of the disturbance a catastrophe was eventually involved. And thus, in what has been said respecting geological events, though they are spoken of as proceeding quietly and with uniformity, it may be understood that sudden crises are also contemplated. Moreover, those who adopt the doctrine of uniformity in an absolute sense must pay a due regard to the variations in intensity of physical acts which their own principles imply. The uniform cooling of a hot body actually means a cooling at first fast, and then slower and slower; and invariability of chemical change actually implies more violent and summary modifications at a high temperature than at one which is low.

But, though it may at first sight have appeared that an admission of the doctrine of catastrophes is in harmony with a providential government of the world, and that the emergence of different organic forms in successive ages is a manifestation of creative intervention, of which it was admitted that as many as from twelve to twenty, if no more, successive instances might be recognized, we may well congratulate ourselves that those important doctrines rest upon a far more substantial basis. Rightly considered, the facts lead to a very different conclusion. [Sidenote: Successive forms a.s.sumed by man.]

Physiological investigations have proved that all animals, even man, during the process of development, pa.s.s in succession through a definite cycle of forms. Starting from a simple cell, form after form, in a definite order is a.s.sumed. In this long line of advance the steps are ever, in all individuals, the same. But no one would surely suppose that the changed aspect at any moment presented is due to a providential interposition. [Sidenote: But they are rigidly determined by law.] On the contrary, it is the inevitable result of what has been taking place under the law of development, and the sure precursor of what is about to follow. In the organic world, the successive orders, and genera, and species are the counterparts of these temporary embryonic forms of the individual. Indeed, we may say of those successive geological beings that they are mere embryos of the latest--embryos that had gained a power of reproduction. How shall we separate the history of the individual from the history of the whole? Do not the fortunes and way of progress of the one follow the fortunes and way of progress of the other? If, in a transitory manner, these forms are a.s.sumed by the individual, equally in a transitory manner are they a.s.sumed by the race.

Nor would it be philosophical to suppose that the management in the one instance differs from the management in the other. If the one is demonstrably the issue of a law in action, so must the other be too. It does not matter that the entire cycle is pa.s.sed through by the individual in the course of a few months, while in the race it demands ages. [Sidenote: Individual and race development conducted in the same way.] The standard of time that ought to be applied is the respective duration of life. In man it is much if he attains to threescore years and ten; but the entire period of human record, embracing several thousand years, offers not a single instance of the birth, maturity, and death of a species. They, therefore, who think they find, in the successive species that have in an orderly manner replaced each other in the life of the earth, the sure proof of Divine intervention, would do well to determine at what point the production of such forms by law ceases, and at what point their production by the immediate act of G.o.d begins. Their task will be as hard to tell where one colour in the rainbow ends and where the next commences. They will also do well to remember that, in great mundane events, the scale of time is ample, and that there may be no essential difference between a course that is run over in a few days and one that requires for its completion thousands of centuries.

[Sidenote: Catastrophes disproved by the co-existence of types.] The co-existence of different types in the organic series was the incontrovertible fact by which was demonstrated the gradual pa.s.sage from form to form without catastrophes, the argument relied upon gathering strength from such circ.u.mstances as these, that even the fossil sh.e.l.ls of the modern Italian tuffs which are not extinct exhibit a slight want of correspondence when compared with those now inhabiting the Mediterranean, some of the old ones being twice and a half as large as the present, and that there is a numerical pa.s.sage from strata containing seventy per cent. of recent sh.e.l.ls to those that are altogether recent, or contain one hundred per cent. This is manifestly indicative of a continually changing impression bringing on a corresponding modelling. It is the proof of a slow merging into, or of a measured a.s.sumption of, the new form--a transition, for the completion of which probably a very long time is required. That the existing reindeer is found in the same fluviatile deposits with an extinct hippopotamus seemed certainly to prove that there was a condition of things in which the co-life of those animals was possible in the same locality, and that, as the physical causes slowly changed, the one might be eliminated and the other might be left. That the regulating conditions were altogether physical was obvious from such facts as that in the bone-caves of Australia all the mammals are marsupial, and in the pampas of South America they are allied to such forms as are indigenous, armadilloes, sloths, etc., showing the tokens of lineage or hereditary transmission. For still more remote times numerous instances of a similar nature were detected; thus, throughout the whole Secondary period, the essential characteristic was the wonderful development of reptile life, while in the Tertiary it was the development of mammals.

