I am not certain that any one can give you a satisfactory answer to that question. a.s.suredly I cannot. All that can be said, for certain, is, that such movements are part of the ordinary course of nature, inasmuch as they are going on at the present time. Direct proof may be given, that some parts of the land of the northern hemisphere are at this moment insensibly rising and others insensibly sinking; and there is indirect, but perfectly satisfactory, proof, that an enormous area now covered by the Pacific has been deepened thousands of feet, since the present inhabitants of that sea came into existence.
Thus there is not a shadow of a reason for believing that the physical changes of the globe, in past times, have been effected by other than natural causes.
Is there any more reason for believing that the concomitant modifications in the forms of the living inhabitants of the globe have been brought about in other ways?
Before attempting to answer this question, let us try to form a distinct mental picture of what has happened in some special case.
The crocodiles are animals which, as a group, have a very vast antiquity. They abounded ages before the chalk was deposited; they throng the rivers in warm climates, at the present day. There is a difference in the form of the joints of the back-bone, and in some minor particulars, between the crocodiles of the present epoch and those which lived before the chalk; but, in the cretaceous epoch, as I have already mentioned, the crocodiles had a.s.sumed the modern type of structure.
Notwithstanding this, the crocodiles of the chalk are not identically the same as those which lived in the times called "older tertiary,"
which succeeded the cretaceous epoch; and the crocodiles of the older tertiaries are not identical with those of the newer tertiaries, nor are these identical with existing forms. I leave open the question whether particular species may have lived on from epoch to epoch. But each epoch has had its peculiar crocodiles; though all, since the chalk, have belonged to the modern type, and differ simply in their proportions, and in such structural particulars as are discernible only to trained eyes.
How is the existence of this long succession of different species of crocodiles to be accounted for?
Only two suppositions seem to be open to us--Either each species of crocodile has been specially created, or it has arisen out of some pre-existing form by the operation of natural causes.
Choose your hypothesis; I have chosen mine. I can find no warranty for believing in the distinct creation of a score of successive species of crocodiles in the course of countless ages of time. Science gives no countenance to such a wild fancy; nor can even the perverse ingenuity of a commentator pretend to discover this sense, in the simple words in which the writer of Genesis records the proceedings of the fifth and sixth days of the Creation.
On the other hand, I see no good reason for doubting the necessary alternative, that all these varied species have been evolved from pre-existing crocodilian forms, by the operation of causes as completely a part of the common order of nature, as those which have effected the changes of the inorganic world.
Few will venture to affirm that the reasoning which applies to crocodiles loses its force among other animals, or among plants. If one series of species has come into existence by the operation of natural causes, it seems folly to deny that all may have arisen in the same way.
A small beginning has led us to a great ending. If I were to put the bit of chalk with which we started into the hot but obscure flame of burning hydrogen, it would presently shine like the sun. It seems to me that this physical metamorphosis is no false image of what has been the result of our subjecting it to a jet of fervent, though nowise brilliant, thought to-night. It has become luminous, and its clear rays, penetrating the abyss of the remote past, have brought within our ken some stages of the evolution of the earth. And in the shifting "without haste, but without rest" of the land and sea, as in the endless variation of the forms a.s.sumed by living beings, we have observed nothing but the natural product of the forces originally possessed by the substance of the universe.
FOOTNOTES:
[29] See Appendix to Captain Dayman"s "Deep Sea Soundings in the North Atlantic Ocean, between Ireland and Newfoundland, made in H.M.S.
_Cyclops_. Published by order of the Lords Commissioners of the Admiralty, 1858." They have since formed the subject of an elaborate Memoir by Messrs. Parker and Jones, published in the _Philosophical Transactions_ for 1865.
[30] During the cruise of H.M.S. _Bull-dog_, commanded by Sir Leopold M"Clintock, in 1860, living star-fish were brought up, clinging to the lowest part of the sounding-line, from a depth of 1,260 fathoms, midway between Cape Farewell, in Greenland, and the Rockall banks. Dr. Wallich ascertained that the sea-bottom at this point consisted of the ordinary _Globigerina_ ooze, and that the stomachs of the star-fishes were full of _Globigerinae_. This discovery removes all objections to the existence of living _Globigerinae_ at great depths, which are based upon the supposed difficulty of maintaining animal life under such conditions; and it throws the burden of proof upon those who object to the supposition that the _Globigerinae_ live and die where they are found.
