But while rejecting trans.m.u.tation, I was equally opposed to the popular theory that the creative power had diminished in energy, or that it had been in abeyance ever since Man had entered upon the scene. That a renovating force which had been in full operation for millions of years should cease to act while the causes of extinction were still in full activity, or even intensified by the accession of Man's destroying power, seemed to me in the highest degree improbable. The only point on which I doubted was whether the force might not be intermittent instead of being, as Lamarck supposed, in ceaseless operation. Might not the births of new species, like the deaths of old ones, be sudden? Might they not still escape our observation? If the coming in of one new species, and the loss of one other which had endured for ages, should take place annually, still, a.s.suming that there are a million of animals and plants living on the globe, it would require, I observed, a million of years to bring about a complete revolution in the fauna and flora. In that case, I imagined that, although the first appearance of a new form might be as abrupt as the disappearance of an old one, yet naturalists might never yet have witnessed the first entrance on the stage of a large and conspicuous animal or plant, and as to the smaller kinds, many of them may be conceived to have stolen in unseen, and to have spread gradually over a wide area, like species migrating into new provinces.*

(* "Principles of Geology" 1st edition 1832 volume 2 chapter 11; and 9th edition page 706.)

It may now be useful to offer some remarks on the very different reception which the twin branches of Lamarck's development theory, namely, progression and trans.m.u.tation, have met with, and to inquire into the causes of the popularity of the one and the great unpopularity of the other. We usually test the value of a scientific hypothesis by the number and variety of the phenomena of which it offers a fair or plausible explanation. If trans.m.u.tation, when thus tested, has decidedly the advantage over progression and yet is comparatively in disfavour, we may reasonably suspect that its reception is r.e.t.a.r.ded, not so much by its own inherent demerits, as by some apprehended consequences which it is supposed to involve and which run counter to our preconceived opinions.

THEORY OF PROGRESSION.

In treating of this question, I shall begin with the doctrine of progression, a concise statement of which, so far as it relates to the animal kingdom, was thus given twelve years ago by Professor Sedgwick, in the preface to his "Discourse on the Studies of the University of Cambridge."

"There are traces," he says, "among the old deposits of the earth of an organic progression among the successive forms of life. They are to be seen in the absence of mammalia in the older, and their very rare appearance in the newer Secondary groups; in the diffusion of warm-blooded quadrupeds (frequently of unknown genera) in the older Tertiary system, and in their great abundance (and frequently of known genera) in the upper portions of the same series; and lastly, in the recent appearance of Man on the surface of the earth."

"This historical development," continues the same author, of the forms and functions of organic life during successive epochs, "seems to mark a gradual evolution of creative power, manifested by a gradual ascent towards a higher type of being." "But the elevation of the fauna of successive periods was not made by trans.m.u.tation, but by creative additions; and it is by watching these additions that we get some insight into Nature's true historical progress, and learn that there was a time when Cephalopoda were the highest types of animal life, the primates of this world; that Fishes next took the lead, then Reptiles; and that during the secondary period they were anatomically raised far above any forms of the reptile cla.s.s now living in the world. Mammals were added next, until Nature became what she now is, by the addition of Man."*

(* Professor Sedgwick's "Discourse on the Studies of the University of Cambridge" Preface to 5th edition pages 44, 154, 216, 1850.)

Although in the half century which has elapsed between the time of Lamarck and the publication of the above summary, new discoveries have caused geologists to a.s.sign a higher antiquity both to Man and the oldest fossil mammalia, fish, and reptiles than formerly, yet the generalisation, as laid down by the Woodwardian Professor, as to progression, still holds good in all essential particulars.

The progressive theory was propounded in the following terms by the late Hugh Miller in his "Footprints of the Creator."

"It is of itself an extraordinary fact without reference to other considerations, that the order adopted by Cuvier in his "Animal Kingdom," as that in which the four great cla.s.ses of vertebrate animals, when marshalled according to their rank and standing, naturally range, should be also that in which they occur in order of time. The brain, which bears an average proportion to the spinal cord of not more than two to one, comes first--it is the brain of the fish; that which bears to the spinal cord an average proportion of two and a half to one succeeded it--it is the brain of the reptile; then came the brain averaging as three to one--it is that of the bird. Next in succession came the brain that averages as four to one--it is that of the mammal; and last of all there appeared a brain that averages as twenty-three to one--reasoning, calculating Man had come upon the scene."*

(* "Footprints of the Creator" Edinburgh 1849 page 283.)

