[Footnote 178: See _Island Life_, p. 251.]
[Footnote 179: Mr. Hemsley suggests that it is not so much the difficulty of transmission by floating, as the bad conditions the seeds are usually exposed to when they reach land. Many, even if they germinate, are destroyed by the waves, as Burch.e.l.l noticed at St.
Helena; while even a flat and sheltered sh.o.r.e would be an unsuitable position for many inland plants. Air-borne seeds, on the other hand, may be carried far inland, and so scattered that some of them are likely to reach suitable stations.]
[Footnote 180: For fuller particulars, see Sir J. Hooker"s _Introduction to Floras of New Zealand and Australia_, and a summary in my _Island Life_, chaps. xxii. xxiii.]
[Footnote 181: For a fuller discussion of this subject, see my _Island Life_, chap. xxiii.]
[Footnote 182: A very remarkable case of wind conveyance of seeds on a large scale is described in a letter from Mr. Thomas Hanbury to his brother, the late Daniel Hanbury, which has been kindly communicated to me by Mr. Hemsley of Kew. The letter is dated "Shanghai, 1st May 1856,"
and the pa.s.sage referred to is as follows:--
"For the past three days we have had very warm weather for this time of year, in fact almost as warm as the middle of summer. Last evening the wind suddenly changed round to the north and blew all night with considerable violence, making a great change in the atmosphere.
"This morning, myriads of small white particles are floating about in the air; there is not a single cloud and no mist, yet the sun is quite obscured by this substance, and it looks like a white fog in England. I enclose thee a sample, thinking it may interest. It is evidently a vegetable production; I think, apparently, some kind of seed."
Mr. Hemsley adds, that this substance proves to be the plumose seeds of a poplar or willow. In order to produce the effects described--_quite obscuring the sun like a white fog_,--the seeds must have filled the air to a very great height; and they must have been brought from some district where there were extensive tracts covered with the tree which produced them.]
CHAPTER XIII
THE GEOLOGICAL EVIDENCES OF EVOLUTION
What we may expect--The number of known species of extinct animals--Causes of the imperfection of the geological record--Geological evidences of evolution--Sh.e.l.ls--Crocodiles--The rhinoceros tribe--The pedigree of the horse tribe--Development of deer"s horns--Brain development--Local relations of fossil and living animals--Cause of extinction of large animals--Indications of general progress in plants and animals--The progressive development of plants--Possible cause of sudden late appearance of exogens--Geological distribution of insects--Geological succession of vertebrata--Concluding remarks.
The theory of evolution in the organic world necessarily implies that the forms of animals and plants have, broadly speaking, progressed from a more generalised to a more specialised structure, and from simpler to more complex forms. We know, however, that this progression has been by no means regular, but has been accompanied by repeated degradation and degeneration; while extinction on an enormous scale has again and again stopped all progress in certain directions, and has often compelled a fresh start in development from some comparatively low and imperfect type.
The enormous extension of geological research in recent times has made us acquainted with a vast number of extinct organisms, so vast that in some important groups--such as the mollusca--the fossil are more numerous than the living species; while in the mammalia they are not much less numerous, the preponderance of living species being chiefly in the smaller and in the arboreal forms which have not been so well preserved as the members of the larger groups. With such a wealth of material to ill.u.s.trate the successive stages through which animals have pa.s.sed, it will naturally be expected that we should find important evidence of evolution. We should hope to learn the steps by which some isolated forms have been connected with their nearest allies, and in many cases to have the gaps filled up which now separate genus from genus, or species from species. In some cases these expectations are fulfilled, but in many other cases we seek in vain for evidence of the kind we desire; and this absence of evidence with such an apparent wealth of material is held by many persons to throw doubt on the theory of evolution itself. They urge, with much appearance of reason, that all the arguments we have hitherto adduced fall short of demonstration, and that the crucial test consists in being able to show, in a great number of cases, those connecting links which we say must have existed. Many of the gaps that still remain are so vast that it seems incredible to these writers that they could ever have been filled up by a close succession of species, since these must have spread over so many ages, and have existed in such numbers, that it seems impossible to account for their total absence from deposits in which great numbers of species belonging to other groups are preserved and have been discovered. In order to appreciate the force, or weakness, of these objections, we must inquire into the character and completeness of that record of the past life of the earth which geology has unfolded, and ascertain the nature and amount of the evidence which, under actual conditions, we may expect to find.
