Popular Science Monthly/Volume 10/November 1876/Professor Huxley's Lectures I
THE THREE HYPOTHESES OF THE HISTORY OF NATURE.
WE live and form part of a system of things of immense diversity and perplexity, which we call Nature, and it is a matter of the deepest interest to all of us that we should form just conceptions of the constitution of that system and of its past history. With relation to this universe, man is, in extent, little more than a mathematical point; in duration, but a fleeting shadow. He is a reed shaken in the winds of force; but, as Pascal long ago remarked, although a reed, he is a thinking reed, and, in virtue of that wonderful capacity of thought, he has a power of framing to himself a symbolic conception of the universe, which, although doubtless highly imperfect, and although wholly inadequate as a picture of that great whole, is yet sufficient to serve him as a guide-book in his practical affairs. It has taken long ages of accumulated and often fruitless labor to enable man to look steadily at the. shifting scenes, phantasmagoria of Nature, to notice what is fixed among her fluctuations, and what is regular among her apparent irregularities; and it is only comparatively lately, within the last few centuries, that there has emerged the conception of a pervading order and a definite course of things, which we term the course of Nature.
But out of this contemplation of Nature, and out of man's thought concerning her, there has in these later times arisen that conception of the constancy of Nature to which I have referred, and which at length has become the guiding conception of modern thought. It has ceased to be almost conceivable to any person who is familiar with the facts upon which that conception is based, that chance should have any place in the universe, or that events should follow anything but the natural order of cause and effect. We have come to look upon the present as the child of the past and as the parent of the future; and, as we have excluded chance from any share or part in the order of things, so in the present order of Nature men have come to neglect, even as a possibility, the notion of any interference with that order. And, whatever may be men's speculative notions upon those points, it is quite certain that every intelligent person guides his life and risks his fortune upon the belief that the order of Nature is constant, and the relation of cause to effect unchanged.
In fact, there is no belief which we entertain which has so complete a logical basis as that to which I have just referred. It tacitly underlies every process of reasoning; it is the foundation of every act of the will. It is based upon the broadest induction, and it is verified by the most constant, regular, and universal of deductive processes. But we must recollect that any human belief, however broad its basis, however defensible it may seem, is, after all, only a probable belief, and that our broadest generalizations are simply statements of the highest degrees of probability. Though we are quite clear about the constancy of Nature at the present time, and in the present order of things, it by no means follows necessarily that we are justified in expanding this generalization into the past, and in denying absolutely that there may have been a time when Nature did not follow a fixed order, when the relations of cause and effect were not definite, and when external agencies did not intervene in the general course of Nature. Cautious men will admit that such a change in the order of Nature may have been possible, just as a very candid thinker may admit that there may be a world in which two and two do not make four, and in which two straight lines do not inclose a space. In fact, this question with which I have to deal in the three lectures I shall have the honor of delivering before you, this question as to the past order of Nature, is essentially an historical question, and it is one that must be dealt with in the same way as any other historical problem.
I will, if you please, in the first place, state to you what are the views which have been entertained respecting the order of Nature in the past, and then I will consider what evidence is in our possession bearing upon these views, and by what light of criticism that evidence is to be interpreted. So far as I know, there are only three hypotheses which ever have been entertained, or which well can be entertained, respecting the past history of Nature.
Upon the first of these the assumption is, that the order of Nature which now obtains has always obtained; in other words, that the present course of Nature, the present order of things, has existed from all eternity. The second hypothesis is, that the present state of things has had only a limited duration, and that at some period in the past the state of things which we now know (substantially, though not, of course, in all its details) arose and came into existence without any precedent condition from which it could have naturally proceeded. The third hypothesis also assumes that the present order of Nature has had but a limited duration, but it supposes that the present order of things proceeded by a natural process from an antecedent order, and that from another antecedent order, and so on; and on this hypothesis the attempt to fix any limit at which we could assign the commencement of this series of changes is given up. I am very anxious that you shall realize what these three hypotheses actually mean; that is to say; what they involve, if you can imagine a spectator to have been present during the period to which they refer. On the first hypothesis, however far back in time you place that spectator, he would have seen a world essentially, though not perhaps in all its details, similar to that which now exists. The animals which existed would be the ancestors of those which now exist, and like them; the plants in like manner would be such as we have now; and the supposition is that, at however distant a period of time you place your observer, he would still find mountains, plains, and waters, with animal and vegetable products flourishing upon them and sporting in them just as he finds now. That view has been held. It was a favorite fancy of antiquity, and has survived toward the present day. It is worthy of remark that it is an hypothesis which is not inconsistent with what geologists are familiar with as the doctrine of Uniformitarianism. That doctrine was held by Hutton, and in his earlier days by Lyell. For Hutton was struck with the demonstration of astronomers that the perturbations of the planetary bodies, however great they may be, yet sooner or later right themselves, and that the solar system contained within itself a self-adjusting power by which these aberrations were all brought back to an equilibrium.
