No attempt will be made by the writer to discuss fully the biological relations of fossils. Such an undertaking would afford matter for a separate volume. All that I purpose in this chapter is to indicate very cursorily the principal points of palæontological teaching, so that my readers can acquire some idea of the progression from lower to higher types that life has made throughout the geological ages. Preliminary to the purpose held in view, let it be understood that the vast majority of fossil animals and plants are extinct, or, differently and perhaps more intelligently expressed, belong to species that no longer exist. So far from there being any truth in the old idea that there have been periodic destructions of all the living beings in existence upon the earth, followed by a corresponding number of new creations of plants and animals, the actual facts indicate that the extinction of old and introduction of new forms have been processes that have been continually going on throughout the whole of geologic time. Every species seems to come into existence at a definite point of time, and to disappear finally at another definite period, though there are few, if any, instances, in which the times of entrance and exit could be fixed with any degree of certainty or precision. Marked differences in the actual time during which different species have remained in existence are noticeable, and therefore corresponding differences in their vertical range, or in the actual amount and thickness of strata through which they present themselves as fossils, some species being found to extend through two or three formations, and even a few have had a more prolonged existence. More commonly, however, the species which begin in the commencement of a great formation die out at or before its close, while those which are introduced for the first time near its middle or end may either become extinct or pass into the next succeeding formation, animals of the lowest and simplest organization as a rule having the longest range in time. Microscopic or minute dimensions seem to favor longevity, for some of the Foraminifera appear to have survived, with little or no perceptible alteration, from the Silurian Period to the present day, whereas largely and highly-organized animals, though long-lived as individuals, rarely seem to live long specifically, and consequently have a restricted vertical range. Exceptions to this rule are, however, occasionally found in some persistent types, the Lampshells of the genus Lingula being little changed from the Lingulæ that swarmed in the Lower Silurian seas, while the existing Pearly Nautilus is the last descendant of a clan nearly as old. Some forms, on the other hand, the Ammonites, which are closely related to the Nautilus, and mostly restricted to certain zones of strata, seem to have enjoyed a comparatively brief lease of life. [Illustration: LIFE IN THE PRIMORDIAL SEA.