CHAPTER XXIII.

THE SUCCESSION OF LIFE UPON THE GLOBE.

In conclusion, it may not be out of place if we attempt to summarise, in the briefest possible manner, some of the princ.i.p.al results which may be deduced as to the succession of life upon the earth from the facts which have in the preceding portion of this work been pa.s.sed in review. That there was a time when the earth was void of life is universally admitted, though it may be that the geological record gives us no direct evidence of this. That the globe of to-day is peopled with innumerable forms of life whose term of existence has been, for the most part, but as it were of yesterday, is likewise an a.s.sertion beyond dispute. Can we in any way connect the present with the remote past, and can we indicate even imperfectly the conditions and laws under which the existing order was brought about? The long series of fossiliferous deposits, with their almost countless organic remains, is the link between what has been and what is; and if any answer to the above question can be arrived at, it will be by the careful and conscientious study of the facts of Palaeontology. In the present state of our knowledge, it may be safely said that anything like a dogmatic or positive opinion as to the precise sequence of living forms upon the globe, and still more as to the manner in which this sequence may have been brought about, is incapable of scientific proof. There are, however, certain general deductions from the known facts which may be regarded as certainly established.

In the first place, it is certain that there has been a _succession_ of life upon the earth, different specific and generic types succeeding one another in successive periods. It follows from this, that the animals and plants with which we are familiar as living, were not always upon the earth, but that they have been preceded by numerous races more or less differing from them.

What is true of the species of animals and plants, is true also of the higher zoological divisions; and it is, in the second place, quite certain that there has been a similar _succession_ in the order of appearance of the primary groups ("sub-kingdoms,"

"cla.s.ses," &c.) of animals and vegetables. These great groups did not all come into existence at once, but they made their appearance successively. It is true that we cannot be said to be certainly acquainted with the first _absolute_ appearance of any great group of animals. No one dare a.s.sert positively that the apparent first appearance of Fishes in the Upper Silurian is really their first introduction upon the earth: indeed, there is a strong probability against any such supposition. To whatever extent, however, future discoveries may push back the first advent of any or of all of the great groups of life, there is no likelihood that anything will be found out which will materially alter the _relative_ succession of these groups as at present known to us.

It is not likely, for example, that the future has in store for us any discovery by which it would be shown that Fishes were in existence before Molluscs, or that Mammals made their appearance before Fishes. The sub-kingdoms of Invertebrate animals were all represented in Cambrian times--and it might therefore be inferred that _these_ had all come simultaneously into existence; but it is clear that this inference, though incapable of actual disproof, is in the last degree improbable. Anterior to the Cambrian is the great series of the Laurentian, which, owing to the metamorphism to which it has been subjected, has so far yielded but the singular _Eozoon_. We may be certain, however, that others of the Invertebrate sub-kingdoms besides the Protozoa were in existence in the Laurentian period; and we may infer from known a.n.a.logies that they appeared successively, and not simultaneously.

When we come to smaller divisions than the sub-kingdoms--such as cla.s.ses, orders, and families--a similar succession of groups is observable. The different cla.s.ses of any given sub-kingdom, or the different orders of any given cla.s.s, do not make their appearance together and all at once, but they are introduced upon the earth in _succession_. More than this, the different cla.s.ses of a sub-kingdom, or the different orders of a cla.s.s, _in the main succeed one another in the relative order of their zoological rank--the lower groups appearing first and the higher groups last_. It is true that in the Cambrian formation--the earliest series of sediments in which fossils are abundant--we find numerous groups, some very low, others very high, in the zoological scale, which _appear_ to have simultaneously flashed into existence. For reasons stated above, however, we cannot accept this appearance as real; and we must believe that many of the Cambrian groups of animals really came into being long before the commencement of the Cambrian period. At any rate, in the long series of fossiliferous deposits of later date than the Cambrian the above-stated rule holds good as a broad generalisation--that the lower groups, namely, precede the higher in point of time; and though there are apparent exceptions to the rule, there are none of such a nature as not to admit of explanation. Some of the leading facts upon which this generalisarion is founded will be enumerated immediately; but it will be well, in the first place, to consider briefly what we precisely mean when we speak of "higher" and "lower" groups.

