It ought now to be understood that not only is the present environment changing, but also that it has been changing from the earliest geologic times. What, then, is to be said about the living creatures that have existed in the changing environment during all these geologic ages? Have they been rigid, unyielding forms? By no means! We know that they can be modified by altering the conditions at present; and a study of the fossils in the rock formations of the different ages of the world shows conclusively that animals and plants have altered in the past with the changing environment. The living creatures in the _Silurian ages_ differ from those in the succeeding _Devonian ages_, and these latter differ from those in the still later _Carboniferous ages_; and so on, to the present. Changing physical geography and climate are associated with changing forms in animal and plant life. The growing amplitude and complexity of a continent are associated with increasing complexity and specialization of its living forms. Just as the North American Continent of the Tertiary period differs from that of the Silurian ages, so also do the animal and plant forms of the Tertiary period differ from those of the Silurian ages. Just as there has been a continuity in the growth of the Silurian continent to that of the Tertiary ages, and the present, so, also, there has been a continuity of living creatures from Silurian to Tertiary and present times. Changing conditions of life have compelled modifications in living forms, and those creatures that were unable to adapt themselves to the altering conditions of life have perished, while those that did adapt themselves, through useful variations, lived and progressed in organization. A study of the fossils in the rock formations (see page 95) of different ages reveals the fact that thousands of species have lived and flourished in one age and then perished, never to appear in succeeding ages. A study of the fossils also reveals that life-forms have passed on from age to age, ever changing with the changing continent, some advancing to higher and higher levels, while others remained lowly. The facts enumerated above may be instructively illustrated by a hasty reference to the history of organic life as unfolded by the rocks of different ages.[7] =Archæan Era.= No evidence of life has been discovered in the Archæan rocks, but, inasmuch as with the dawning of the Palæozoic time, the waters of the sea were peopled with plants and animals living in great numbers and considerable variety, it is evident that the ancestors of these creatures must have lived during the Archæan ages. TABLE OF STRATIFIED ROCKS AND THE SUCCESSIVE APPEARANCE OF TYPICAL ANIMAL LIFE-FORMS.