But there are still other evidences of the existence of earthquakes during the geological past. There are found in the different strata of the earth's crust fossil remains of the plants and animals that lived on the earth long before the creation of man. By a careful study of these fossils we know positively the kinds of animals and plants that lived on the earth, in its waters, or in its atmosphere, when these strata were being deposited. It is in this way possible for a geologist to trace the life of the earth and its development as it is written on the great book of which the earth's different strata form the separate pages. Now, a careful study of the earth's fauna and flora during the geological past, shows, beyond any question, that occasionally great changes have occurred in the earth; for, here and there, during different times, we find that certain species of animals and plants have completely disappeared, to be followed, after certain intervals, by entirely different species. It is evident, therefore, that changes have occurred that have made it impossible for the animals and plants that formerly lived on the earth to exist under the changed conditions. These occurrences are known to geologists as _exterminations_, _catastrophes_, or _cataclysms_. They are also sometimes called _revolutions_, for they mark a more or less complete wiping-out of the animals living at the time they occurred. If you will try to think you will readily understand how great a catastrophe must be, that would be able to wipe out or completely destroy an entire race of animals. You have doubtless read with astonishment the terrible catastrophe that accompanied the eruption of Krakatoa, especially at the loss of life and property caused by the great waves that were set up in the ocean, but far reaching as these losses were they have nevertheless affected but a limited portion of the earth. The plain truth is even more stupendous, for catastrophes of the geological past appear to have been so far-reaching and powerful as to affect the whole surface of the earth, and to have annihilated entire races of animals and plants as if they had never existed. Geologists are all practically agreed that there are only two ways in which such exterminations of the earth's life could have been caused, and these are changes in the earth's climate, or the starting of waves in the sea by great earthquakes. In the sea; for it must be borne in mind that in the geological past the greater part of the earth's surface was covered by water, and the land areas were comparatively small and low, so that waves created by earthquakes might easily have overwhelmed the entire land surface.