Hutton imagined that something of the same kind may go on in the earth, although no one recognized more clearly than he the fact that the dry land is being constantly washed down by rain and rivers and deposited in the sea, and that thus in a certain length of time, greater or shorter, the inequalities of the earth's surface must be leveled, and its high lands brought down to the sea. Then, taking into account the internal forces of Nature, by which upheavals of the sea-bottom give rise to new land, he thought that these operations might naturally compensate each other, and thus, for any assignable time, the general features of the earth might remain what they are. And, inasmuch as there need be no limit under these circumstances to the propagation of animals and plants, it is clear that the logical development of this idea might lead to the conception of the eternity of the world. Not that I mean to say that either Hutton or Lyell held this conception—assuredly not; they would have been the first to repudiate it. But, by the arguments they used, it might have been possible to justify this hypothesis.
The second hypothesis is that to which I have referred as the hypothesis which supposes that the present order of things had at some no very remote time a sudden origin, so that the world, such as it now is, arose. That is the doctrine which you will find stated most fully and clearly in the immortal poem of John Milton, the English "Divina Commedia," "Paradise Lost." I believe it is largely to the influence of that remarkable work, combined with daily teachings to which we have all listened in our childhood, that this hypothesis owes its general wide diffusion as one of the current beliefs of English speaking people. If you turn to the seventh book of "Paradise Lost" you will find there stated the hypothesis to which I refer, which is briefly this: That this visible universe of ours made its appearance at no great distance of time from the present day, and that the parts of which it is composed made their appearance in a certain definite order in the space of six natural days, in such a manner that in the first of these days light appeared; in the second, the firmament or sky separated the water above from the water beneath it; on the third day the waters drew away from the dry land, and upon it a varied vegetable life similar to that which now exists made its appearance; that the fourth day was devoted to the apparition of the sun, the stars, the moon, and the planets; that on the fifth day aquatic animals originated within the waters; that on the sixth day the earth gave rise to our four-footed terrestrial creatures, and to all varieties of terrestrial animals except birds, which had appeared on the preceding day; and, finally, that man appeared upon the earth, and the work of fashioning the universe was finished. John Milton, as I have said, leaves no doubt whatever as to how, in his judgment, these marvelous processes occurred. I doubt not that his immortal poem is familiar to all of you, but I should like to recall one passage to your minds, in order that I may be justified in what I have said regarding the perfectly concrete, definite conception which Milton had respecting the origin of the animal world. He says:
"The sixth, and of creation last, arose
With evening harps and matin; when God said,
Pasturing at once, and in broad hoards upsprung.
The grassy clods now calved; now half appears
There is no doubt as to the meaning of this statement, or as to what a man of Milton's genius expected would have been actually visible to one who could witness the process of the origination of living things.
The third hypothesis, or the hypothesis of evolution, supposes that at any given period in the past we should meet with a state of things more or less similar to the present, but less similar in proportion as we go back in time; that the physical form of the earth could be traced back in this way to a condition in which its parts were separated, as little more than a nebulous cloud making part of a whole in which we should find the sun and the other planetary bodies also resolved; and that, if we traced back the animal world and the vegetable world, we should find preceding what now exist animals and plants not identical with them, but like them, only increasing their differences as we go back in time, and at the same time becoming simpler and simpler, until finally we should arrive at that gelatinous mass which, so far as our present knowledge goes, is the common foundation of all life.
The hypothesis of evolution supposes that in all this vast progression there would be no breach of continuity, no point at which we could say "This is a natural process," and "This is not a natural process," but that the whole might be compared to that wonderful series of changes which may be seen going on every day under our eyes, in virtue of which there arises, out of that semifluid, homogeneous substance which we call an egg, the complicated organization of one of the higher animals. That, in a few words, is what is meant by the hypothesis of evolution.
