« PreviousContinue »
reveal any such finely-graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against my theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.
In the first place it should always be borne in mind what sort of intermediate forms must, on my theory, have formerly existed. I have found it difficult, when looking at any two species, to avoid picturing to myself forms directly intermediate between them. But this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all its modified descendants. To give a simple illustration: the fantail and pouter pigeons have both descended from the rock-pigeon ; if we possessed all the intermediate varieties which have ever existed, we should have an extremely close series between both and the rock-pigeon; but we should have no varieties directly intermediate between the fantail and pouter; none, for instance, combining a tail somewhat expanded with a crop somewhat enlarged, the characteristic features of these two breeds. These two breeds, moreover, have become so much modified, that if we had no historical or indirect evidence regarding their origin, it would not have been possible to have determined from a mere comparison of their structure with that of the rock-pigeon (C. livia), whether they had descended from this species or from some other allied species, such as C. oenas.
~ So with natural species, if we look to forms very distinct, for instance to the horse and tapir, we have no reason to suppose that links ever existed directly intermediate between them, but between each and an unknown common parent. The common parent will have had in its whole organisation much general resemblance to the tapir and to the horse; but in some points of structure may have differed considerably from both, even perhaps more than they differ from each other. Hence, in all such cases, we should be unable to recognise the parent-form of any two or more species, even if we closely compared the structure of the parent with that of its modified descendants, unless at the same
time we had a nearly perfect chain of the intermediate links.
It isjust possible by my theory, that one of two living forms might have descended from the other ; for instance, a horse from a tapir; and in this case direct intermediate links will have existed between-them» But such a case would imply that one form had remained for a Very long period unaltered, whilst its descendants had undergone a vast amount of change; and the principle of competition between organism and organism, between child and parent, will render this a very rare event; for in all cases the new and improved forms of life tend to supplant the old and unimproved forms.
By the theory of natural selection all living species have been connected with the parent-species of each genus, by differences not greater than we see between the varieties of the same species at the present day; and these parent-species, now generally extinct, have in their turn been similarly connected with more ancient species; and so on backwards, always converging to the common ancestor of each great class. So that the number of intermediate and transitional links, between all living and extinct species, must have been ineonceivably great. But assuredly, if this theory be true, such have lived upon the earth.
On the'lapse of Time.—~Independently of our not finding fossil remains of such infinitely numerous connecting links, it may be objected, that time will not have sufficed for so great an amount of organic change, all changes having been effected very slowly through natural selection. It is hardly possible for me even to recall to the reader who may not be a practical geologist, the facts leading the mind feebly to comprehend the lapse of time. He who can read Sir Charles Lyell’s grand work on the Principles of Geology, which the future historian will recognise as having produced a revolution in natural science, yet does not admit how incomprehensibly vast have been the past periods of time, may at once close this volume. Not that it suffices to study the Principles of Geology, or to read special treatises by different observers on separate) formations, and to mark how each author attempts to give an inadequate idea of the duration of each formation or even each stratum. A- man must for years examine for himself great piles 0f superimposed strata, and watch the sea at Work, grinding down old rocks and making. fresh usediment, before he can hope to
comprehend anything of the lapse of time, the monuments of which we see around us.
It is good to wander along lines of sea-coast, when formed'of moderately: hard] rocks,;and mark the process of degradation. The tides'in‘most cases reach thevcliffs only for a short time twice a day, and the waves eat into them only when they are charged with 'sand or pebbles; for there is good evidence that pure water effectsnothing in wearing away rock. At last the base of the cliff is undermined, huge fragments fall down, and these remaining fixed, have to be worn away atom by-atom, until, reduced in size, theycan be rolled about by the waves, and then are more quickly ground into pebbles, sand, or mud. But how often dowe see along the bases of retreating cliffs rounded boulders, all thickly clothed by marine productions, showing how little they are abraded and how seldom they are rolled aboutl Moreover, if we follow for a few; miles any line of.rocky cliff, which is undergoing degradation, we find that it is only here and there, along a short length or round a promontory, that the cliffs are. at the present timesuffering. nThe appearance, oh the surface and .the vegetation show that elsewhere years have elapsed since the waters washed their base.
