The great length of geologic time inferred from the slow change of species.  Life forms, like land forms, are thus subject to change under the influence of their changing environment and of forces acting from within.  How slowly they change may be seen in the apparent stability of existing species.  In the lifetime of the observer and even in the recorded history of man, species seem as stable as the mountain and the river.  But life forms and land forms are alike variable, both in nature and still more under the shaping hand of man.  As man has modified the face of the earth with his great engineering works, so he has produced widely different varieties of many kinds of domesticated plants and animals, such as the varieties of the dog and the horse, the apple and the rose, which may be regarded in some respects as new species in the making.  We have assumed that land forms have changed in the past under the influence of forces now in operation.  Assuming also that life forms have always changed as they are changing at present, we come to realize something of the immensity of geologic time required for the evolution of life from its earliest lowly forms up to man.

It is because the onward march of life has taken the same general course the world over that we are able to use it as a universal time scale and divide geologic time into ages and minor subdivisions according to the ruling or characteristic organisms then living on the earth.  Thus, since vertebrates appeared, we have in succession the Age of Fishes, the Age of Amphibians, the Age of Reptiles, and the Age of Mammals.

The chart given on page 295 is thus based on the law of superposition and the law of the evolution of organisms.  The first law gives the succession of the formations in local areas.  The fossils which they contain demonstrate the law of the progressive appearance of organisms, and by means of this law the formations of different countries are correlated and set each in its place in a universal time scale and grouped together according to the affinities of their imbedded organic remains.

Geologic time divisions compared with those of human history.  We may compare the division of geologic time into eras, periods, and other divisions according to the dominant life of the time, to the ill-defined ages into which human history is divided according to the dominance of some nation, ruler, or other characteristic feature.  Thus we speak of the dark ages, the age of Elizabeth, and the age of electricity.  These crude divisions would be of much value if, as in the case of geologic time, we had no exact reckoning of human history by years.