M. Aga.s.siz, in his "Essay on Cla.s.sification," has devoted a chapter to the "Parallelism between the Geological Succession of Animals and Plants and their present relative Standing;" in which he has expressed a decided opinion that within the limits of the orders of each great cla.s.s there is a coincidence between their relative rank in organisation and the order of succession of their representatives in time.*

(* "Contributions to the Natural History of the United States" Part 1.--Essay on Cla.s.sification page 108.)

Professor Owen, in his Palaeontology, has advanced similar views, and has remarked, in regard to the vertebrata that there is much positive as well as negative evidence in support of the doctrine of an advance in the scale of being, from ancient to more modern geological periods. We observe, for example, in the Tria.s.sic, Oolitic, and Cretaceous strata, not only an absence of placental mammalia, but the presence of innumerable reptiles, some of large size, terrestrial and aquatic, herbivorous and predaceous, fitted to perform the functions now discharged by the mammalia.

The late Professor Bronn, of Heidelberg, after pa.s.sing in review more than 24,000 fossil animals and plants, which he had cla.s.sified and referred each to their geological position in his "Index Palaeontologicus," came to the conclusion that, in the course of time, there had been introduced into the earth more and more highly organised types of animal and vegetable life; the modern species being, on the whole, more specialised, i.e. having separate organs, or parts of the body, to perform different functions, which, in the earlier periods and in beings of simpler structure, were discharged in common by a single part or organ.

Professor Adolphe Brongniart, in an essay published in 1849 on the botanical cla.s.sification and geological distribution of the genera of fossil plants,* arrives at similar results as to the progress of the vegetable world from the earliest periods to the present.

(* Tableau des Genres de Vegetaux fossiles, etc.

"Dictionnaire Universel d'Histoire Naturelle" Paris 1849.)

He does not pretend to trace an exact historical series from the sea-weed to the fern, or from the fern again to the conifers and cycads, and lastly from those families to the palms and oaks, but he, nevertheless, points out that the cryptogamic forms, especially the acrogens, predominate among the fossils of the primary formations, the Carboniferous especially, while the gymnosperms or coniferous and cycadeous plants abound in all the strata, from the Trias to the Wealden inclusive; and lastly, the more highly developed angiosperms, both monocotyledonous and dicotyledonous, do not become abundant until the Tertiary period. It is a remarkable fact, as he justly observes, that the angiospermous exogens, which comprise four-fifths of living plants--a division to which all our native European trees, except the Coniferae, belong, and which embrace all the Compositae, Leguminosae, Umbelliferae, Cruciferae, Heaths, and so many other families--are wholly unrepresented by any fossils. .h.i.therto discovered in the Primary and Secondary formations from the Silurian to the Oolitic inclusive. It is not till we arrive at the Cretaceous period that they begin to appear, sparingly at first, and only playing a conspicuous part, together with the palms and other endogens, in the Tertiary epoch.

When commenting on the eagerness with which the doctrine of progression was embraced from the close of the last century to the time when I first attempted, in 1830, to give some account of the prevailing theories in geology, I observed that far too much reliance was commonly placed on the received dates of the first appearances of certain orders or cla.s.ses of animals or plants, such dates being determined by the age of the stratum in which we then happened to have discovered the earliest memorials of such types. At that time (1830), it was taken for granted that Man had not co-existed with the mammoth and other extinct mammalia, yet now that we have traced back the signs of his existence to the Pleistocene era, and may antic.i.p.ate the finding of his remains on some future day in the Pliocene period, the theory of progression is not shaken; for we cannot expect to meet with human bones in the Miocene formations, where all the species and nearly all the genera of mammalia belong to types widely differing from those now living; and had some other rational being, representing Man, then flourished, some signs of his existence could hardly have escaped unnoticed, in the shape of implements of stone or metal, more frequent and more durable than the osseous remains of any of the mammalia.

