ALTERNATION OF GENERATIONS.

Hitherto, no rival hypothesis has been proposed as a subst.i.tute for the doctrine of trans.m.u.tation; for what we term "independent creation," or the direct intervention of the Supreme Cause, must simply be considered as an avowal that we deem the question to lie beyond the domain of science.

The discovery by Steenstrup of alternate generation enlarges our views of the range of metamorphosis through which a species may pa.s.s, so that some of its stages (as when a Sertularia and a Medusa interchange) deviate so far from others as to have been referred by able zoologists to distinct genera, or even families. But in all these cases the organism, after running through a certain cycle of change, returns to the exact point from which it set out, and no new form or species is thereby introduced into the world. The only secondary cause therefore which has as yet been even conjecturally brought forward, to explain how in the ordinary course of nature a new specific form may be generated is, as Lamarck declared, "variation," and this has been rendered a far more probable hypothesis by the way in which "natural selection" is shown to give intensity and permanency to certain varieties.

INDEPENDENT CREATION.

When I formerly advocated the doctrine that species were primordial creations and not derivative, I endeavoured to explain the manner of their geographical distribution, and the affinity of living forms to the fossil types nearest akin to them in the Tertiary strata of the same part of the globe, by supposing that the creative power, which originally adapts certain types to aquatic and others to terrestrial conditions, has at successive geological epochs introduced new forms best suited to each area and climate, so as to fill the places of those which may have died out.

In that case, although the new species would differ from the old (for these would not be revived, having been already proved by the fact of their extinction to be incapable of holding their ground), still they would resemble their predecessors generically. For, as Mr. Darwin states in regard to new races, those of a dominant type inherit the advantages which made their parent species flourish in the same country, and they likewise partake in those general advantages which made the genus to which the parent species belonged a large genus in its own country.

We might therefore, by parity of reasoning, have antic.i.p.ated that the creative power, adapting the new types to the new combination of organic and inorganic conditions of a given region, such as its soil, climate, and inhabitants, would introduce new modifications of the old types--marsupials, for example, in Australia, new sloths and armadilloes in South America, new heaths at the Cape, new roses in the northern and new calceolarias in the southern hemisphere. But to this line of argument Mr. Darwin and Dr. Hooker reply that when animals or plants migrate into new countries, whether a.s.sisted by man or without his aid, the most successful colonisers appertain by no means to those types which are most allied to the old indigenous species. On the contrary it more frequently happens that members of genera, orders, or even cla.s.ses, distinct and foreign to the invaded country, make their way most rapidly and become dominant at the expense of the endemic species. Such is the case with the placental quadrupeds in Australia, and with horses and many foreign plants in the pampas of South America, and numberless instances in the United States and elsewhere which might easily be enumerated. Hence the trans.m.u.tationists infer that the reason why these foreign types, so peculiarly fitted for these regions, have never before been developed there is simply that they were excluded by natural barriers. But these barriers of sea or desert or mountain could never have been of the least avail had the creative force acted independently of material laws or had it not pleased the Author of Nature that the origin of new species should be governed by some secondary causes a.n.a.logous to those which we see preside over the appearance of new varieties, which never appear except as the offspring of a parent stock very closely resembling them.

CHAPTER 22. -- OBJECTIONS TO THE HYPOTHESIS OF TRANs.m.u.tATION CONSIDERED.

Statement of Objections to the Hypothesis of Trans.m.u.tation founded on the Absence of Intermediate Forms.

Genera of which the Species are closely allied.

Occasional Discovery of the missing Links in a Fossil State.

Davidson's Monograph on the Brachiopoda.

Why the Gradational Forms, when found, are not accepted as Evidence of Trans.m.u.tation.

Gaps caused by Extinction of Races and Species.

Vast Tertiary Periods during which this Extinction has been going on in the Fauna and Flora now existing.

Genealogical Bond between Miocene and Recent Plants and Insects.

Fossils of Oeningen.

Species of Insects in Britain and North America represented by distinct Varieties.

