Page:The New International Encyclopædia 1st ed. v. 15.djvu/283

PALEONTOLOGY. by subsequent iuVLstigations. During the Ter- tiary, liowiver, climatic zones certainly existed, as is indicated by the fossil floras and faunas, but a lowering of the temperature began during Eocene time in Xorth America and during Mio- cene time in Europe, and culminated in the ice age of the l^leistocene. arious interesting at- tempts have been made by Neuiuayr, buess, Chamberlin, and others to correlate the evolu- tion of animals and plants with the changes of climate in r^st times.

It will be seen from what has been said re- garding facies development that the study of the distribution and migration of fossil faunas leads to conclusions regarding the physical geography, paleogeography, of ancient times; these conclu- sions must of course be tested in the light of the evidence derived from the study of the tectonic features. Much has been accomplished in this field of research by Heer, Xeumayr. Suess, De l.apparent, (,'anu, Smith, Chamberlin, Weiler, Orlmann. Schuchcrt, Ulrich, and Clarke.

The Specie:s and Genis in Jf.u;oNTOLOGT. The early conceptions of species held by the botanists and zoologists of the non-transforraist school were held also by paleontologists. Each fossil species was considered to represent a dis- tinct entity separate from its allies and specially created. At that time transitional forms between species were gnmpcd as varieties under one or the other specific head, or were even in many cases tlirown away and destroyed, as they inter- fered with the hard and fast delimitations .sought after in the early classifications. Sulisequentlj-, as the evolution doctrine became better understood, these transitional individuals were recognized as alVording examples of the varial)ility of species, and they are now considered to be of equal impor- tance.with the norm of the species itself, as afford- ingevidence upon theoriginof new varieties. A fos- sil species differs from a living species in one im- jmrtant respect. The living species of the present day is distinguished by certain particular char- acters which dilTerentiatc it from its allies, per- tain physiological tests determine its individual- ity, and it has a more or less limited area of geographical distribution. The species of the paleontologist is a far different conceiition. In addition to its gcogra]tliical distribution, it has geologic range; for it lived during the period of deposition of perhaps several successive forma- tions, and it is represented in these formations by a series of fossil forms of more or less unmis- takable continuity until it disappears at some higher horizon. The physiological tests are im- possible, and hence the paleontologist nuist rely upon likeness of form and upon continuity of occurrence, and he groups under a single specific denomination those individuals which resendde each other in essential characters and which dif- fer only in secondary characters. Some species were evidently very shortlived, others enjoyed long lives and underwent little if any change of form, while still others varied consi<lerably during their ])eriods of existence, and in their later stages present such wide departure from the original form that, were the intermediate transitory phases absent, they would be con- sidered to constitute distinct species or perhaps event distinct genera. Examples of such series of variable species are furnished by the Planorbis of the Upper Miocene at Steinheim, Wiirttem- berg, described by Hyatt and HilgendorfT; the Paludinaus of the Lower Pliocene of Slavonia, de- scrilicd liy NCumayr; the Ammonites of the Op- pelio auhradiatua type of the Jurassic limestones, studied by Waagen.

The existence of such series of transitional forms, the members of which occur in successive horizons and all of w'hich have apparentlj- been derived from an original common ancestor, forces recognition of the fact that the term species in paleontology is a very arbitrary one, and that the limitation of a species is determineil not by an.y stricfly definable form, but rather by the absence of transitional forms that would serve to link it through scarcely distinguishable grades of variation to its nearest ally. Two species found in formations of different ages, and now considered distinct, may through future discov- ery of intermediate transitional stages prove to be but the earliest and latest stages of a single race. The same juinciples are true with respect of genera, families, etc. (see HoR.SE, Fcs.siL), our conceptions of which change as intermediate forms are discovered, and as the gaps in the classification are filled up. The inevitable con- clusion drawn is that species and genera and even the larger groups are mere stages in the life history of organisms, that they have no real existence in nature, and that they are arbitrary concepts of the stages of evolution attained by a race of organisms at a particular moment or during a more extended period of its history. Peuslstent .io Aueruant Types. Persistent or conservative types are conmion among fossils, and they include those types that have escaped all changes of environment, and also those mem- bers of an original stock which have not responded to the influences of changed environments, and which have perpetuated the characters of their more primitive ancestral type through several geological fornu<tions or epochs. P^vamples are found among the Foraminifera, Globigerina and Orbulina (Camlirian lo recent); Nautilus among cephalopods (Ordovician to recent); Lingula, Crania, and Phynclionella among brachiopods; and Cidaris among echinoderms.

Aberrant types, or forms in which organs have been developed to an extent not found in the other normal members of the group, are common among fossils. Eucalyptocrinus among crinoids, the Rudistae, PholadidiC, and Tercdinidie among pelecypods, Ampy.x among triloliites, Triceratops and Xaosaurus among reptiles, and the titano- theres among mammals, serve as examples. The most aberrant forms are usually found at the ends of short lines of descent, and they seem to mark approaching extinction of these side lines. They seem to indicate extreme adaptation of the organism to special modes of life and appear to have lost their powers of adaptation in other directions.

Gexebalized and Synthetic Tyt>e.s. In the early history of a subkingdom there is often found to be a group of fossil organisms which combine in more or less marked degree the char- acters that distinguish a number of distinct classes or orders of later date. Such a gen- eralized type or synthetic group is consi<lcred to resemble closely if not actually to represent the ancestral type of the entire subkingdom. An excellent example is afforded by the Cystoidea of Cambrian and Ordovician origin, which com- bine the characters of the Crinoidea, Blastoidea, Asteroidea, and Echinoidea of later origin. (See