I have already suggested that in dealing with these three hypotheses, in endeavoring to form a judgment as to which of them is the more worthy of belief; or whether none is worthy of belief—and our condition of mind should be that suspension of judgment which is so difficult to all but trained minds—we should be indifferent to all a priori considerations. The question is a question of fact, historical fact. The universe has come into existence somehow or other, and the question is, whether it came into existence in one fashion, or whether it came into existence in another; and, as an essential preliminary to our further discussion, permit me to say two or three words as to the nature of historical evidence, and the kinds of historical evidence. The evidence as to the occurrence of any fact in past time is of one or two kinds, which, for convenience' sake, I will speak of on the one hand as testimonial evidence, and on the other as circumstantial evidence. By testimonial evidence I mean human testimony; and by circumstantial evidence I mean evidence which is not human testimony. Let me illustrate by a familiar figure what I mean by these two kinds of evidence, and what is to be said respecting their value:
Suppose that a man tells you that he saw a person strike another and kill him; that is testimonial evidence of the fact of murder. But it is possible to have circumstantial evidence of the fact of murder; that is to say, you may find a man dying with a wound upon his head having exactly the form and character of the wound which is made by an axe, and, with due care to take surrounding circumstances into account, you may conclude with the utmost certainty that the man has been murdered—is dying in consequence of the violence inflicted by that implement. We are very much in the habit of considering circumstantial evidence as of less value than testimonial evidence, and it may be in many cases, where the circumstances are not perfectly clear and perfectly intelligible, that it is a dangerous and uncertain kind of evidence; but it must not be forgotten that in many cases it is quite as good as testimonial evidence, and that notit is a great deal better than testimonial evidence. For example, take the case to which I referred just now. The circumstantial evidence is better and more convincing than the testimonial evidence, for it is impossible, under the circumstances that I have mentioned, to suppose that the man had met his death from any cause but the violent blow of an axe wielded by another man. The circumstantial evidence in favor of a murder having been committed, in that case, is as complete and as convincing as evidence can be. It is evidence which is open to no doubt and no falsification. But the testimony of the witness is open to multitudinous doubts. He may have been mistaken. He may have been actuated by malice. It has constantly happened that even an accurate man has declared a thing has happened in this, that, or the other way, when a careful analysis of the circumstantial evidence has shown that it did not happen in that way, but in some other way.
Now we must turn to our three hypotheses. Let me first direct your attention to what is to be said about the hypothesis of the eternity of this state of things in which we now are. What will first strike you is, that that is an hypothesis which, whether true or false, is not capable of verification by evidence; for, in order to secure testimony to an eternity of duration, you must have an eternity of witnesses or an infinity of circumstances, and neither of these is attainable. It is utterly impossible that such evidence should be carried beyond a certain point of time, and all that could be said at most should be that there was nothing to contradict the hypothesis. But when you look, not to the testimonial evidence—which might not be good for much in this case—but to the circumstantial evidence, then you find that this hypothesis is absolutely incompatible with that circumstantial evidence, and the evidence is of so plain and so simple a character that it is impossible in any way to escape from the conclusions which it forces upon us.
IDEAL SECTION OF THE CRUST OF THE EARTH.
On further examination, it is found that these beds of solid material are of exactly the same nature as these which are at present being formed under known conditions at the surface of the earth; that the chalk, for example, which forms a great part of the Cretaceous formation in some parts of the world, is identical in its physical and chemical characters, or practically so, with a substance which is now being formed at the bottom of the Atlantic Ocean, and covers an enormous area; that other bodies of rock are comparable with the sands which are being formed upon sea-shores, packed together, and so on. Thus it comes to be certain that, omitting rocks of igneous origin, all these beds of stone, of which a total of not less than seventy thousand feet is known, have been deposited and formed by natural agencies, either out of the waste and washing of the dry land, or else as the product of plants and animals. Now, these rocks or strata are full of the remains of animals and plants. Countless thousands of species of animals and plants, as perfectly recognizable as those which you meet with in museums at the present day, or as the shells and remains which you pick up upon the beach—countless thousands of species of these creatures have been imbedded in the sand or mud, or limestone, just as they are being imbedded now. They furnish us with a record, the general nature of which cannot be subject to any misinterpretation, as to the kind of things that have lived upon the surface of the earth during the time that is registered by this great thickness of stratified rock. The most superficial study of these remains shows us that the animals and plants which live at the present time have had only a temporary duration; that you will find them and such as they are now, for the most part, only in those uppermost of the strata called Tertiary. As you go back in time their places are taken by other forms as numerous and diversified, but different, and you will find yet others different from the Cretaceous or Tertiary, and from those of the present day, and so on, as you go further and further back. Thus, the circumstantial evidence absolutely negatives the conception of the eternity of the present condition of things. We can say with certainty that such has not been the course of Nature. We can say with certainty that the present condition of things has existed for a comparatively short period; and that, so far as animal and vegetable nature are concerned, it has been preceded by a different condition of things. We can pursue this evidence until we reach the lowest of stratified rocks, in which we lose the indications of life altogether. The hypothesis of the eternity of the present condition of things may, therefore, be put out of court.