But the appearance of mammals had commenced long before that of reptiles had ceased. Indeed, the latter event is incomplete in our times; for, though the marine Saurians have been almost entirely removed, the fluviatile and terrestrial ones maintain themselves, though diminished both in species and individuals. Now such an overlapping of reptiles and mammals was altogether irreconcilable with the doctrine of a crisis or catastrophe, and, in fact, it demonstrated the changing of organisms in the changing of physical states.

[Sidenote: Cuvier's doctrine of permanence of species.] Cuvier maintained the doctrine of the permanence of animal species from the facts that the oldest known do not appear to have undergone any modification, and that every existing one shows a resistance to change.

If his observations are restricted to periods not exceeding human history, they may perhaps be maintained, but that duration cannot be looked upon as more than a moment in the limitless progress we are considering, and it was in this view that Cuvier's doctrine proved to be incapable of defence. [Sidenote: Imperfection of evidence in its support.] What does it signify if our domestic animals show no variations when compared with the corresponding images depicted on the hieroglyphic monuments of Egypt, or with the descriptions left by ancient authors? Evidence of that kind is valueless. Does the geologist ask of the architect his opinion whether there have ever been upliftings and down-sinkings of the earth? If he did, would not every structure in Europe be brought forward as an evidence that nothing of the kind had ever occurred? A leaning tower, or a church with inclining walls in Italy, might pa.s.s for nothing; the Pyramids would testify that Egypt itself had never undergone any disturbance--they remain solid on their bases, undisturbed. But what is the weight of all this when placed in opposition with the ma.s.s of evidence offered by inclined and fractured strata? And yet such is precisely the proof offered in behalf of the permanence of animals. The facts with which the zoologist deals, like those on which the architect depends, are insufficient for the purpose--they are wanting in extent of time. There have been movements in the crust of the earth, though every building in the world may be perpendicular; there have been transformations of organisms, though for four thousand years there may have been no perceptible change.

[Sidenote: Control of organisms by physical conditions.] If ever there had been a universal creation of all possible organic forms or combinations, forthwith vast numbers of them must have disappeared, every type being eliminated which was not in correspondence with the external conditions or with the medium in which it was placed. If the environment or the physical conditions underwent a variation, a corresponding variation in the forms that could by possibility exist must ensue, and, from a thorough study of those not eliminated, the physical conditions might be ascertained; and conversely, from a thorough knowledge of the physical conditions, the forms that could escape elimination might be designated. The facts on which Cuvier rested did not demonstrate what he supposed. His immobility of species was no consequence of an innate or intrinsic resistance possessed by them, but merely an ill.u.s.tration that external physical agents had not undergone any well-marked variation in the time with which he was concerned.

[Sidenote: Nature of variation of physical conditions.] What is here meant by variation in physical forces or condition is not any intrinsic change in their nature, but the varied manner in which they may work by interfering with one another, or experiencing declines of intensity.

From the fact that we may read in the fixed stars, through the progressive motion of light, the history of a million of past years, we may be sure that the forces of nature have undergone no intrinsic change; that light was propagated at the same rate, was capable of producing the same optical and chemical effects, and varied in its intensity by distance as it does now; that heat determined corporeal magnitudes. These are things that in their nature are absolutely unchangeable. Always, as now, the freezing of water, and its boiling under a given pressure, must have been the same; there must have been a thermometric zero of life and an upward limit, no animal process ever going on below 32 Fahrenheit or above 212 Fahrenheit.

[Sidenote: Effect thereof on organisms.] But out of this invariability of natural causes variations in their condition of action arise, and it is these that affect organic forms. Of such forms, some become at length incapable of maintaining themselves in the slow progress of change; others acclimatize, or accommodate, or suit themselves thereto by undergoing modifications, and this was at last discerned to be the true explanation of extinctions and appearances, events taking place very slowly in untold periods of time, and rather by imperceptible degrees than by a sudden catastrophe or crisis.

[Sidenote: Trans.m.u.tation of species.] The doctrine of the trans.m.u.tation of species has met with no little resistance. They who have refused to receive it as one of the truths of Nature have perhaps not given full weight to physiological evidence. When they ask, Has any one ever witnessed such an event as the trans.m.u.tation of one species into another? has any experimenter ever accomplished it by artificial means?

they do not take a due account of time. In the Fables it is related that when the flowers were one evening conversing, "Our gardener," said the rose to the lily, "will live for ever. I have not seen any change in him. The tulip, who died yesterday, told me that she had remarked the same thing; she believed that he must be immortal. I am sure that he never was born."

[Sidenote: Two modes of action.] Two modes have been presented by which we may conceive of the influence of physical agents upon organic forms.