It is well known that naturalists are in the habit of "cla.s.sifying"

the innumerable animals which now exist upon the globe; or, in other words, of systematically arranging them into groups. The precise arrangement adopted by one naturalist may differ in minor details from that adopted by another; but all are agreed as to the fundamental points of cla.s.sification, and all, therefore, agree in placing certain groups in a certain sequence. What, then, is the principle upon which this sequence is based? Why, for example, are the Sponges placed below the Corals; these below the Sea-urchins; and these, again, below the Sh.e.l.l-fish? Without entering into a discussion of the principles of zoological cla.s.sification, which would here be out of place, it must be sufficient to say that the sequence in question is based upon the _relative type of organisation_ of the groups of animals cla.s.sified. The Corals are placed above the Sponges upon the ground that, regarded as a whole, the _plan or type of structure_ of a Coral is more complex than that of a Sponge. It is not in the slightest degree that the Sponge is in any respect less highly organised or less perfect, as a Sponge, than is the Coral as a Coral. Each is equally perfect in its own way; but the structural pattern of the Coral is the highest, and therefore it occupies a higher place in the zoological scale. It is upon this principle, then, that the primary subdivisions of the animal kingdom (the so-called "sub-kingdoms") are arranged in a certain order. Coming, again, to the minor subdivisions (cla.s.ses, orders, &c.) of each sub-kingdom, we find a different but entirely a.n.a.logous principle employed as a means of cla.s.sification. The numerous animals belonging to any given sub-kingdom are formed upon the same fundamental plan of structure; but they nevertheless admit of being arranged in a regular series of groups. All the Sh.e.l.l-fish, for example, are built upon a common plan, this plan representing the ideal Mollusc; but there are at the same time various groups of the _Mollusca_, and these groups admit of an arrangement in a given sequence. The principle adopted in this case is simply of _the relative elaboration of the common type_. The Oyster is built upon the same ground-plan as the Cuttle-fish; but this plan is carried out with much greater elaboration, and with many more complexities, in the latter than in the former: and in accordance with this, the _Cephalopoda_ const.i.tute a higher group than the Bivalve Sh.e.l.l-fish. As in the case of superiority of structural type, so in this case also, it is not in the least that the Oyster is an _imperfect_ animal. On the contrary, it is just as perfectly adapted by its organisation to fill its own sphere and to meet the exigencies of its own existence as is the Cuttle-fish; but the latter lives a life which is, physiologically, higher than the former, and its organisation is correspondingly increased in complexity.

This being understood, it may be repeated that, in the main, the succession of life upon the globe in point of _time_ has corresponded with the relative order of succession of the great groups of animals in _zoological rank_; and some of the more striking examples of this may be here alluded to. Amongst the _Echinoderms_, for instance, the two orders generally admitted to be the "lowest" in the zoological scale--namely, the _Crinoids_ and the _Cystoids_--are likewise the oldest, both, appearing in the Cambrian, the former slowly dying out as we approach the Recent period, and the latter disappearing wholly before the close of the Palaeozoic period. Amongst the _Crustaceans_, the ancient groups of the Trilobites, Ostracodes, Phyllopods, Eurypterids, and Limuloids, some of which exist at the present day, are all "low" types; whereas the highly-organised Decapods do not make their appearance till near the close of the Palaeozoic epoch, and they do not become abundant till we reach Mesozoic times. Amongst the _Mollusca_, those Bivalves which possess breathing-tubes (the "siphonate" Bivalves) are generally admitted to be higher than those which are dest.i.tute of these organs (the "asiphonate" Bivalves); and the latter are especially characteristic of the Palaeozoic period, whilst the former abound in Mesozoic and Kainozoic formations. Similarly, the Univalves with breathing-tubes and a corresponding notch in the mouth of the sh.e.l.l ("siphonostomatous" Univalves) are regarded as higher in the scale than the round-mouthed vegetable-eating Sea-snails, in which no respiratory siphons exist ("holostomatous" Univalves); but the latter abound in the Palaeozoic rocks--whereas the former do not make their appearance till the Jura.s.sic period, and their higher groups do not seem to have existed till the close of the Cretaceous. The _Cephalopods_, again--the highest of all the groups of Mollusca--are represented in the Palaeozoic rocks exclusively by Tetrabranchiate forms, which const.i.tute the lowest of the two orders of this cla.s.s; whereas the more highly specialised Dibranchiates do not make their appearance till the commencement of the Mesozoic. The Palaeozoic Tetrabranchiates, also, are of a much simpler type than the highly complex _Ammonitidoe_ of the Mesozoic.