We now come to what I may call Milton's hypothesis—the hypothesis that the present condition of things has endured for a comparatively moderate time, and at the commencement of that time came into existence within the course of six days. I doubt not that it may have excited some surprise in your minds that I should have spoken of this as Milton's hypothesis, rather than that I should choose the terms which are much more familiar to you, such as "the doctrine of creation," or "the Biblical doctrine," or "the doctrine of Moses," all of which denominations, as applied to the hypothesis to which I have just referred, are certainly much more familiar to you than the title of the Miltonic hypothesis. But I have had what I cannot but think are very weighty reasons for taking the course which I have pursued. For example, I have discarded the title of the hypothesis of creation, because my present business is not with the question as to how Nature has originated, as to the causes which have led to her origination, but as to the manner and order of the appearance of natural objects. Our present inquiry is not why the objects which constitute Nature came into existence, but when they came into existence, and in what order. This is a strictly historical question, a question as completely historical as that about the date at which the Angles and the Jutes invaded England. But the other question about creation is a philosophical question, and one which cannot be solved or even approached by the historical method. What we want to know is, whether the facts, so far as they are known, afford evidence that things arose in the way described by Milton, or not; and, when that question is settled, it will be time enough to inquire as to the causes of their origination.
In the second place, I have not spoken of this doctrine as the Biblical hypothesis. It is quite true that persons as diverse in their general views as Milton the Protestant and the celebrated Jesuit Father Suarez, each read in the first chapter of Genesis the interpretation adopted by Milton. It is quite true that that interpretation, unless I mistake, is that which has been instilled into every one of us in our childhood; but I do not for one moment venture to say that it can properly be called the Biblical doctrine. In the first place, it is not my business to say what the Hebrew text contains, and what it does not; in the second place, were I to say that this is the Biblical hypothesis, I should be met by the authority of many eminent scholars, to say nothing of men of science, who in recent times have absolutely denied that this doctrine is to be found in Genesis at all. If we are to listen to them, we must believe what seems so clearly defined in Genesis—as if very great pains had been taken so that there should be no possibility of mistake—is not the meaning of the text at all. The account is divided into periods that we may make just as long as convenience requires. We are also to understand that it is consistent with that phraseology to believe that plants and animals may have been evolved by natural processes, lasting for millions of years, out of similar rudiments. A person who is not a Hebrew scholar can only stand by and admire the marvelous flexibility of a language which admits of such diverse interpretations.
Assuredly, in the face of such contradictory authority upon matters upon which he is competent to form no judgment, he will abstain from giving any opinion, as I do. In the third place, I have carefully abstained from speaking of this as a Mosaic doctrine, because Ave are now assured upon the authority of the highest critics, and even of dignitaries in the Church, that there is no evidence whatever that Moses ever wrote this chapter, or knew anything about it. You will understand that I give no opinion—it would be an impertinence upon my part to volunteer an opinion upon such a subject. But, that being the state of opinion among the scholars and the clergy, it is well for us the laity, who stand outside, to avoid entangling ourselves in such a vexed question. So, as happily Milton leaves us no conceivable ambiguity as to what he means, I will continue to speak of the opinion in question as the Miltonian hypothesis.