_The Number of known Species of Extinct Animals._
When we state that the known fossil mollusca are considerably more numerous than those which now live on the earth, it appears at first sight that our knowledge is very complete, but this is far from being the case. The species have been continually changing throughout geological time, and at each period have probably been as numerous as they are now. If we divide the fossiliferous strata into twelve great divisions--the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias, Trias, Permian, Carboniferous, Devonian, Silurian, and Cambrian,--we find not only that each has a very distinct and characteristic molluscan fauna, but that the different subdivisions often present a widely different series of species; so that although a certain number of species are common to two or more of the great divisions, the totality of the species that have lived upon the earth must be very much more than twelve times--perhaps even thirty or forty times--the number now living. In like manner, although the species of fossil mammals now recognised by more or less fragmentary fossil remains may not be much less numerous than the living species, yet the duration of existence of these was comparatively so short that they were almost completely changed, perhaps six or seven times, during the Tertiary period; and this is certainly only a fragment of the geological time during which mammalia existed on the globe.
There is also reason to believe that the higher animals were much more abundant in species during past geological epochs than now, owing to the greater equability of the climate which rendered even the arctic regions as habitable as the temperate zones are in our time.
The same equable climate would probably cause a more uniform distribution of moisture, and render what are now desert regions capable of supporting abundance of animal life. This is indicated by the number and variety of the species of large animals that have been found fossil in very limited areas which they evidently inhabited at one period. M.
Albert Gaudry found, in the deposits of a mountain stream at Pikermi in Greece, an abundance of large mammalia such as are nowhere to be found living together at the present time. Among them were two species of Mastodon, two different rhinoceroses, a gigantic wild boar, a camel and a giraffe larger than those now living, several monkeys, carnivora ranging from martens and civets to lions and hyaenas of the largest size, numerous antelopes of at least five distinct genera, and besides these many forms altogether extinct. Such were the great herds of Hipparion, an ancestral form of horse; the h.e.l.ladotherium, a huge animal bigger than the giraffe; the Ancylotherium, one of the Edentata; the huge Dinotherium; the Aceratherium, allied to the rhinoceros; and the monstrous Chalicotherium, allied to the swine and ruminants, but as large as a rhinoceros; and to prey upon these, the great Machairodus or sabre-toothed tiger. And all these remains were found in a s.p.a.ce 300 paces long by 60 paces broad, many of the species existing in enormous quant.i.ties.
The Pikermi fossils belong to the Upper Miocene formation, but an equally rich deposit of Upper Eocene age has been discovered in South-Western France at Quercy, where M. Filhol has determined the presence of no less than forty-two species of beasts of prey alone.
Equally remarkable are the various discoveries of mammalian fossils in North America, especially in the old lake bottoms now forming what are called the "bad lands" of Dakota and Nebraska, belonging to the Miocene period. Here are found an enormous a.s.semblage of remains, often perfect skeletons, of herbivora and carnivora, as varied and interesting as those from the localities already referred to in Europe; but altogether distinct, and far exceeding, in number and variety of species of the larger animals, the whole existing fauna of North America. Very similar phenomena occur in South America and in Australia, leading us to the conclusion that the earth at the present time is impoverished as regards the larger animals, and that at each successive period of Tertiary time, at all events, it contained a far greater number of species than now inhabit it. The very richness and abundance of the remains which we find in limited areas, serve to convince us how imperfect and fragmentary must be our knowledge of the earth"s fauna at any one past epoch; since we cannot believe that all, or nearly all, of the animals which inhabited any district were entombed in a single lake, or overwhelmed by the floods of a single river.
But the spots where such rich deposits occur are exceedingly few and far between when compared with the vast areas of continental land, and we have every reason to believe that in past ages, as now, numbers of curious species were rare or local, the commoner and more abundant species giving a very imperfect idea of the existing series of animal forms. Yet more important, as showing the imperfection of our knowledge, is the enormous lapse of time between the several formations in which we find organic remains in any abundance, so vast that in many cases we find ourselves almost in a new world, all the species and most of the genera of the higher animals having undergone a complete change.