[31] I have recently traced out the development of the "coccoliths" from a diameter of 1/7000th of an inch up to their largest size (which is about 1/1600th), and no longer doubt that they are produced by independent organisms, which, like the _Globigerinae_, live and die at the bottom of the sea.
[32] "Elements of Geology," by Sir Charles Lyell, Bart. F.R.S., p. 23.
X.
GEOLOGICAL CONTEMPORANEITY AND PERSISTENT TYPES OF LIFE.
Merchants occasionally go through a wholesome, though troublesome and not always satisfactory, process which they term "taking stock." After all the excitement of speculation, the pleasure of gain, and the pain of loss, the trader makes up his mind to face facts and to learn the exact quant.i.ty and quality of his solid and reliable possessions.
The man of science does well sometimes to imitate this procedure; and, forgetting for the time the importance of his own small winnings, to re-examine the common stock in trade, so that he may make sure how far the stock of bullion in the cellar--on the faith of whose existence so much paper has been circulating--is really the solid gold of truth.
The Anniversary Meeting of the Geological Society seems to be an occasion well suited for an undertaking of this kind--for an inquiry, in fact, into the nature and value of the present results of palaeontological investigation; and the more so, as all those who have paid close attention to the late mult.i.tudinous discussions in which palaeontology is implicated, must have felt the urgent necessity of some such scrutiny.
First in order, as the most definite and unquestionable of all the results of palaeontology, must be mentioned the immense extension and impulse given to botany, zoology, and comparative anatomy, by the investigation of fossil remains. Indeed, the ma.s.s of biological facts has been so greatly increased, and the range of biological speculation has been so vastly widened, by the researches of the geologist and palaeontologist, that it is to be feared there are naturalists in existence who look upon geology as Brindley regarded rivers. "Rivers,"
said the great engineer, "were made to feed ca.n.a.ls;" and geology, some seem to think, was solely created to advance comparative anatomy.
Were such a thought justifiable, it could hardly expect to be received with favour by this a.s.sembly. But it is not justifiable. Your favourite science has her own great aims independent of all others; and if, notwithstanding her steady devotion to her own progress, she can scatter such rich alms among her sisters, it should be remembered that her charity is of the sort that does not impoverish, but "blesseth him that gives and him that takes."
Regard the matter as we will, however, the facts remain. Nearly 40,000 species of animals and plants have been added to the Systema Naturae by palaeontological research. This is a living population equivalent to that of a new continent in mere number; equivalent to that of a new hemisphere, if we take into account the small population of insects as yet found fossil, and the large proportion and peculiar organization of many of the Vertebrata.
But, beyond this, it is perhaps not too much to say that, except for the necessity of interpreting palaeontological facts, the laws of distribution would have received less careful study; while few comparative anatomists (and those not of the first order) would have been induced by mere love of detail, as such, to study the minutiae of osteology, were it not that in such minutiae lie the only keys to the most interesting riddles offered by the extinct animal world.
These a.s.suredly are great and solid gains. Surely it is matter for no small congratulation that in half a century (for palaeontology, though it dawned earlier, came into full day only with Cuvier) a subordinate branch of biology should have doubled the value and the interest of the whole group of sciences to which it belongs.
But this is not all. Allied with geology, palaeontology has established two laws of inestimable importance: the first, that one and the same area of the earth"s surface has been successively occupied by very different kinds of living beings; the second, that the order of succession established in one locality holds good, approximately, in all.
The first of these laws is universal and irreversible; the second is an induction from a vast number of observations, though it may possibly, and even probably, have to admit of exceptions. As a consequence of the second law, it follows that a peculiar relation frequently subsists between series of strata, containing organic remains, in different localities. The series resemble one another, not only in virtue of a general resemblance of the organic remains in the two, but also in virtue of a resemblance in the order and character of the serial succession in each. There is a resemblance of arrangement; so that the separate terms of each series, as well as the whole series, exhibit a correspondence.
Succession implies time; the lower members of a series of sedimentary rocks are certainly older than the upper; and when the notion of age was once introduced as the equivalent of succession, it was no wonder that correspondence in succession came to be looked upon as correspondence in age, or "contemporaneity." And, indeed, so long as relative age only is spoken of, correspondence in succession _is_ correspondence in age; it is _relative_ contemporaneity.
But it would have been very much better for geology if so loose and ambiguous a word as "contemporaneous" had been excluded from her terminology, and if, in its stead, some term expressing similarity of serial relation, and excluding the notion of time altogether, had been employed to denote correspondence in position in two or more series of strata.