He who most closely studies the action of the sea on our shores, will, I 'belieVe, be most ;deeply impressed with the slowness with which rocky.coasts are worn away. Theobservationson this head by Hugh Miller, and by that excellent-observer Mr. Smith, of Jordan Hill, are most impressive; With the mind -thus.impressed, let any one examine ,beds of conglomerate many thousand feet in thickness, which, thoughv probably formedr'at a quicker rate than (many other deposits, yet, from-being formedjof worn and rounded- pebbles, each of which bears the-stamp of time, are good to show how slowly the mass has been accumulated. InitheCordillera I estimated one pile of conglomerate at ten thousand feet in thickness. Let the observer remember-Lyell’s profound remark- that the thickness .and extent of sedimentary formations are theJresult‘ and measure of the degradation \which 'the earth’s. crust has elsewhere suffered, (And .what an amount of degradationis implied DY'IhQ sedimentary deposits of many countriesl Professor Ramsay has givew me the maximum thickness,» in .most cases from actual measurement, in a few. cases from restimate, of each
formation in different parts of Great Britain ; and this
—-making altogether 72,584 feetythat is, very nearly thirteen and three-quarters British miles. _Some of the formations, which are represented in England by thin beds, are thousands of feet in thickness on the Continent. Moreover, between each successive formation, we have, in the opinion of most geologists, enormously long blank periods. So that the lofty pile of sedimentary racks in Britain giVeS but an inadequate idea ,of the time_ which has elapsed during their accumulation; yet what time this must have consumed 1 Good observers have estimated that sediment is deposited by the great Mississippi river at the rate of only 600 feet in a hundred thousand years. This estimate has no pretension to strict exactness; yet considering over what wide spaces very fine sediment is transported by the currents of the sea, the process of accumulation over any one extensive area must be extremely slow. ,
But the amount of denudation which the strata have in many places suffered, independently of the rate of accumulation of the degraded matter, probably offers the best evidence of the lapse of time. I remember having been much struck with the evidence of denudation, when viewing volcanic islands, which have been worn by the waves and pared all round into perpendicular cliffs of _one or, two thousand feet in height; for the gentle slope of the lava-streams, due to their formerly liquid state, showed at a glance how far the hard, rocky beds had once extended into the open ocean. The same story is still more plainly told by faults,—those great cracks along which the strata have been upheaved on one side, or thrown down on the other, to the height or depth of thousands of feet; for since the crust- cracked (it makes no great difference whether the upheaval was sudden, or, as most geologists now believe, was very slow and effected by many starts), the surface of the land has been so completely planed down by the action of the sea, that no trace of these vast dislocations is externally visible.
The Craven fault, for instance, extends for ,upwards of 30- miles, and along this line the vertical displacement
of the‘ strata has varied from 600 to 3000 feet. Prof. Ramsay has published an account of a downthrow in Anglesea of 2300 feet; and he informs me that be fully believes there is one in Merionethshire of 12,000 feet; yet in these cases there is nothing on the surface of the land to show such prodigious movements; the pile of rocks on the one or other side having been sm othly swept away. The consideration of these facts impresses the mind almost in the same manner as does the vain endeavour to grapple with the idea of eternity.
I have -made these few remarks because it is highly important for us to gain some notion, however imperfect, of the lapse of time. During each year, over the whole world, the land and the water have been peopled by
, hosts of living forms. 'What an infinite number of generations, which the mind cannot grasp, must have succeeded each other in the long roll of years! Now turn to our richest geological museums, and what a paltry display we behold! ~
On the poorness of our Palaeontological collections.— That our palaeontological collections are very imperfect, is admitted by every one. The remark of that admirable palaeontologist, the late Edward Forbes, should not be forgotten, namely, that numbers of our fossil species are known and named from single and often broken specimens, or from a few specimens collected on some one spot. Only a small portion of the surface of the earth has been geologically explored, and no part with sufficient care, as the important discoveries made every year in Europe prove. No organism wholly soft can be
<preserved. Shells and bones will decay and disappear when left on the bottom of the sea, where .,_S€Qiment is not-accumulating. I believe we are continuallf'tfiking a most erroneous view, when we tacitly admit to ourselves that sediment is being deposited over nearly the whole bed of the sea, at a rate sufficiently quick to embed and preserve fossil remains. Throughout an enormously large proportion of the ocean, the bright blue tint ‘of the water bespeaks its purity. The many cases on record of a formation conformably covered, after an enormous interval of time, by another and later formation, without the underlying bed having suffered in the interval any wear and tear, seem explieable'only on the view of the bottom of the sea not rarely lying for ages in an unaltered