Now we have to test that hypothesis. For my part, I have no prejudice one way or the other. If there is evidence in favor of this view, I have no sort of theoretical difficulties in the way of accepting it, but there must be evidence. We scientific men get an awkward habit—no, I won't call it that, for it is a valuable habit—of reasoning, so that we believe nothing unless there is evidence for it; and we have a way of looking upon belief which is not based upon evidence, not only as illogical, but as immoral. We will, if you please, test this view in the light of facts, for by what I have said you will understand that I don't propose to discuss the question of what testimonial evidence is to be adduced in favor of this view. If those whose business it is to judge are not at one as to the authenticity of the document, or as to the facts to which it bears witness, the discussion of testimonial evidence is superfluous. But one regards this less because the circumstantial evidence, if carefully considered, leads to the conclusion that the hypothesis is altogether inadequate, and cannot be sustained. And the considerations upon which I base that conclusion are of the simplest possible character. Whatever the flexibility of interpretation of the statement on which Milton's hypothesis is based, it is quite impossible to deny that it contains assertions of a very definite character relating to the succession of living forms. It is stated that plants, for example, made their appearance upon the third day, and not before. And you will understand that what was meant by plants are plants which now live—the trees and shrubs which we now have. One of two things—either the existing plants have been the result of a separate origination of which we have no record or ground for supposition, or else they have arisen by process of evolution from the original stock. And, in the second place, it is clear that there was no animal life before the fourth day, and that on the fourth day marine animals and birds appeared. And it is further clear that terrestrial life made its appearance upon the sixth day, and not before. Hence, it follows that, if in the large mass of circumstantial evidence as to what really has happened in the past history of the globe—if in that we find down to a certain point indications of the existence of terrestrial animals, it is perfectly certain that all that has taken place since that time must be referred to the sixth day.
In this great Carboniferous formation whence America has derived so vast a proportion of her actual and potential wealth, in that formation and in the beds of coal which are formed from the vegetation of that period, we find abundant evidence of the existence of terrestrial animals. They have been described not only by European naturalists, but by your own naturalists. There are to be found in the coal of your own coal-fields numerous insects allied to our cockroaches. There are to be found there spiders and scorpions of large size, and so similar to existing scorpions that it requires the practised eye of the naturalist to distinguish them. Inasmuch as these things can be proved to have been alive in the Carboniferous epoch, it is. perfectly clear that, if the Miltonic account is correct, the huge mass of rocks extending from the middle of the Palæozoic formations to the end of the series must belong to the day or period which is termed by Milton the sixth day of the creation. But, further, it is expressly stated that aquatic animals took their origin upon the fifth day, and did not exist before; hence all formations in which aquatic animals can be proved to exist, and which therefore lived at the time these formations were deposited, must have been deposited during the time of the period which Milton speaks of as the fifth day. But there is absolutely no fossiliferous rock in which you do not find the remains of marine animals. The lowest forms of life in the Silurian are marine animals, and, if the view which is entertained by Principal Dawson and Dr. Carpenter respecting the nature of the Eozoön be correct, if it is true that animal remains exist at a period as far antecedent to the deposit in the coal as the coal is from us, at the bottom of the series of stratified rocks in the Laurentian strata, it follows plainly enough that the whole series of stratified rocks, if they are to be brought into harmony with Milton at all, must be referred to the sixth day, and we cannot hope to find the slightest trace of the work of the other days in our stratified formations. When one comes to consider this, one sees how absolutely futile are the attempts that have been made to run a parallel between the story told by the stratified rocks as we know them and the account which Milton gives of it. The whole series of stratified rocks must be referred to the last two periods, and neither the Carboniferous nor any other formation can afford evidence of the work of the third day. Not only is there this objection to any attempt to run a parallel between the Miltonic account and the actual facts, but there is a further difficulty. In the Miltonic account the order in which animals should have made their appearance in the stratified rock would be this: Fishes, including the great whales, and birds; after that, all varieties of terrestrial animals. Nothing could be further from the facts as we find them. As a matter of fact we know of not the slightest evidence of the existence of birds before the Jurassic and perhaps the Triassic formations.
If there were any parallel between the Miltonic account and the circumstantial evidence, we ought to have abundant evidence of the existence of birds in the Devonian, the Silurian, and the Carboniferous rocks. I need hardly tell you that this is not the case, and that not a trace of birds makes its appearance until the far later period which I have mentioned.