_Causes of the Imperfection of the Geological Record._
These facts are quite in accordance with the conclusions of geologists as to the necessary imperfection of the geological record, since it requires the concurrence of a number of favourable conditions to preserve any adequate representation of the life of a given epoch. In the first place, the animals to be preserved must not die a natural death by disease, or old age, or by being the prey of other animals, but must be destroyed by some accident which shall lead to their being embedded in the soil. They must be either carried away by floods, sink into bogs or quicksands, or be enveloped in the mud or ashes of a volcanic eruption; and when thus embedded they must remain undisturbed amid all the future changes of the earth"s surface.
But the chances against this are enormous, because denudation is always going on, and the rocks we now find at the earth"s surface are only a small fragment of those which were originally laid down. The alternations of marine and freshwater deposits, and the frequent unconformability of strata with those which overlie them, tell us plainly of repeated elevations and depressions of the surface, and of denudation on an enormous scale. Almost every mountain range, with its peaks, ridges, and valleys, is but the remnant of some vast plateau eaten away by sub-aerial agencies; every range of sea-cliffs tell us of long slopes of land destroyed by the waves; while almost all the older rocks which now form the surface of the earth have been once covered with newer deposits which have long since disappeared. Nowhere are the evidences of this denudation more apparent than in North and South America, where granitic or metamorphic rocks cover an area hardly less than that of all Europe. The same rocks are largely developed in Central Africa and Eastern Asia; while, besides those portions that appear exposed on the surface, areas of unknown extent are buried under strata which rest on them uncomformably, and could not, therefore, const.i.tute the original capping under which the whole of these rocks must once have been deeply buried; because granite can only be formed, and metamorphism can only go on, deep down in the crust of the earth. What an overwhelming idea does this give us of the destruction of whole piles of rock, miles in thickness and covering areas comparable with those of continents; and how great must have been the loss of the innumerable fossil forms which those rocks contained! In view of such destruction we are forced to conclude that our palaeontological collections, rich though they may appear, are really but small and random samples, giving no adequate idea of the mighty series of organism which have lived upon the earth.[183]
Admitting, however, the extreme imperfection of the geological record as a whole, it may be urged that certain limited portions of it are fairly complete--as, for example, the various Miocene deposits of India, Europe, and North America,--and that in these we ought to find many examples of species and genera linked together by intermediate forms. It may be replied that in several cases this really occurs; and the reason why it does not occur more often is, that the theory of evolution requires that distinct genera should be linked together, not by a direct pa.s.sage, but by the descent of both from a common ancestor, which may have lived in some much earlier age the record of which is either wanting or very incomplete. An ill.u.s.tration given by Mr. Darwin will make this more clear to those who have not studied the subject. The fantail and pouter pigeons are two very distinct and unlike breeds, which we yet know to have been both derived from the common wild rock-pigeon. Now, if we had every variety of living pigeon before us, or even all those which have lived during the present century, we should find no intermediate types between these two--none combining in any degree the characters of the pouter with that of the fantail. Neither should we ever find such an intermediate form, even had there been preserved a specimen of every breed of pigeon since the ancestral rock-pigeon was first tamed by man--a period of probably several thousand years. We thus see that a complete pa.s.sage from one very distinct species to another could not be expected even had we a complete record of the life of any one period. What we require is a complete record of all the species that have existed since the two forms began to diverge from their common ancestor, and this the known imperfection of the record renders it almost impossible that we should ever attain.
All that we have a right to expect is, that, as we multiply the fossil forms in any group, the gaps that at first existed in that group shall become less wide and less numerous; and also that, in some cases, a tolerably direct series shall be found, by which the more specialised forms of the present day shall be connected with more generalised ancestral types. We might also expect that when a country is now characterised by special groups of animals, the fossil forms that immediately preceded them shall, for the most part, belong to the same groups; and further, that, comparing the more ancient with the more modern types, we should find indications of progression, the earlier forms being, on the whole, lower in organisation, and less specialised in structure than the later. Now evidence of evolution of these varied kinds is what we do find, and almost every fresh discovery adds to their number and cogency. In order, therefore, to show that the testimony given by geology is entirely in favour of the theory of descent with modification, some of the more striking of the facts will now be given.