Their long persistent action upon the individual may give rise to modifications, developing one part, stunting another; and such variations, being transmitted in an hereditary way, may become firmly fixed at last. Thus a given plant may, in the course of ages, under the influence of unremittingly acting physical conditions, undergo a permanent change, and a really new plant arise as soon as, through the repet.i.tions of successive generations, the modifications have become so thorough, so profound, as to be capable of transmission with certainty.

Perhaps this is what has taken place with many of our kitchen-garden plants, of which the special varieties may be propagated by seeds. But there is another mode by which that result may be reached, even if we decline the doctrine of St. Augustine, who, in his work "De Civitate Dei," shows how islands may be peopled with animals by "spontaneous generation." All organic forms originally spring from a simple cell, the development of which, as indicated by the final form attained, is manifestly dependent on the physical conditions it has been exposed to during its course. If those conditions change, that final form must change correspondingly; and in this manner, since all organic beings come from the same starting-point--the same cell, as has been said, which helplessly submits to whatever impression may be put upon it--the issue is the same as though a transformation or trans.m.u.tation had occurred, since the descendant is not like its ancestors. Such a manner of considering these changes is in harmony with our best physiological knowledge, since it does not limit itself to a small portion of the life of an individual, but embraces its whole cycle or career. For the more complete examination of this view I may refer to the second chapter of the second book of my "Physiology."

[Sidenote: Problem of the modification of forms.] But here has arisen the inquiry, Does the modification of organic forms depend exclusively on the impressions of external influences, or is it due to a nisus or force of development residing in the forms themselves?

Whether we consider the entire organic series in its succession, or the progress of an individual in his development, the orderly course presented might seem to indicate that the operation is taking place under a law--an orderly progression being always suggestive of the operation of law. But a philosophical caution must, however, be here exercised; for deceptive appearances may lead us into the error of imputing to such a law, impressed by the Creator on the developing organism, that which really belongs to external physical conditions, which, on their part, are following a law of their own. What is here meant may be ill.u.s.trated by the facts that occur on the habitable surface of a planet suffering a gradual decline of heat. [Sidenote: Three solutions of it.] On such a surface a succession of vegetable types might make its appearance, and, as these different types emerged or were eliminated, we might speak of the events as creations and extinctions, and therefore as the acts of G.o.d. Or, in the second place, we might refer them to an intrinsic force of development imparted to each germ, which reached in due season its maximum, and then declined and died out; and, comparing each type with its preceding and succeeding ones, the interrelation might be suggested to us of the operation of a controlling law. Or, in the third place, we might look to the external physical condition--the decline of heat--itself taking place at a determinate rate under a mathematical law, and drawing in its consequences the organic variations observed.

Now the first of these explanations in reality means the arbitrary and unchallengeable will of G.o.d, who calls into existence, and extinguishes according to his sovereign pleasure, whatever he pleases; the orderly progression we notice becoming an evidence that his volitions are not erratic, but are according to pure reason. The second implies that there has been impressed upon every germ a law of continuous organic variation--it might have been through the arbitrary fiat of G.o.d. The third implies that the successive types owe their appearance and elimination to a physical influence, which is itself varying under a strict mathematical necessity; for the law of cooling, which the circ.u.mstances force on our attention, is such a strict mathematical necessity.

[Sidenote: Their relative probability.] If at this point we balance the probabilities of these three explanations, we shall perhaps find ourselves bia.s.sed toward the last, as physiologists have been, because of its rigorous scientific aspect, and should not be surprised to find it supported by an array of facts depending on the principle that the appearance of new forms does not observe a certain inevitable order, or stand in a certain relation to time. From individual development it might seem as if the advancing procession of an organism is such that specific forms ever appear in a certain order one after another, and at certain intervals; but the fallacy of such a conclusion is apparent when we attend to the orderly procedure of the physical conditions to which the developing organism is exposed. [Sidenote: Development is in place, not in time.] The pa.s.sing through a given form at a given epoch is due to the relation being to s.p.a.ce and its conditions, not to time. And so in the life of the earth, if development were according to time, we should have an orderly succession of grades as the earth grew older, and in all localities, at a given moment, the contemporary organisms would be similar; but if it were according to s.p.a.ce, that rigorous procedure would not occur; in its stead we should have a broken series, the affiliation being dependent on the secularly continuous variation of the physical condition.

Now this was discovered to be the case. For instance, throughout the northern hemisphere, during the Tertiary period, an extinct placental Fauna was contemporaneous with an extinct marsupial Fauna in Australia.

