PALEONTOLOGY
412
PALEONTOLOGY
for many of them, tli;it tlii\v owed their existence to
volcanic action. Kqually inconclusive are the earliest
limestones, now that we know that these are still
being produced chemically in the ocean. In short,
pala-ontology tells us nothing about the origin of life;
the whole series of organisms, from the simplest pro-
toplasmic m;usses to the differentiated fonns found in
the Cambrian rocks is missing.
If we survey the fossils so far known in historical order, the following facts are ascertained: The earliest or primary jieriod of the earth is the era of the Pteri- dophyta, the ferns, horsetails, and club-mosses; in the Triassic and .lurassic periods the gymnosperms pre- vail, and beginning with the cretaceous period the angiosperms. TIk^ history of the animal kingdom is similar. Of the arliculala, only the Crustacea appear in the earliest formations, insects and spiders are not found unl il the Upper Carboniferous. The first verte- brates are found in the Upper Silurian, these are some trunk-fish or ostraciid;K, which reached their most flourishing period in the Upper Devonian. The first vertebrates living on Land ajjpcar in the Carboniferous period; these were amphibians represented by the stegocephala, and th(> first reptiles. The Triassic al.so yields the first small mammals, which, however, do not become important until the Old Tertiary period, while true birds are already known in the Jurassic. Man, who appears in the Quaternary, concludes the series. Thus, starling from geological antiquity, the fossils of which still in part seem strange to us, although in al- most all cases they can be inserted without difficulty in the existing orders and classes of tlic animal and vegetable kingdoms, thi i^' is I'l.iind a prof^rrssive ap- proximation to the oiuaniMiis now existing which is completed by the gradual and imliroken succession of beings more and more highly differentiated.
At the first glance this seems to be a brilliant con- firmation of the theory of development, but when more closely examined it is seen that the guiding- thread, which should lead from one point to another, is continually broken and the loose ends cannot readily be connected. Vertebrates first appear in the Silurian and angiosperms in the cretaceous, but there are no organisms leading up to these groui)s. Thus we are met by the broad fact that both vertebrates and flow- ering plants with covered seed appear without inter- mediate links. The same thing is true of each one of the clas.scs in the animal and vegetable kingdoms. We see them, indeed, appear one after another in time, but we always miss the intervening links which would indicate genetic relations among the several orders. It is true that at times animal remains are found which, it is believed, may rightly be claimed as the missing links. The best known of these is prob- ably the aboriginal bird, the archa;opteryx, which ranks midway Ix^t ween reptile and bird. Its plumage, its bird-like fool, and the closed capsule of its .skull characterize it :us a bird, while the structure of the vertebra^ th(^ teeth, and the long, lizard-like tail point to the reptiles. Since, however, it has been found that these reptile-like peculiarities also appear in embrj-onic birds, there is no longer any doubt that the species under consideration are real birds, the highly-ditTerentiated last link of an extinct class of birds. In the same way the opinion that the thero- morpha, a kinfl of reptile, are the aboriginal form of \ertebrates, has not proved tenable. At the same lime we now and then find in the record of successive geological strata forms that may be regarded as the common starting-point of two or of several orders. \\ e know, for instance, the connecting links between the four-branched and six-branched corals, or between the ganoids, and the teleosts fbony fish), also between the two great groups of camivoro\is .and insectivorous marsupials on the one side and the herbivorous mar- supials on the other. At the base of the i)lacental mammalia are found forms which unite the character-
istics of hoofed animals, beasts of prey, and insectiv-
orous animals. Such collective types as they are
called, however, are very rare, where.os according to
the theory of descent they should be found in large
numbers.
In the smallest classified case of minute systematic units it is true pateontological series of descent may be recognized, for here individual species by imper- ceptible mutations lead to new species. The best known line of descent of this kind is probably the ancestral tree of the horse, published long ago by Huxley; but this very ca,se illustrates the dilliculties of such problems, for just now it is very doubtful if some of the links should be inserted in the series. Moreover, such proofs always c<intain hypothetical elements. Besides, connecting links are often lack- ing; or parts separately found, such as teeth or bones, are the only means of completing a line of ancestral descent. A special obstacle to the recognition of true relationship is the phenomenon called convergence. By convergence is meant the fact that, in consequence of similar conditions of life, uniformity of organs or even of the entire structure can be developed by ani- mals far apart in systematic classification. Thus, for example, a mollusc of the cretaceous period, a brachi- opod of the Carboniferous, and a coral of the Devo- nian externally are much alike. Or, again, in Mesozoic times the repiilia prevailed in water, air, and on land. There existed in this period beasts of prey, along with herbivorous and insectivorous animals, cheiroptera in the air, and fish-like carnivora in the ocean. In the latest geological periods the mammals took the lead, and placental mammals took possession of all three elements. Alongside of these there existed carnivor- ous, insectivorous, and rodent marsupials.
If we study the fossils of successive strata we will notice along with the forms which are gradually changed, numerous new forms unconnected with pre- viously-existing forms. There is, therefore, a gap which cannot be filled up by means of small, inappre- ciable changes, as the Darwinian theory of descent de- mands, because there is not time enough for numerous intermediate members of the series. Hackel, there- fore, assumes a process of change which he calls meta- kinesis, by this he understands "an almost violent and always far-reaching cliange in the forms, which cer- tainly cannot take place in the adult form of the or- ganism, but only in its earlier younger stages when the individual organs are not yet histologically si)ecialized and therefore possess a more or less independent i)la,s- ticity". In the shortest space of time such nietaki- netic processes can completely change the a]ii)earaiice of the entire fauna and flora, and in the history of life periods of relative constancy alternate with those of violent change and new formation. Under these con- ditions the individual genera act very ditTcrentlv. Many genera of the brachiopods, the foraniiiiifera, the echinoderms, gasterojxKls, as well as the iiiolliisca, the cephaloiKjds, and the Crustacea extend almost without change from geological antiiiuity up into the present time. Other genera, on the contrary, have only a life of very brief tiuration. In these latter is perceived, at times, a very gradual remodelling by mutations, mu- tations which being separated into fragments by a vio- lent metakinetic break-up, afterwards give rise to a large number of species; thus the vital energy of the genera is soon exhausted. This phenomenon brings us, therefore, face to face with a new problem, com- monly called the "extinction of species".
One circumstance must, however, still be pointed out, namely that the variability of the forni groups does not appear to be unlimited in all directions, but that this variability in different families frequently moves independently in the same direction. For in- stance, there was a tendency toward bilateral sym- metry in the animal kingdom at a fairly early period, and individual echinoderms attained it; but it was not