_Geological Evidences of Evolution._
In an article in _Nature_ (vol. xiv. p. 275), Professor Judd calls attention to some recent discoveries in the Hungarian plains, of fossil lacustrine sh.e.l.ls, and their careful study by Dr. Neumayr and M. Paul of the Austrian Geological Survey. The beds in which they occur have acc.u.mulated to the thickness of 2000 feet, containing throughout abundance of fossils, and divisible into eight zones, each of which exhibits a well-marked and characteristic fauna. Professor Judd then describes the bearing of these discoveries as follows--
"The group of sh.e.l.ls which affords the most interesting evidence of the origin of new forms through descent with modification is that of the genus Vivipara or Paludina, which occurs in prodigious abundance throughout the whole series of freshwater strata. We shall not, of course, attempt in this place to enter into any details concerning the forty distinct _forms_ of this genus (Dr. Neumayr very properly hesitates to call them all _species_), which are named and described in this monograph, and between which, as the authors show, so many connecting links, clearly ill.u.s.trating the derivation of the newer from the older types, have been detected. On the minds of those who carefully examine the admirably engraved figures given in the plates accompanying this valuable memoir, or still better, the very large series of specimens from among which the subjects of these figures are selected, and which are now in the museum of the Reichsanstalt of Vienna, but little doubt will, we suspect, remain that the authors have fully made out their case, and have demonstrated that, beyond all controversy, the series with highly complicated ornamentation were variously derived by descent--the lines of which are in most cases perfectly clear and obvious--from the simple and unornamented Vivipara achatinoides of the Congerien-Schichten (the lower division of the series of strata). It is interesting to notice that a large portion of these unquestionably derived forms depart so widely from the type of the genus Vivipara, that they have been separated on so high an authority as that of Sandberger, as a new genus, under the name of Tulotoma. And hence we are led to the conclusion that a vast number of forms, certainly exhibiting specific distinctions, and according to some naturalists, differences even ent.i.tled to be regarded of generic value, have all a common ancestry."
It is, as Professor Judd remarks, owing to the exceptionally favourable circ.u.mstances of a long-continued and unbroken series of deposits being formed under physical conditions either identical or very slowly changing, that we owe so complete a record of the process of organic change. Usually, some disturbing elements, such as a sudden change of physical conditions, or the immigration of new sets of forms from other areas and the consequent retreat or partial extinction of the older fauna, interferes with the continuity of organic development, and produces those puzzling discordances so generally met with in geological formations of marine origin. While a case of the kind now described affords evidence of the origin of species complete and conclusive, though on a necessarily very limited scale, the very rarity of the conditions which are essential to such completeness serves to explain why it is that in most cases the direct evidence of evolution is not to be obtained.
Another ill.u.s.tration of the filling up of gaps between existing groups is afforded by Professor Huxley"s researches on fossil crocodiles. The gap between the existing crocodiles and the lizards is very wide, but as we go back in geological time we meet with fossil forms which are to some extent intermediate and form a connected series. The three living genera--Crocodilus, Alligator, and Gavialis--are found in the Eocene formation, and allied forms of another genus, Holops, in the Chalk. From the Chalk backward to the Lias another group of genera occurs, having anatomical characteristics intermediate between the living crocodiles and the most ancient forms. These, forming two genera Belodon and Stagonolepis, are found in a still older formation, the Trias. They have characters resembling some lizards, especially the remarkable Hatteria of New Zealand, and have also some resemblances to the Dinosaurians--reptiles which in some respects approach birds.
Considering how comparatively few are the remains of this group of animals, the evidence which it affords of progressive development is remarkably clear.[184]
Among the higher animals the rhinoceros, the horse, and the deer afford good evidence of advance in organisation and of the filling up of the gaps which separate the living forms from their nearest allies. The earliest ancestral forms of the rhinoceroses occur in the Middle Eocene of the United States, and were to some extent intermediate between the rhinoceros and tapir families, having like the latter four toes to the front feet, and three to those behind. These are followed in the Upper Eocene by the genus Amynodon, in which the skull a.s.sumes more distinctly the rhinocerotic type. Following this in the Lower Miocene we have the Aceratherium, like the last in its feet, but still more decidedly a rhinoceros in its general structure. From this there are two diverging lines--one in the Old World, the other in the New. In the former, to which the Aceratherium is supposed to have migrated in early Miocene times, when a mild climate and luxuriant vegetation prevailed far within the arctic circle, it gave rise to the Ceratorhinus and the various horned rhinoceroses of late Tertiary times and of those now living. In America a number of large hornless rhinoceroses were developed--they are found in the Upper Miocene, Pliocene, and Post-Pliocene formations--and then became extinct. The true rhinoceroses have three toes on all the feet.[185]
_The Pedigree of the Horse Tribe._
Yet more remarkable is the evidence afforded by the ancestral forms of the horse tribe which have been discovered in the American tertiaries.
