Darwin, that while the conditions of life were ever changing, those animals and plants which were not sufficiently plastic to conform to a new state of things died out, while those which were more capable of modification, so as to be in harmony with their surroundings, became what we know as new species, genera, families, orders.
Breaks in the Succession accompanied by
Geographical Changes.
The succession of living forms has been uninterrupted, although, from errors of observation, as well as from the fragmentary nature of the evidence, it appears to be broken. Each break may be likened to places from which pages, or chapters, or whole volumes, as the case may be, have been torn out from the record by the hand of time, or not yet discovered by man. It must also be observed that the breaks in the succession are invariably accompanied by geographical change of corresponding magnitude. That, for example, which took place at the close of the Secondary period consisted in the elevation of the chalk rocks, which were accumulated at the bottom of the Cretaceous ocean, extending from southern and eastern Britain, through France into the Mediterranean area, Greece, Palestine, Asia Minor, Germany, Russia, and far into Asia, so as to form a continent on which the Eocene mammalia make their appearance. In this case the conditions of life must have been profoundly modified by the geographical change. The climate, must have been altered, and forms of life, which had been previously elaborated outside Europe, would enter into competition with the old