If the development was proceeding according to time, by an innate nisus, and not according to external influences, the types for the same epoch in the two hemispheres should be the same; if under external influences, irrespective of time, they should be, as they were found to be, different.

If true-going clocks, which owe their motion to their own internal mechanism, were started in all countries of the earth at the same instant, they would strike their successive hours simultaneously. But sun-dials, which owe their indications to an exterior cause, would in different longitudes tell different times, or, when the needful light was absent, their shadows would altogether fail.

As to the vegetable kingdom, the principles that hold for the animal again apply. At a very early period, even before the deposit of the coal, all the distinct forms of vegetable tissue were in existence, and nothing to prevent, so far as time was concerned, their being united together all over the world into similar structural combinations. And, in truth, as the botany of the Coal period proves, there was a far more extensive sameness than we see at present, simply because the distribution of heat was more uniform and climates were less marked. But from this point the diversity of form in climate distribution becomes more and more conspicuous, though we must descend, perhaps, as late as the Wealden before we discover any flowering plants, except Gymnosperms, as Conifers and Cycads. All this is what might be expected on the doctrine of external influence, but not on the doctrine of an innate and interior developmental force.

If, at this stage, attention is once again turned to the animal kingdom, we find our opinion confirmed. The diminution of carbonic acid in the atmosphere, the deposit of coal in the earth, the precipitation of carbonate of lime in the sea, the disengagement of an increased quant.i.ty of oxygen in the air, and the reduction of atmospheric pressure--different effects contemporaneously occurring--were soon followed by the consequence which they made possible--the appearance of hot-blooded mammals. [Sidenote: Cold and hot-blooded animals.] Perhaps those first arising might, like our hibernates, lead a sluggish existence, with imperfect respiration; but, as the media improved and the temperature declined, more vigorous forms of life emerged, though we have probably to descend to the Tertiary epoch before we meet with birds, which of all animals have the most energetic respiration, and possess the highest heat.

[Sidenote: The organisms of the sea.] As with the atmosphere, so with the sea. Variations in its composition must control the organisms it contains. With its saline const.i.tuents its life must change. Before the sunlight had removed from the atmosphere so much of its carbonic acid, decomposing it through the agency of plants, the weight of carbonate of lime held in solution by the highly carbonated water was far greater than was subsequently possible, and the occurrence of limestone became a necessary event. With such a disturbance in the composition of the sea-water, its inhabiting organisms were necessarily disturbed. And so again, subsequently, when the solar heat began to preponderate on the surface over the subsiding interior heat, the const.i.tution of the sea-water, as respects its salinity, was altered through difference of evaporation in different lat.i.tudes, an effect inevitably making a profound impression on marine animal life.

[Sidenote: Nature of hereditary transmission.] Supported by the facts that have been mentioned respecting the later fossils of Australia and Brazil, and their a.n.a.logy to forms now existing in those countries, much stress was laid on the hereditary transmission of structure, and hence the inference was drawn that such examples are of a mixed nature, depending in part on external agency, in part on an interior developmental force. From marsupial animals, marsupials will issue; from placental ones, those that are placental. But here, perhaps, an ill.u.s.tration drawn from the inorganic kingdom may not be without interest and use. Two pieces of carbonate of lime may be rolling among the pebbles at the bottom of a brook, one perpetually splitting into rhomboids, the other into arragonitic prisms. The fragments differ from one another not only thus in their crystalline form, but in their physical qualities, as density and hardness, and in their optical qualities also. We might say that the calc-spar crystals gave birth to calc-spar crystals, and the arragonitic to arragonite; we might admit that there is an interior propensity, an intrinsic tendency to produce that result, just as we say that there is a tendency in the marsupial to engender a marsupial; but if, in our ill.u.s.tration, we look for the cause of that cause, we find it in a physical impression long antecedently made, that the carbonate of lime, crystallizing at 212 Fahr., produces arragonite, and, at a lower temperature, calc-spar; and that the physical impression thus accomplished, though it may have been thousands of years ago, was never cast off, but perpetually manifested itself in all the future history of the two samples. That which we sometimes speak of as hereditary transmission, and refer to an interior property, peculiarity, or force, may be nothing more than the manifestation of a physical impression long antecedently made.

