EXTINCTION OF ORGANISMS, One of the most impressive phenomena brought to the attention of the student of paleontology is the extinction or disappearance of species, families, and even whole orders, as well as faunas, of fossil animals and plants. In sonic cases the rate of disappearance is gradual, the group diminishing in importance before final extinction occurs; in other eases ex tinction is sudden, as if due to some catastrophe. Extinctions of the latter type are usually asso ciated with important changes in the sediments or with unconformities, and occur at levels which have been used to mark the limits of geological systems or formations. In many cases such ex tinction is only apparent, and is due to migration of the fauna into some distant or unknown re gion. In sonic eases the extinction is very real, entire groups of animals having been,. as it were, suddenly annihilated. Some types of organisms after having enjoyed a more or less extended pe riod of life have slowly died ont and have become extinct, apparently uninfluenced by any physio graphic catastrophe, after having passed through a maximum period of evolution and a subsequent pbylogerontie decline. They have grown old and
died apparently through lack of growth force.
Without doubt the most far-reaching causes of extinction of marine animals during past times have been those changes in the relative levels of land and sea by which the water has been largely drawn off from the epieontinental shallow seas and from the continental shelf or littoral zone during intervals of isontatic readjustment. As a consequence of such readjustments, which also mark the openings of hew Geological periods, the faunas inhabiting the shallow seas have been driven into deeper water and have suffered an nihilation. with the exception of scattered parties which found shelter in some harbors of refuge and thus served to furnish the nuclei for new faunas which evolved under future more favorable con ditions.