Page:Catholic Encyclopedia, volume 5.djvu/742

 EVOLUTION

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EVOLUTION

out leaving the slightest trace of their antecedents, origin, birth, or age. Some SOO sp)ecies of this remo- test period are known to us. They belong almost without exception to marine fauna, and are distributed over all the chief groups of the invertebrates. Nearly one-half of them are arthropods. They are the well- known trilobites which occupy a position about the middle of the scale of animal development. Other groups belong to ccelenterates, brachiopods, gastro- pods, and cephalopods. Sponges, too, and traces of worms are found, as also verj- imperfect fragments of scorpions and other insects. Moreover, there can be no doubt that various types of fishes must have ex- isted, since in the Silurian age numerous representa- t ives, such as selachians, ganoids, marsipobranchs, dip- noans, are found from the verj" beginning side by side. Where are the ancestors of these highly specialized beings? The one thing we may affirm is that we know absolutely nothing whatever of a primitive faima and of the nimiberless series of organisms which must have followed them up to the Cambrian era, for the simple reason that we possess absolutely no evidence. More- over, there is not the least trace of pahtontological evidence in favour of the spontaneous awakening of life or of the ascending development out of primitive protoplasmic masses up to the time of the Cambrian era. The Cambrian t^-pes were all of them specialized forms perfectly adapted to time and environments, and not generalized types of zoological systems. The origin of the plant world is also shrouded in impene- trable darkness for the palaeontologist. The enor- mous layers of anthracite and graphite are, according to the most recent investigations, of inorganic origin. Clearly established evidence of plant life only dates from post-SUurian times, and consists of contents of the oldest turf moors — giant-ferns and horsetails, plants akin to the club-mosses, like the Le pidodendron. and Gjinnosperms. like the slender Cordaites. One is astoimded at the rich forms of this long-lost flora, and we search in vain for their ancestors.

It is certainly remarkable, and a fact which clearly proves the transformation of species, that plants be- longing to these remote times vary considerably from their later representatives. But, as Kerner von Mari- laun insists, the "fundamental structure of the tj-pe" is never obliterated, and the degree of organization has at least remained the same. In particular, the pres- ent dwarf-forms of the horse-tails and club-mosses are but miserable remains of their mighty ancestors, and the Cordaites. though different from the present coni- fers, were as highly organized as they. To this must be added the recently discovered fact that seed- bearing plants, which constitute a considerable part of the fern flora of the Carboniferous, are found among the ferns of the Devonian era.

(3) A'lgiosjx-rms and Vertebrates. But how did the imdoubtedly higher forms of a later period originate? To begin with the angiosperms, we are confronted with the fact that these organisms appear quite sud- denly ill the Cretaceous era and, what is more remark- able, in forms as highly organized as their present repre.'^^ntatives. It Ls a fact that principally the di- cotyledons (at least those in the more recent strata) correspond more and more to the present-day forms, clearly indicating the relationship they bear to one an- other. But whence the earliest forms of the creta- ceous came, is shrouded in mystery. Similarly, the gradual transformation of one species into another cannot be proved in any concrete case. Only this much is certain, that if evolution took place, it in- volved a change which did not imply attainment to a higher stage of organization. It must be borne in mind, moreover, that we know of no intermediate forms capable of justifying even as much as a hypoth- esis that angiosperms were evolved from lower plants. If the origin of the angiosperms is for the present an insoluble problem, the genesis of the verte-

brates is no less so. However, in order not to pass en- tirely over the post-Cambrian historj' of the inverte- brates, we must at least make mention of the sig- nificant fact that this fauna seems to be constantly changing, but without ascending to higher forms of organization. The modification is especially manifest in the shell-bearing groups, owing to the changed size, form, and ornamentation of their shells, and in this offers a verj' acceptable basis for the establishment of a series of kindred forms — e. g., with the gastropod genus Paludina of the Slavonian tertian.- strata. But since such structures depend almost entirely on the calcareous nature of the mediimi. and on the varying kind and amount of movement, we can scarcely be in- clined to regard an increased ornamentation of the shell as a mark of real progress in organization, but at most as a temporarj* development of actual disposi- tions due to varj-ing conditions of life.

The first authenticated ancestors of the vertebrates are the fish-remains of the lower Siliu-ian era. Widely removed from them we find in the carboniferous strata the oldest remains of the amphibian quadru- peds and, associated with them, forms of reptiles whose sudden appearance and equally sudden disap- pearance belong to the unsolved problems of paleontol- ogy. Among the Mesozoic fishes we encounter old forms together with teleosts which suddenly appear in the Jurassic strata without producing any transitional forms. It is generally supposed that the teleosts represent a higher grade of organization than the ganoids; as a matter of fact, the teleosts, it would seem, have no structural advantage over the cartilagi- nous fishes in the lesser hardness of the scale and the greater hardness of the skeleton. This is, however, but a shifting, as it were, of development, as the disap- pearance of the rigid body-covering is compensated for by the ossification of the skeleton. At any rate, the origin of the teleosts is an unsolved problem, as is that of the Silurian ganoids. The appearance of birds and mammals is likewise ver>' mysterious. The first known bird is the famous "bird-reptile" Archaop- teryx of the Jurassic strata at Soluhofen. In spite of some characteristics that remind one of reptiles — as for instance the twenty homologous caudal vertebne, the talons, the separated metacarpal bones and the toothed jaw — yet the true bird natiu^ is evinced by the plumage, the pinions, and the bill. In fact Archa:- opterijx is far removed from the reptiles, nor does it constitute any connecting link with the later birds, not even with the toothed Ichthyornis and Hesperonis of the upper Cretaceous era. Certainly the two iso- lated specimens from Soluhofen indicate that birds must have existed a long time before; but where their place of origin is, none can tell.

Paheontologj- is silent likewise about the early his- tory of mammals. The mesozoic representation of this class may have some connexion with marsupials, monotremes, and insectivorous animals, but as to the early historj- of the great majority of placental mam- mals we have no evidence whatever. A vast number of intermediate forms would certainly be required to connect the mammals with the reptiles. No such series of forms is known. Even the genealogj' of the horse, which is considered the most striking example of an evolutionary series within a mammalian family, is scarcely more than a verj' moderately supported hypothesis. Let the reader consider the accompany- ing table of differences in the pala?ontological repre- .seiitativesof the Equida-. Upon the facts embodied in this table, which chiefly refer to fossils found in North .Vmerican strata, the following comments are suggested: The genera of the En(e with the Palceotherinee, we know only