In the beginning of this century it was one of the canons of the popular geological creed that the first warm-blooded quadrupeds which had inhabited this planet were those derived from the Eocene gypsum of Montmartre in the suburbs of Paris, almost all of which Cuvier had shown to belong to extinct genera. This dogma continued in force for more than a quarter of a century, in spite of the discovery in 1818 of a marsupial quadruped in the Stonesfield strata, a member of the Lower Oolite, near Oxford. Some disputed the authority of Cuvier himself as to the mammalian character of the fossil; others, the accuracy of those who had a.s.signed to it so ancient a place in the chronological series of rocks.

In 1832 I pointed out that the occurrence of this single fossil in the Oolite was "fatal to the theory of successive development" as then propounded.*

(* "Principles of Geology" 2nd edition 1 173.)

Since that period great additions have been made to our knowledge of the existence of land quadrupeds in the olden times. We have ascertained that, in Eocene strata older than the gypsum of Paris, no less than four distinct sets of placental mammalia have flourished; namely, first, those of the Headon series in the Isle of Wight, from which fourteen species have been procured; secondly, those of the antecedent Bagshot and Bracklesham beds, which have yielded, together with the contemporaneous "calcaire grossier" of Paris, twenty species; thirdly, the still older beds of Kyson, near Ipswich, and those of Herne Bay, at the mouth of the Thames, in which seven species have been found; and fourthly, the Woolwich and Reading beds, which have supplied ten species.*

(* Lyell's supplement to 5th edition of "Elements" 1857.)

We can scarcely doubt that we should already have traced back the evidence of this cla.s.s of fossils much farther had not our inquiries been arrested, first by the vast gap between the Tertiary and Secondary formations, and then by the marine nature of the Cretaceous rocks.

The mammalia next in antiquity, of which we have any cognisance, are those of the Upper Oolite of Purbeck, discovered between the years 1854 and 1857, and comprising no less than fourteen species, referable to eight or nine genera; one of them, Plagiaulax, considered by Dr.

Falconer to have been a herbivorous marsupial. The whole a.s.semblage appear, from the joint observations of Professor Owen and Dr. Falconer, to indicate a low grade of quadruped, probably of the marsupial type.

They were, for the most part, diminutive, the two largest not much exceeding our common hedgehog and polecat in size.

Next anterior in age are the mammalia of the Lower Oolite of Stonesfield, of which four species are known, also very small and probably marsupial, with one exception, the Stereognathus ooliticus, which, according to Professor Owen's conjecture, may have been a hoofed quadruped and placental, though, as we have only half of the lower jaw with teeth, and the molars are unlike any living type, such an opinion is of course hazarded with due caution.

Still older than the above are some fossil quadrupeds of small size, found in the Upper Trias of Stuttgart in Germany, and more lately by Mr.

C. Moore in beds of corresponding age near Frome, which are also of a very low grade, like the living Myrmecobius of Australia. Beyond this limit our knowledge of the highest cla.s.s of vertebrata does not as yet extend into the past, but the frequent shifting back of the old landmarks, nearly all of them once supposed in their turn to indicate the date of the first appearance of warm-blooded quadrupeds on this planet, should serve as a warning to us not to consider the goal at present reached by palaeontology as one beyond which they who come after us are never destined to pa.s.s.

On the other hand, it may be truly said in favour of progression that after all these discoveries the doctrine is not gainsaid, for the less advanced marsupials precede the more perfect placental mammalia in the order of their appearance on the earth.

If the three localities where the most ancient mammalia have been found--Purbeck, Stonesfield, and Stuttgart--had belonged all of them to formations of the same age, we might well have imagined so limited an area to have been peopled exclusively with pouched quadrupeds, just as Australia now is, while other parts of the globe were inhabited by placentals, for Australia now supports one hundred and sixty species of marsupials, while the rest of the continents and islands are tenanted by about seventeen hundred species of mammalia, of which only forty-six are marsupial, namely, the opossums of North and South America. But the great difference of age of the strata in each of these three localities seems to indicate the predominance throughout a vast lapse of time (from the era of the Upper Trias to that of the Purbeck beds) of a low grade of quadrupeds; and this persistency of similar generic and ordinal types in Europe while the species were changing, and while the fish, reptiles, and mollusca were undergoing vast modifications, raises a strong presumption that there was also a vast extension in s.p.a.ce of the same marsupial forms during that portion of the Secondary epoch which has been termed "the age of reptiles."