Falconer's Monograph on living and fossil Elephants.

Fossil Species and Genera of the Horse Tribe in North and South America.

Relation of the Pliocene Mammalia of North America, Asia, and Europe.

Species of Mammalia, though less persistent than the Mollusca, change slowly.

Arguments for and against Trans.m.u.tation derived from the Absence of Mammalia in Islands.

Imperfection of the Geological Record.

Intercalation of newly discovered Formation of intermediate Age in the chronological Series.

Reference of the St. Ca.s.sian Beds to the Tria.s.sic Periods.

Discovery of new organic Types.

Feathered Archaeopteryx of the Oolite.

THEORY OF TRANs.m.u.tATION--ABSENCE OF INTERMEDIATE LINKS.

The most obvious and popular of the objections urged against the theory of trans.m.u.tation may be thus expressed: If the extinct species of plants and animals of the later geological periods were the progenitors of the living species, and gave origin to them by variation and natural selection, where are all the intermediate forms, fossil and living, through which the lost types must have pa.s.sed during their conversion into the living ones? And why do we not find almost everywhere pa.s.sages between the nearest allied species and genera, instead of such strong lines of demarcation and often wide intervening gaps?

We may consider this objection under two heads:--

First. To what extent are the gradational links really wanting in the living creation or in the fossil world, and how far may we expect to discover such as are missing by future research?

Secondly. Are the gaps more numerous than we ought to antic.i.p.ate, allowing for the original defective state of the geological records, their subsequent dilapidation and our slight acquaintance with such parts of them as are extant, and allowing also for the rate of extinction of races and species now going on, and which has been going on since the commencement of the Tertiary period?

First. As to the alleged absence of intermediate varieties connecting one species with another, every zoologist and botanist who has engaged in the task of cla.s.sification has been occasionally thrown into this dilemma--if I make more than one species in this group, I must, to be consistent, make a great many. Even in a limited region like the British Isles this embarra.s.sment is continually felt.

Scarcely any two botanists, for example, can agree as to the number of roses, still less as to how many species of bramble we possess. Of the latter genus, Rubus, there is one set of forms respecting which it is still a question whether it ought to be regarded as const.i.tuting three species or thirty-seven. Mr. Bentham adopts the first alternative and Mr. Babington the second, in their well-known treatises on British plants.

We learn from Dr. Hooker that at the antipodes, both in New Zealand and Australia, this same genus Rubus is represented by several species rich in individuals and remarkable for their variability. When we consider how, as we extend our knowledge of the same plant over a wider area, new geographical varieties commonly present themselves, and then endeavour to imagine the number of forms of the genus Rubus which may now exist, or probably have existed, in Europe and in regions intervening between Europe and Australia, comprehending all which may have flourished in Tertiary and Post-Tertiary periods, we shall perceive how little stress should be laid on arguments founded on the a.s.sumed absence of missing links in the flora as it now exists.

If in the battle of life the compet.i.tion is keenest between closely allied varieties and species, as Mr. Darwin contends, many forms can never be of long duration, nor have a wide range, and these must often pa.s.s away without leaving behind them any fossil memorials. In this manner we may account for many breaks in the series which no future researches will ever fill up.

DAVIDSON ON FOSSIL BRACHIOPODA.

It is from fossil conchology more than from any other department of the organic world that we may hope to derive traces of a transition from certain types to others, and fossil memorials of all the intermediate shades of form. We may especially hope to gain this information from the study of some of the lower groups, such as the Brachiopoda, which are persistent in type, so that the thread of our inquiry is less likely to be interrupted by breaks in the sequence of the fossiliferous rocks.

