History of the Intellectual Development of Europe, Volume II (of 2)

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. Successive forms assumed by man. Physiological investigations have proved that all animals, even man, during the process of development, pass in succession through a definite cycle of forms. Starting from a simple cell, form after form, in a definite order is assumed. 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. 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 assumed by the individual, equally in a transitory manner are they assumed 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 passed through by the individual in the course of a few months, while in the race it demands ages. 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 God 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.

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 passage from form to form without catastrophes, the argument relied upon gathering strength from such circumstances as these, that even the fossil shells 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 passage from strata containing seventy per cent. of recent shells 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 assumption 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.

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. 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 pass 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 mass 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.

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 illustration that external physical agents had not undergone any well-marked variation in the time with which he was concerned.

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.

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.

Transmutation of species. The doctrine of the transmutation 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 transmutation 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."

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 repetitions 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 transmutation 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."

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 illustrated by the facts that occur on the habitable surface of a planet suffering a gradual decline of heat. 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 God. 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 God, 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 God. 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 circumstances force on our attention, is such a strict mathematical necessity.

Their relative probability. If at this point we balance the probabilities of these three explanations, we shall perhaps find ourselves biassed 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. Development is in place, not in time. The passing through a given form at a given epoch is due to the relation being to space 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 space, 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 quantity 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. 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.

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 constituents 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 constitution of the sea-water, as respects its salinity, was altered through difference of evaporation in different latitudes, an effect inevitably making a profound impression on marine animal life.

Nature of hereditary transmission. Supported by the facts that have been mentioned respecting the later fossils of Australia and Brazil, and their analogy 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 illustration 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 illustration, 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. 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.

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.