And again, if it be true that all varieties of fishes and the great whales, and the like, made their appearance on the fifth day, then we ought to find the remains of these things in the older rocks—in those which preceded the Carboniferous epoch. Fishes, it is true, we find, and numerous ones; but the great whales are absent, and the fishes are not such as now live. Not one solitary species of fish now in existence is to be found there, and hence you are introduced again to the dilemma that either the creatures which were created then, which came into existence the sixth day, were not those which are found at present, are not the direct and immediate predecessors of those which now exist; in which case you must either have had a fresh creation of which nothing is said, or a process of evolution; or else the whole story must be given up, as not only devoid of any circumstantial evidence, but contrary to that evidence.
I placed before you in a few words, some little time ago, a statement of the sum and substance of Milton's hypothesis. Let me try now to put before you as briefly the effect of the circumstantial evidence as to the past history of the earth which is written without the possibility of mistake, with no chance of error, in the stratified rocks. What we find is, that that great series of formations represents a period of time of which our human chronologies hardly afford us a unit of measure. I will not pretend to say how we ought to measure this time, in millions or in billions of years. Happily for my purpose, that is wholly unessential. But that the time was enormous, there is no sort of question.
It results from the simplest methods of interpretation, that all that is now dry land has once been at the bottom of the waters. Leaving out of view certain patches of metamorphosed rocks, certain-volcanic products, it is perfectly certain that at a comparatively recent period of the world's history—the Cretaceous epoch—none of the great physical features which at present mark the surface of the globe existed. It is certain that the Rocky Mountains were not. It is certain that the Himalaya Mountains were not. It is certain that the Alps and the Pyrenees had no existence. The evidence is of the plainest possible character, and is simply this: We find raised up on the flanks of these mountains, elevated by the forces of upheaval which have given rise to them, masses of cretaceous rock which formed the bottom of the sea before those mountains existed. It is therefore perfectly clear that the elementary forces which gave rise to the mountains operated subsequently to the Cretaceous epoch; that the mountains themselves are largely made up of the materials deposited in the sea which once occupied their place. We meet as we go back in time with constant alternations of sea and land, of estuary and open ocean, and in correspondence with these alternations we meet with changes in the fauna and flora of the kind I have stated.
But no inspection of these changes gives us the slightest right to believe that there has been any discontinuity in natural processes. There is no trace of cataclysm, of great sweeping deluges or universal destructions of organic life. The appearances which were formerly interpreted that way have all been shown to be delusive as our knowledge has increased and as the blanks between the different formations have been filled up. It can now be shown that there is no absolute break between formation and formation, that there has been no sudden disappearance of all the forms of life at one time and replacement by another, but that everything has gone on slowly and gradually, that one form has died out and another has taken its place, and that thus by slow degrees one fauna has been replaced by another. So that, within the whole of the immense period indicated by these stratified rocks, there is assuredly—leaving evolution out of the question altogether—not the slightest trace of any break in the uniformity of Nature's operations, not a shadow of indication that events have followed other than their natural and orderly sequence.
That, I say, is the most natural teaching of the circumstantial evidence contained in the stratified rock. I leave you* to consider how far by any ingenuity of interpretation, by any stretching of the meaning of language, it can be brought into the smallest similarity with that view which I have put before you as the Miltonic doctrine.
There remains the third hypothesis—what I have spoken of as the hypothesis of evolution; and I propose that in lectures to come we should consider that as carefully as we have considered the other two hypotheses. I need not say that it is quite hopeless to look for testimonial evidence of evolution. The very nature of the case precludes the possibility of such evidence. Our sole inquiry is, what foundation circumstantial evidence lends to that hypothesis, or whether it lends any, or whether it controverts it; and I should deal with the matter entirely as a question of history. I shall not indulge in the discussion of any speculative probabilities. I shall not attempt to show that Nature is unintelligible unless we adopt some such hypothesis: for anything I know about it, it may be the way of Nature to be unintelligible. She is often puzzling, and I have no reason to suppose she is bound to fit herself to our notions; but I shall deal with the matter entirely from the point of view of history, and I shall place before you three kinds of evidence entirely based upon what we know of the forms of animal life which are contained in the series of stratified rock. I shall endeavor to show you that there is one kind of evidence which is neutral, which neither helps evolution nor is inconsistent with it. I shall then endeavor to show you that there is a second kind of evidence which indicates a strong probability in favor of evolution, but does not prove it; and, lastly, I shall endeavor to show that there is a third kind of evidence which, being as complete as any evidence which we can hope to obtain upon such a subject, and being wholly and entirely in favor of evolution, may be fairly called demonstrative evidence of its having occurred.