Similar facts are observable amongst the _Vertebrate animals_.

The Fishes are the lowest cla.s.s of Vertebrates, and they are the first to appear, their first certain occurrence being in the Upper Silurian; whilst, even if the Lower Silurian and Upper Cambrian "Conodonts" were shown to be the teeth of Fishes, there would still remain the enormously long periods of the Laurentian and Lower Cambrian, during which there were Invertebrates, but no Vertebrates. The _Amphibians_, the next cla.s.s in zoological order, appears later than the Fishes, and is not represented till the Carboniferous; whilst its highest group (that of the Frogs and Toads) does not make its entrance upon the scene till Tertiary times are reached. The cla.s.s of the _Reptiles_, again, the next in order, does not appear till the Permian, and therefore not till after Amphibians of very varied forms had been in existence for a protracted period. The _Birds_ seem to be undoubtedly later than the Reptiles; but, owing to the uncertainty as to the exact point of their first appearance, it cannot be positively a.s.serted that they preceded Mammals, as they should have done. Finally, the Mesozoic types of _Mammals_ are mainly, if not exclusively, referable to the _Marsupials_, one of the lowest orders of the cla.s.s; whilst the higher orders of the "Placental" Quadrupeds are not with certainty known to have existed prior to the commencement of the Tertiary period.

Facts of a very similar nature are offered by the succession of Plants upon the globe. Thus the vegetation of the Palaeozoic period consisted princ.i.p.ally of the lowly-organised groups of the Cryptogamous or Flowerless plants. The Mesozoic formations, up to the Chalk, are especially characterised by the naked-seeded Flowering plants--the Conifers and the Cycads; whilst the higher groups of the Angiospermous Exogens and Monocotyledons characterise the Upper Cretaceous and Tertiary rocks.

Facts of the above nature--and they could be greatly multiplied--seem to point clearly to the existence of some law of progression, though we certainly are not yet in a position to formulate this law, or to indicate the precise manner in which it has operated.

Two considerations, also, must not be overlooked. In the first place, there are various groups, some of them highly organised, which make their appearance at an extremely ancient date, but which continue throughout geological time almost unchanged, and certainly unprogressive. Many of these "persistent types" are known--such as various of the _Foraminifera_, the _Linguloe_, the _Nautili_, &c.; and they indicate that under given conditions, at present unknown to us, it is possible for a life-form to subsist for an almost indefinite period without any important modification of its structure. In the second place, whilst the facts above mentioned point to some general law of progression of the great zoological groups, it cannot be a.s.serted that the primeval types _of any given group_ are necessarily "lower," zoologically speaking, than their modern representatives. Nor does this seem to be at all necessary for the establishment of the law in question. It cannot be a.s.serted, for example, that the Ganoid and Placoid Fishes of the Upper Silurian are in themselves less highly organised than their existing representatives; nor can it even be a.s.serted that the Ganoid and Placoid orders are low _groups_ of the cla.s.s _Pisces_. On the contrary, they are high groups; but then it must be remembered that these are probably not really the first Fishes, and that if we meet with Fishes at some future time in the Lower Silurian or Cambrian, these may easily prove to be representatives of the lower orders of the cla.s.s. This question cannot be further entered into here, as its discussion could be carried out to an almost unlimited length; but whilst there are facts pointing both ways, it appears that at present we are not justified in a.s.serting that the earlier types of each group--so far as these are known to us, or really are without predecessors--are _necessarily_ or _invariably_ more "degraded" or "embryonic" in their structure than their more modern representatives.