As to the cla.s.s Reptilia, some of the orders which prevailed when the Secondary rocks were formed are confessedly much higher in their organisation than any of the same cla.s.s now living. If the less perfect ophidians, or snakes, which now abound on the earth had taken the lead in those ancient days among the land reptiles, and the Deinosaurians had been contemporary with Man, there can be no doubt that the progressionist would have seized upon this fact with unfeigned satisfaction as confirmatory of his views. Now that the order of succession is precisely reversed, and that the age of the Iguanodon was long anterior to that of the Eocene Palaeophis and living boa, while the crocodile is in our own times the highest representative of its cla.s.s, a retrograde movement in this important division of the vertebrata must be admitted. It may perhaps be accounted for by the power acquired by the placental mammalia, when they became dominant, a power before which the cla.s.s of vertebrata next below them, as coming most directly in compet.i.tion with them, may more than any other have given way.

For no less than thirty-four years it had been a received axiom in palaeontology that reptiles had never existed before the Permian or Magnesian Limestone period, when at length in 1844 this supposed barrier was thrown down, and Carboniferous reptiles, terrestrial and aquatic, of several genera were brought to light; and discussions are now going on as to whether some remains of an Enaliosaur (perhaps a large Labyrinthodon) have not been detected in the coal of Nova Scotia, and whether certain sandstones near Elgin in Scotland, containing the bones of lacertian, crocodilian, and rhynchosaurian reptiles, may not be referable to the "Old Red" or Devonian group. Still, no traces of this cla.s.s have yet been detected in rocks as ancient as those in which the oldest fish have been found. [38]

As to fossil representatives of the ichthyic type, the most ancient were not supposed before 1838 to be of a date anterior to the Coal, but they have since been traced back, first to the Devonian, and then to the Silurian rocks. No remains, however, of them or of any vertebrate animal have yet been discovered in the Ordovician strata, rich as these are in invertebrate fossils, nor in the still older Cambrian; so that we seem authorised to conclude, though not without considerable reserve, that the vertebrate type was extremely scarce, if not wholly wanting, in those epochs often spoken of as "primitive," but which, if the Development Theory be true, were probably the last of a long series of antecedent ages in which living beings flourished.

As to the Mollusca, which afford the most unbroken series of geological medals, the highest of that cla.s.s, the Cephalopoda, abounded in older Silurian times, comprising several hundred species of chambered univalves. Had there been strong prepossessions against the progressive theory, it would probably have been argued that when these cephalopods abounded, and the siphonated gasteropods were absent, a higher order of zoophagous mollusca discharged the functions afterwards performed by an inferior order in the Secondary, Tertiary, and Post-Tertiary seas. But I have never seen this view suggested as adverse to the doctrine of progress, although much stress has been laid on the fact that the Silurian Brachiopoda, creatures of a lower grade, formerly discharged the functions of the existing lamellibranchiate bivalves, which are higher in the scale.

It is said truly that the Ammonite, Orthoceras, and Nautilus of these ancient rocks were of the tetrabranchiate division, and none of them so highly organised as the Belemnite and other dibranchiate cephalopods which afterwards appeared, and some of which now flourish in our seas.

Therefore, we may infer that the simplest forms of the Cephalopoda took precedence of the more complex in time. But if we embrace this view, we must not forget that there are living Cephalopoda, such as the Octopods, which are devoid of any hard parts, whether external or internal, and which could leave behind them no fossil memorials of their existence, so that we must make a somewhat arbitrary a.s.sumption, namely, that at a remote era, no such Dibranchiata were in being, in order to avail ourselves of this argument in favour of progression. On the other hand, it is true that in the Lower Cambrian not even the sh.e.l.l-bearing tetrabranchiates have yet been discovered.