In the last place, the idea of an intrinsic force of development is in connexion with time and a progression, and only comes into prominence when we examine a limited portion or number of the things under consideration. The earth, though very beautiful, is very far from being perfect. [Sidenote: The broken organic chain.] The plants and animals we see are only the wrecks of a broken series, an incomplete, and, therefore, unworthy testimonial of the Almighty power. We should judge very inadequately of some great author if only here and there a fragmentary paragraph of his work remained; and so, in the book of organization, we must combine what is left with what we can recover from past ages and buried strata before we can rise to a comprehension of the grand argument, and intelligibly grasp the whole work.

[Sidenote: Enormous age of the earth.] Of that book it is immaterial to what page we turn. It tells us of effects of such magnitude as imply prodigiously long periods of time for their accomplishment. Its moments look to us as if they were eternities. What shall we say when we read in it that there are fossiliferous rocks which have been slowly raised ten thousand feet above the level of the sea so lately as since the commencement of the Tertiary times; that the Purbeck beds of the upper oolite are in themselves the memorials of an enormous lapse of time; that, since a forest in a thousand years can scarce produce more than two or three feet of vegetable soil, each dirt-bed is the work of hundreds of centuries. What shall we say when it tells us that the delta of the Mississippi could only be formed in many tens of thousands of years, and yet that is only as yesterday when compared with the date of the inland terraces; that the recession of the Falls of Niagara from Queenstown to the present site consumed thirty thousand years; that if the depression of the carboniferous strata of Nova Scotia took place at the rate of four feet in a century, there were demanded 375,000 years for its completion--such a movement in the upward direction would have raised Mont Blanc; that it would take as great a river as the Mississippi two millions of years to convey into the Gulf of Mexico as much sediment as is found in those strata. Such statements may appear to us, who with difficulty shake off the absurdities of the patristic chronology, wild and impossible to be maintained, and yet they are the conclusions that the most learned and profound geologists draw from their reading of the Book of Nature.

[Sidenote: Summary as respects the world in time.] Thus, as respects the age of the earth and her relations in time, we approach the doctrine of Orientals, who long ago ascertained that the scales of time and of s.p.a.ce correspond to each other. More fortunate than we, they had but one point of resistance to encounter, but that resistance they met with dissimulation, and not in an open way. They attempted to conceal the tendency of their doctrine by allying or affiliating it with detected errors. According to their national superst.i.tion, the earth is supported on the back of an elephant, and this on a succession of animals, the last of which is a tortoise. It is not to be supposed that the Brahmans, who wrote commentaries on the Surya Siddhanta, should for a moment have accepted these preposterous delusions--that was impossible for such great geometers; yet led, perhaps, by a wish to do nothing that might disturb public feeling, they engaged in the hopeless task of showing that their profound philosophical discoveries were not inconsistent with the ancient traditions; that a globular and revolving earth might be sustained on a descending succession of supporting beasts. But they had the signal advantage over us that those popular traditions conceded to them that limitless time for which we have had to struggle.

[Sidenote: The life of the universe.] The progression of life on the surface of our planet is under the guidance of pre-ordained and resistless law--it is affiliated with material and correspondingly changing conditions. It suggests that the succession of organic forms which, in a due series, the earth's surface in the long lapse of time has presented, is the counterpart of a like progress which other planets in the solar system exhibit in myriads of years, and leads us to the conception of the rise, development, and extinction of a multiplicity of such living forms in other systems--a march of life through the universe, and its pa.s.sing away.

[Sidenote: Multiplicity of worlds implies succession of worlds.]

Magnitudes and times, therefore, go parallel with one another. With the abandonment of the geocentric theory, and of the doctrine of the human destiny of the universe, have vanished the unworthy hypotheses of the recent date of creation and the approaching end of all things. In their stead are subst.i.tuted more n.o.ble ideas. The multiplicity of worlds in infinite s.p.a.ce leads to the conception of a succession of worlds in infinite time. This existing universe, with all its splendours, had a beginning, and will have an end; it had its predecessors, and will have its successors; but its march through all its transformations is under the control of laws as unchangeable as destiny. As a cloud, which is composed of myriads of separate and isolated spherules of water, so minute as to be individually invisible, on a summer's afternoon changes its aspect and form, disappearing from the sky, and being replaced in succeeding hours by other clouds of a different aspect and shape, so the universe, which is a cloud of suns and worlds, changes in the immensity of time its form and fashion, and that which is contemporary with us is only an example of countless combinations of a like kind, which in ancient times have one after another vanished away. In periods yet to come the endless succession of metamorphoses will still go on, a series of universes to which there is no end.

CHAPTER X.

THE EUROPEAN AGE OF REASON--(_Continued_).