It remains to consider very briefly how far Palaeontology supports the doctrine of "Evolution," as it is called; and this, too, is a question of almost infinite dimensions, which can but be glanced at here. Does Palaeontology teach us that the almost innumerable kinds of animals and plants which we know to have successively flourished upon the earth in past times were produced separately and wholly independently of each other, at successive periods? or does it point to the theory that a large number of these supposed distinct forms, have been in reality produced by the slow modification of a comparatively small number of primitive types? Upon the whole, it must be unhesitatingly replied that the evidence of Palaeontology is in favour of the view that the succession of life-forms upon the globe has been to a large extent regulated by some orderly and constantly-acting law of modification and evolution. Upon no other theory can we comprehend how the fauna of any given formation is more closely related to that of the formation next below in the series, and to that of the formation next above, than to that of any other series of deposits. Upon no other view can we comprehend why the Post-Tertiary Mammals of South America should consist princ.i.p.ally of Edentates, Llamas, Tapirs, Peccaries, Platyrhine Monkeys, and other forms now characterising this continent; whilst those of Australia should be wholly referable to the order of Marsupials. On no other view can we explain the common occurrence of "intermediate" or "transitional" forms of life, filling in the gaps between groups now widely distinct.

On the other hand, there are facts which point clearly to the existence of some law other than that of evolution, and probably of a deeper and more far-reaching character. Upon no theory of evolution can we find a satisfactory explanation for the constant introduction throughout geological time of new forms of life, which do not appear to have been preceded by pre-existent allied types; The Graptolites and Trilobites have no known predecessors, and leave no known successors. The Insects appear suddenly in the Devonian, and the Arachnides and Myriapods in the Carboniferous, under well-differentiated and highly-specialised types. The Dibranchiate Cephalopods appear with equal apparent suddenness in the older Mesozoic deposits, and no known type of the Palaeozoic period can be pointed to as a possible ancestor. The _Hippuritidoe_ of the Cretaceous burst into a varied life to all appearance almost immediately after their first introduction into existence.

The wonderful Dicotyledonous flora of the Upper Cretaceous period similarly surprises us without any prophetic annunciation from the older Jura.s.sic.

Many other instances could be given; but enough has been said to show that there is a good deal to be said on both sides, and that the problem is one environed with profound difficulties.

One point only seems now to be universally conceded, and that is, that the record of life in past time is not interrupted by gaps other than those due to the necessary imperfections of the fossiliferous series, to the fact that many animals are incapable of preservation in a fossil condition, or to other causes of a like nature. All those who are ent.i.tled to speak on this head are agreed that the introduction of new and the destruction of old species have been slow and gradual processes, in no sense of the term "catastrophistic." Most are also willing to admit that "Evolution" has taken place in the past, to a greater or less extent, and that a greater or less number of so-called species of fossil animals are really the modified descendants of pre-existent forms. _How_ this process of evolution has been effected, to what extent it has taken place, under what conditions and laws it has been carried out, and how far it may be regarded as merely auxiliary and supplemental to some deeper law of change and progress, are questions to which, in spite of the brilliant generalisations of Darwin, no satisfactory answer can as yet be given. In the successful solution of this problem--if soluble with the materials available to our hands--will lie the greatest triumph that Palaeontology can hope to attain; and there is reason to think that, thanks to the guiding-clue afforded by the genius of the author of the 'Origin of Species,' we are at least on the road to a sure, though it may be a far-distant, victory.

APPENDIX.

TABULAR VIEW OF THE CHIEF DIVISIONS OF THE ANIMAL KINGDOM.

(Extinct groups are marked with an asterisk. Groups not represented at all as fossils are marked with two asterisks.)

