THE SPECIES AND GENT'S IN PALEONTOLOGY, The early conceptions of species held by the botanists and zWilogists of the non-transformist school were held also by paleontologists. Each fossil species was considered to represent a dis tinct entity separate front its allies and specially created. At that time transitional forms between species were grouped as varieties under one or the other specific head, or were even in litany eases thrown away and destroyed, as they inter fered with the hard and fast delimitations sought after in the early claSsifications. Subsequently, as the evolution doctrine became better understood, these transitional individuals were recognized as affording examples of the variability of species, and they are now considered to lie of equal impor tance.with the norm of the species itself, as afford ingevidence upon the origin of new varieties. A fos species differs from a living species in one im portant respect. The living species of the present day is distinguished by certain particular elmr acters which differentiate it from its allies, "et. lain 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 conception. In addition to its geograithical 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 must rely upon likeness of form and upon continuity of occurrence, and he groups under it single specific denomination those individuals which resemble each other in essential characters and which dif fer only in secondary characters. Sonic species were evidently very short-lived, others enjoyed long lives and underwent little if any change of form, while still others varied considerably during their periods 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, Viirttem berg, described by Hyatt and Hilgendorff; the Paludinaus of the Lower Pliocene of Slavonia, de scribed by Neumayr; the Ammonites of the Op pelia subradiatus type of the Jurassic limestones, studied by \\ aagen.
The existence of such series of transitional forms, the members of which occur in successive horizons and all of which have apparently 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 determined not by any strictly 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 principles are true with respect of genera, families. etc. (see HonsE. FOSSIL), 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 arc 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.