In regard to plants, although the generalisation above cited of M.

Adolphe Brongniart is probably true, there has been a tendency in the advocates of progression to push the inferences deducible from known facts, in support of their favourite dogma, somewhat beyond the limits which the evidence justifies. Dr. Hooker observes, in his recent "Introductory Essay to the Flora of Australia," that it is impossible to establish a parallel between the successive appearances of vegetable forms in time, and their complexity of structure or specialisation of organs as represented by the successively higher groups in the natural method of cla.s.sification. He also adds that the earliest recognisable Cryptogams are not only the highest now existing, but have more highly differentiated vegetative organs than any subsequently appearing, and that the dicotyledonous embryo and perfect exogenous wood, with the highest specialised tissue known (the coniferous with glandular tissue), preceded the monocotyledonous embryo and endogenous wood in date of appearance on the globe--facts wholly opposed to the doctrine of progression, and which can only be set aside on the supposition that they are fragmentary evidence of a time farther removed from the origin of vegetation than from the present day.*

(* "Introductory Essay to the Flora of Australia," page 31 London 1859. Published separately.) [39]

It would be an easy task to multiply objections to the theory now under consideration; but from this I refrain, as I regard it not only as a useful, but rather in the present state of science as an indispensable hypothesis, and one which though destined hereafter to undergo many and great modifications will never be overthrown.

It may be thought almost paradoxical that writers who are most in favour of trans.m.u.tation (Mr. C. Darwin and Dr. J. Hooker, for example) are nevertheless among those who are most cautious, and one would say timid, in their mode of espousing the doctrine of progression; while, on the other hand, the most zealous advocates of progression are oftener than not very vehement opponents of trans.m.u.tation. We might have antic.i.p.ated a contrary leaning on the part of both, for to what does the theory of progression point? It supposes a gradual elevation in grade of the vertebrate type in the course of ages from the most simple ichthyic form to that of the placental mammalia and the coming upon the stage last in the order of time of the most anthropomorphous mammalia, followed by the human race--this last thus appearing as an integral part of the same continuous series of acts of development, one link in the same chain, the crowning operation as it were of one and the same series of manifestations of creative power. If the dangers apprehended from trans.m.u.tation arise from the too intimate connection which it tends to establish between the human and merely animal natures, it might have been expected that the progressive development of organisation, instinct, and intelligence might have been unpopular, as likely to pioneer the way for the reception of the less favoured doctrine. But the true explanation of the seeming anomaly is this, that no one can believe in trans.m.u.tation who is not profoundly convinced that all we know in palaeontology is as nothing compared with what we have yet to learn, and they who regard the record as so fragmentary, and our acquaintance with the fragments which are extant as so rudimentary, are apt to be astounded at the confidence placed by the progressionists in data which must be defective in the extreme. But exactly in proportion as the completeness of the record and our knowledge of it are overrated, in that same degree are many progressionists unconscious of the goal towards which they are drifting. Their faith in the fullness of the annals leads them to regard all breaks in the series of organic existence, or in the sequence of the fossiliferous rocks, as proofs of original chasms and leaps in the course of nature--signs of the intermittent action of the creational force, or of catastrophes which devastated the habitable surface. They do not doubt that there is a continuity of plan, but they believe that it exists in the Divine mind alone, and they are therefore without apprehension that any facts will be discovered which would imply a material connection between the outgoing organisms and the incoming ones.

CHAPTER 21. -- ON THE ORIGIN OF SPECIES BY VARIATION AND NATURAL SELECTION.

Mr. Darwin's Theory of the Origin of Species by Natural Selection.

Memoir by Mr. Wallace.

Manner in which favoured Races prevail in the Struggle for Existence.

Formation of new Races by breeding.

Hypotheses of definite and indefinite Modifiability equally arbitrary.

Compet.i.tion and Extinction of Races.

Progression not a necessary Accompaniment of Variation.

Distinct Cla.s.ses of Phenomena which Natural Selection explains.

Unity of Type, Rudimentary Organs, Geographical Distribution, Relation of the extinct to the living Fauna and Flora, and mutual Relations of successive Groups of Fossil Forms.