In anatomy, where such correspondence of position has constantly to be spoken of, it is denoted by the word "h.o.m.ology" and its derivatives; and for Geology (which after all is only the anatomy and physiology of the earth) it might be well to invent some single word, such as "h.o.m.otaxis"
(similarity of order), in order to express an essentially similar idea.
This, however, has not been done, and most probably the inquiry will at once be made--To what end burden science with a new and strange term in place of one old, familiar, and part of our common language?
The reply to this question will become obvious as the inquiry into the results of palaeontology is pushed further.
Those whose business it is to acquaint themselves specially with the works of palaeontologists, in fact, will be fully aware that very few, if any, would rest satisfied with such a statement of the conclusions of their branch of biology as that which has just been given.
Our standard repertories of palaeontology profess to teach us far higher things--to disclose the entire succession of living forms upon the surface of the globe; to tell us of a wholly different distribution of climatic conditions in ancient times; to reveal the character of the first of all living existences; and to trace out the law of progress from them to us.
It may not be unprofitable to bestow on these professions a somewhat more critical examination than they have hitherto received, in order to ascertain how far they rest on an irrefragable basis; or whether, after all, it might not be well for palaeontologists to learn a little more carefully that scientific "ars artium," the art of saying "I don"t know." And to this end let us define somewhat more exactly the extent of these pretensions of palaeontology.
Every one is aware that Professor Bronn"s "Untersuchungen" and Professor Pictet"s "Traite de Paleontologie" are works of standard authority, familiarly consulted by every working palaeontologist. It is desirable to speak of these excellent books, and of their distinguished authors, with the utmost respect, and in a tone as far as possible removed from carping criticism; indeed, if they are specially cited in this place, it is merely in justification of the a.s.sertion that the following propositions, which may be found implicitly, or explicitly, in the works in question, are regarded by the ma.s.s of palaeontologists and geologists, not only on the Continent but in this country, as expressing some of the best-established results of palaeontology. Thus:--
Animals and plants began their existence together, not long after the commencement of the deposition of the sedimentary rocks; and then succeeded one another, in such a manner, that totally distinct faunas and florae occupied the whole surface of the earth, one after the other, and during distinct epochs of time.
A geological formation is the sum of all the strata deposited over the whole surface of the earth during one of these epochs: a geological fauna or flora is the sum of all the species of animals or plants which occupied the whole surface of the globe, during one of these epochs.
The population of the earth"s surface was at first very similar in all parts, and only from the middle of the Tertiary epoch onwards, began to show a distinct distribution in zones.
The const.i.tution of the original population, as well as the numerical proportions of its members, indicates a warmer and, on the whole, somewhat tropical climate, which remained tolerably equable throughout the year. The subsequent distribution of living beings in zones is the result of a gradual lowering of the general temperature, which first began to be felt at the poles.
It is not now proposed to inquire whether these doctrines are true or false; but to direct your attention to a much simpler though very essential preliminary question--What is their logical basis? what are the fundamental a.s.sumptions upon which they all logically depend? and what is the evidence on which those fundamental propositions demand our a.s.sent?
These a.s.sumptions are two: the first, that the commencement of the geological record is coeval with the commencement of life on the globe; the second, that geological contemporaneity is the same thing as chronological synchrony. Without the first of these a.s.sumptions there would of course be no ground for any statement respecting the commencement of life; without the second, all the other statements cited, every one of which implies a knowledge of the state of different parts of the earth at one and the same time, will be no less devoid of demonstration.
The first a.s.sumption obviously rests entirely on negative evidence. This is, of course, the only evidence that ever can be available to prove the commencement of any series of phaenomena; but, at the same time, it must be recollected that the value of negative evidence depends entirely on the amount of positive corroboration it receives. If A.B. wishes to prove an _alibi_, it is of no use for him to get a thousand witnesses simply to swear that they did not see him in such and such a place, unless the witnesses are prepared to prove that they must have seen him had he been there. But the evidence that animal life commenced with the Lingula-flags, _e.g._, would seem to be exactly of this unsatisfactory uncorroborated sort. The Cambrian witnesses simply swear they "haven"t seen anybody their way;" upon which the counsel for the other side immediately puts in ten or twelve thousand feet of Devonian sandstones to make oath they never saw a fish or a mollusk, though all the world knows there were plenty in their time.