INVERTEBRATE ANIMALS.

SUB-KINGDOM I.--PROTOZOA.

Animal simple or compound; body composed of "sarcode," not definitely segmented; no nervous system; and no digestive apparatus, beyond occasionally a mouth and gullet.

CLa.s.s I. GREGARINIDae.**

CLa.s.s II. RHIZOPODA.

_Order_ 1. _Monera_.**

" 2. _Amoebea_.**

" 3. _Foraminifera_.

" 4. _Radiolaria_ (Polycystines, &c.) " 5. _Spongida_ (Sponges).

CLa.s.s III. INFUSORIA.**

SUB-KINGDOM II.--COELENTERATA.

Animal simple or compound; body-wall composed of two princ.i.p.al layers; digestive ca.n.a.l freely communicating with the general cavity of the body; no circulating organs, and no nervous system or a rudimentary one; mouth surrounded by tentacles, arranged, like the internal organs, in a "radiate" or star-like manner.

CLa.s.s I. HYDROZOA.

_Sub-cla.s.s_ 1. _Hydroida_ ("Hydroid Zoophytes"). _Ex._ Fresh-water Polypes,** Pipe-corallines (_Tubularia_), Sea-Firs (_Sertularia_).

_Sub-cla.s.s_ 2. _Siphonophora_** ("Oceanic Hydrozoa").

_Ex_. Portuguese Man-of-war (_Physalia_).

_Sub-cla.s.s_ 3. _Discophora_ ("Jelly-fishes"). Only known as fossils by impressions of their stranded carca.s.ses.

_Sub-cla.s.s_ 4. _Lucernarida_ ("Sea-blubbers"). Also only known as fossils by impressions left in fine-grained strata.

_Sub-cla.s.s_ 5. _Graptolitidoe_* ("Graptolites").

CLa.s.s II. ACTINOZOA.

_Order_ 1. _Zoantharia_. _Ex_. Sea-anemones**

(_Actinidoe_), Star-corals (_Astroeidoe_).

_Order_ 2. _Alcyonaria_. _Ex_. Sea-pens (_Pennatula_), Organ-pipe Coral (_Tubipora_), Red Coral (_Corallium_).

_Order_ 3. _Rugosa_ ("Rugose Corals").

" 4. _Ctenophora_.** _Ex_. Venus's Girdle (_Cestum_).

SUB-KINGDOM III.--ANNULOIDA.

Animals in which the digestive ca.n.a.l is completely shut off from the cavity of the body; a distinct nervous system; a system of branched "water-vessels," which usually communicate with the exterior. Body of the adult often "radiate," and never composed of a succession of definite rings.

CLa.s.s I. ECHINODERMATA.

_Order_ 1. _Crinoidea_ ("Sea-lilies"). _Ex_.

Feather-star (_Comatula_), Stone-lily (_Encrinus_*).

_Order_ 2. _Blastoidea_* ("Pentremites").

" 3. _Cystoidea_* ("Globe-lilies").

" 4. _Ophiuroidea_ ("Brittle-stars"). _Ex_.

Sand-stars (_Ophiura_), Brittle-stars (_Ophiocoma_).

_Order_ 5. _Asteroidea_ ("Star-fishes"). Ex. Cross-fish (_Uraster_), Sun-star (_Solaster_).

_Order_ 6. _Echinoidea_ ("Sea-urchins"). Ex. Sea-eggs (_Echinus_), Heart-urchins (_Spatangus_).

_Order_ 7. _Holothuroidea_ ("Sea-cuc.u.mbers"). _Ex_.

Trepangs (_Holothuria_).

CLa.s.s II. SCOLECIDA** (Intestinal Worms, Wheel Animalcules, &c.)

SUB-KINGDOM IV.--ANNULOSA.

Animal composed of numerous definite segments placed one behind the other; nervous system forming a knotted cord placed along the lower (ventral) surface of the body.

_Division A. Anarthropoda_. No jointed limbs.