The splendid monograph just concluded by Mr. Davidson on the British Brachiopoda, ill.u.s.trates, in the first place, the tendency of certain generic forms in this division of the mollusca to be persistent throughout the whole range of geological time yet known to us; for the four genera, Rhynchonella, Crania, Discina, and Lingula, have been traced through the Silurian, Devonian, Carboniferous, Permian, Jura.s.sic, Cretaceous, Tertiary, and Recent periods, and still retain in the existing seas the identical shape and character which they exhibited in the earliest formations. On the other hand, other Brachiopoda have gone through in shorter periods a vast series of transformations, so that distinct specific and even generic names have been given to the same varying form, according to the different aspects and characters it has put on in successive sets of strata.

In proportion as materials of comparison have acc.u.mulated, the necessity of uniting species previously regarded as distinct under one denomination has become more and more apparent. Mr. Davidson, accordingly, after studying not less than 260 reputed species from the British Carboniferous rocks, has been obliged to reduce that number to 100, to which he has added 20 species either entirely new or new to the British strata; but he declares his conviction that, when our knowledge of these 120 Brachiopoda is more complete, a further reduction of species will take place.

Speaking of one of these forms, which he calls Spirifer trigonalis, he says that it is so dissimilar to another extreme of the series, S.

cra.s.sa, that in the first part of his memoir (published some ten years ago) he described them as distinct, and the idea of confounding them together must, he admits, appear absurd to those who have never seen the intermediate links, such as are presented by S. bisulcata, and at least four others with their varieties, most of them sh.e.l.ls formerly recognised as distinct by the most eminent palaeontologists, but respecting which these same authorities now agree with Mr. Davidson in uniting them into one species.*

(* "Monograph on British Brachiopoda" Palaeontographical Society page 222.)

The same species has sometimes continued to exist under slightly modified forms throughout the whole of the Ordovician and Silurian as well as the entire Devonian and Carboniferous periods, as in the case of the sh.e.l.l generally known as Leptaena rhomboidalis, Wahlenberg. No less than fifteen commonly received species are demonstrated by Mr. Davidson by the aid of a long series of transitional forms, to appertain to this one type; and it is acknowledged by some of the best writers that they were induced on purely theoretical grounds to give distinct names to some of the varieties now suppressed, merely because they found them in rocks so widely remote in time that they deemed it contrary to a.n.a.logy to suppose that the same species could have endured so long: a fallacious mode of reasoning, a.n.a.logous to that which leads some zoologists and botanists to distinguish by specific names slight varieties of living plants and animals met with in very remote countries, as in Europe and Australia, for example; it being a.s.sumed that each species has had a single birthplace or area of creation, and that they could not by migration have gone from the northern to the southern hemisphere across the intervening tropics.

Examples are also given by Mr. Davidson of species which pa.s.s from the Devonian into the Carboniferous, and from that again into the Permian rocks. The vast longevity of such specific forms has not been generally recognised in consequence of the change of names which they have undergone when derived from such distant formations, as when Atrypa unguicularis a.s.sumes, when derived from a Carboniferous rock, the name of Spirifer Urei, besides several other synonyms, and then, when it reaches the Permian period, takes the name of Spirifer Clannyana, King; all of which forms the author of the monograph, now under consideration, a.s.serts to be one and the same.

No geologist will deny that the distance of time which separates some of the eras above alluded to, or the dates of the earliest and latest appearances of some of the fossils above mentioned, must be reckoned by millions of years. According to Mr. Darwin's views, it is only by having at our command the records of such enormous periods that we can expect to be able to point out the gradations which unite very distinct specific forms. But the advocate of trans.m.u.tation must not be disappointed if, when he has succeeded in obtaining some of the proofs which he was challenged to produce, they make no impression on the mind of his opponent. All that will be conceded is that specific variation in the Brachiopoda, at least, has a wider range than was formerly suspected. So long as several allied species were brought nearer and nearer to each other, considerable uneasiness might have been felt as to the reality of species in general, but when fifteen or more are once fairly merged in one group, const.i.tuting in the aggregate a single species, one and indivisible, and capable of being readily distinguished from every other group at present known, all misgivings are at an end.