The family Equidae, comprising the living horse, a.s.ses, and zebras, differ widely from all other mammals in the peculiar structure of the feet, all of which terminate in a single large toe forming the hoof.
They have forty teeth, the molars being formed of hard and soft material in crescentic folds, so as to be a powerful agent in grinding up hard gra.s.ses and other vegetable food. The former peculiarities depend upon modifications of the skeleton, which have been thus described by Professor Huxley:--
"Let us turn in the first place to the fore-limb. In most quadrupeds, as in ourselves, the fore-arm contains distinct bones, called the radius and the ulna. The corresponding region in the horse seems at first to possess but one bone. Careful observation, however, enables us to distinguish in this bone a part which clearly answers to the upper end of the ulna. This is closely united with the chief ma.s.s of the bone which represents the radius, and runs out into a slender shaft, which may be traced for some distance downwards upon the back of the radius, and then in most cases thins out and vanishes. It takes still more trouble to make sure of what is nevertheless the fact, that a small part of the lower end of the bone of a horse"s fore-arm, which is only distinct in a very young foal, is really the lower extremity of the ulna.
"What is commonly called the knee of a horse is its wrist. The "cannon bone" answers to the middle bone of the five metacarpal bones which support the palm of the hand in ourselves. The pastern, coronary, and coffin bones of veterinarians answer to the joints of our middle fingers, while the hoof is simply a greatly enlarged and thickened nail. But if what lies below the horse"s "knee" thus corresponds to the middle finger in ourselves, what has become of the four other fingers or digits?
We find in the places of the second and fourth digits only two slender splintlike bones, about two-thirds as long as the cannon bone, which gradually taper to their lower ends and bear no finger joints, or, as they are termed, phalanges. Sometimes, small bony or gristly nodules are to be found at the bases of these two metacarpal splints, and it is probable that these represent rudiments of the first and fifth toes. Thus, the part of the horse"s skeleton which corresponds with that of the human hand, contains one overgrown middle digit, and at least two imperfect lateral digits; and these answer, respectively, to the third, the second, and the fourth fingers in man.
"Corresponding modifications are found in the hind limb. In ourselves, and in most quadrupeds, the leg contains two distinct bones, a large bone, the tibia, and a smaller and more slender bone, the fibula. But, in the horse, the fibula seems, at first, to be reduced to its upper end; a short slender bone united with the tibia, and ending in a point below, occupying its place.
Examination of the lower end of a young foal"s shin-bone, however, shows a distinct portion of osseous matter which is the lower end of the fibula; so that the, apparently single, lower end of the shin-bone is really made up of the coalesced ends of the tibia and fibula, just as the, apparently single, lower end of the fore-arm bone is composed of the coalesced radius and ulna.
"The heel of the horse is the part commonly known as the hock.
The hinder cannon bone answers to the middle metatarsal bone of the human foot, the pastern, coronary, and coffin bones, to the middle toe bones; the hind hoof to the nail; as in the forefoot.
And, as in the forefoot, there are merely two splints to represent the second and the fourth toes. Sometimes a rudiment of a fifth toe appears to be traceable.
"The teeth of a horse are not less peculiar than its limbs. The living engine, like all others, must be well stoked if it is to do its work; and the horse, if it is to make good its wear and tear, and to exert the enormous amount of force required for its propulsion, must be well and rapidly fed. To this end, good cutting instruments and powerful and lasting crushers are needful. Accordingly, the twelve cutting teeth of a horse are close-set and concentrated in the forepart of its mouth, like so many adzes or chisels. The grinders or molars are large, and have an extremely complicated structure, being composed of a number of different substances of unequal hardness. The consequence of this is that they wear away at different rates; and, hence, the surface of each grinder is always as uneven as that of a good millstone."[186]