EXTINCTION, Tribal. Certain broader inferences as to the nature of arrested evolution and extinction of races or types, rather than the species, can be drawn only from a systematic examination of the geologic-paleontologic rec ord. It is now clearly recognized that periodic emergence and subsidence of the continents has been going on as far back as the fossil records extend, with resultant biologic dias trophism. It is also recognized that the initial expansion or climacteric of groups mainly occurs soon after their first appearance.
As a consequence the inadaptive and unfit species are crowded out, not alone by those nearly of their own kind, but by the successful forms of other groups. Earth, sea, and air, as suggested, must tend to hold to some numeri cal mean of population. But the higher a group in the scale of organization, the more delicate is the adjustment to environment. Rapid adaptive change, like over-specialization, much increases liability to extinction by cataclysm. The relatively successful forms are swept away by renewed environmental disturbance if too rapid to neutralize by a further course of direct evolution, or by catagenetic change. For this reason alone the relative tenure of races as such is longest in the lower forms. The higher groups such as Limulids may show a remarkable persistence, or, like the Testudinata, a relatively long life; but °immortal types)) are mainly in conspicuous like the Foraminifers. That such so often occupy abysmal oceanic or other zones least subject to change, sets a further visible boundary to the course of extinction. Still another is set by the fact that higher types also tend to persist in the zones and regions of least change — The inherent organic quality which as fixedly as gravity sets and holds evolution in its course was aptly termed by Cope "bathmism." This is the force that evolves, and also kills; but the bathmic causes of extinction are even harder to scan than are direct bathmic factors. Re duced to its simplest elements, life must be regarded as a property of matter, and plasm certainly exhibits in closed environments a high degree of stability. It may also exhibit char acters little removed from those of strictly in organic substances (Chunder-Bose) ; although wholly senescent or absolutely unchanging types are not theoretically indicated— that is, taking the entire duration of the rock succession as a time unit. However, when the common fac
tors of change, bathmism (or ontogeny), en vironment, heredity and selection, are inactive, the organism shows little progression; and this indicates climatic change, taken in its very broadest sense, as the chief evolutionary stimu lus. There is a further inference of primary value gained from the study of devolution or catagenesis as opposed to anagenesis. No races are exempt from the totality of climatic factors. The precision of the phenomena of plant suc cession would alone warrant this statement. It follows that while surprising modifications may take place when races decline, there is, dur ing the anagenetic movement, a melior mean of form toward which all types strive— each according to its capacity, and in delicate adjust ment to environment. Consequently the idea that lines of descent take the form of the "paleontologic tree,)) and lead back continually to main primitive stems, must as a rule express too much. A far simpler course of parallel development and parallel decline is indicated. As to when, in the course of this universal parallelism, direct evolution or ascendancy reached its high noon would at present be only a subject of speculation. But from a physical point of view there must be such a point of time, probably denoted by the appearance of persistent bipolar ice caps. Morphologically the appearance of the oak in the lowermost Cre taceous may denote the evolutionary crest. Be yond are palms and grasses.
Patten, 'Evolution of Ver tebrates and their Kin' (1912) ; Scott, W. B., 'History of Land Mammals on the Western Hemisphere' (1913) ; Wieland, 'American Fos sil Cycads' (Vol. II, 1916) ; Clements, 'Plant Succession, an Analysis of the Development of Vegetation' (1916) ; Lull, 'Organic Evolution' (1917) ; Osborn,