Implicit trust in the immutability of species is then restored, and the more insensible the shades from one extreme to the other, in a word, the more complete the evidence of transition, the more nugatory does the argument derived from it appear. It then simply resolves itself into one of those exceptional instances of what is called a protean form.

Thirty years ago a great London dealer in sh.e.l.ls, himself an able naturalist, told me that there was nothing he had so much reason to dread, as tending to depreciate his stock in trade, as the appearance of a good monograph on some large genus of mollusca; for, in proportion as the work was executed in a philosophical spirit, it was sure to injure him, every reputed species p.r.o.nounced to be a mere variety becoming from that time unsaleable. Fortunately, so much progress has since been made in England in estimating the true ends and aims of science, that specimens indicating a pa.s.sage between forms usually separated by wide gaps, whether in the Recent or fossil fauna, are eagerly sought for, and often more prized than the mere normal or typical forms.

It is clear that the more ancient the existing mollusca, or the farther back into the past we can trace the remains of sh.e.l.ls still living, the more easy it becomes to reconcile with the doctrine of trans.m.u.tation the distinctness in character of the majority of living species. For, what we want is time, first, for the gradual formation, and then for the extinction of races and allied species, occasioning gaps between the survivors.

In the year 1830 I announced, on the authority of M. Deshayes, that about one-fifth of the mollusca of the Falunian or Upper Miocene strata of Europe, belonged to living species. Although the soundness of that conclusion was afterwards called in question by two or three eminent conchologists (and by the late M. Alcide d'Orbigny among others), it has since been confirmed by the majority of living naturalists and is well borne out by the copious evidence on the subject laid before the public in the magnificent work edited by Dr. h.o.e.rnes, and published under the auspices of the Austrian Government, "On the Fossil Sh.e.l.ls of the Vienna Basin."

The collection of Tertiary sh.e.l.ls from which those descriptions and beautiful figures were taken is almost unexampled for the fine state of preservation of the specimens, and the care with which all the varieties have been compared. It is now admitted that about one-third of these Miocene forms, univalves and bivalves included, agree specifically with living mollusca, so that much more than the enormous interval which divides the Miocene from the Recent period must be taken into our account when we speculate on the origin by trans.m.u.tation of the sh.e.l.ls now living, and the disappearance by extinction of intermediate varieties and species.

MIOCENE PLANTS AND INSECTS RELATED TO RECENT SPECIES.

Geologists were acquainted with about three hundred species of marine sh.e.l.ls from the Falunian strata on the banks of the Loire, before they knew anything of the contemporary insects and plants. At length, as if to warn us against inferring from negative evidence the poverty of any ancient set of strata in organic remains proper to the land, a rich flora and entomological fauna was suddenly revealed to us characteristic of Central Europe during the Upper Miocene period. This result followed the determination of the true position of the Oeningen beds in Switzerland, and of certain formations of "Brown Coal" in Germany.

Professor Heer, who has described nearly five hundred species of fossil plants from Oeningen, besides many more from other Miocene localities in Switzerland,* estimates the phanerogamous species which must have flourished in Central Europe at that time at 3000, and the insects as having been more numerous in the same proportion as they now exceed the plants in all lat.i.tudes.

(* Heer, "Flora tertiaria Helvetiae" 1859; and Gaudin's French translation, with additions, 1861.)

This European Miocene flora was remarkable for the preponderance of arborescent and shrubby evergreens, and comprised many generic types no longer a.s.sociated together in any existing flora or geographical province. Some genera, for example, which are at present restricted to America, co-existed in Switzerland with forms now peculiar to Asia, and with others at present confined to Australia.

Professor Heer has not ventured to identify any of this vast a.s.semblage of Miocene plants and insects with living species, so far at least as to a.s.sign to them the same specific names, but he presents us with a list of what he terms h.o.m.ologous forms, which are so like the living ones that he supposes the one to have been derived genealogically from the others. He hesitates indeed as to the manner of the transformation or the precise nature of the relationship, "whether the changes were brought about by some influence exerted continually for ages, or whether at some given moment the old types were struck with a new image."