A first book in organic evolution

by D. Kerfoot Shute

From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.

A later epoch still in the history of the globe is known as the Quaternary period, the period that immediately preceded our present epoch. The great features of this period, which is divided into Glacial, Champlain and Terrace epochs, are the wide-spread oscillations of the earth’s crust in high latitudes towards the north and south poles, attended with great changes of climate from temperateness to extreme cold.

The Glacial epoch was characterized by upward crust movements, the land becoming over one thousand feet higher than at present. The land was covered with ice, and an arctic severity of climate extended almost to the Gulf of Mexico. The Champlain epoch was characterized by a downward movement of the coast until it became five hundred feet or more below the present level, so that many lower portions of the continent became covered with sea. At this time there was a moderation of the temperature, a melting of the vast sheets of ice, and consequently a flooding of rivers and lakes, with many icebergs floating in them. The last or Terrace epoch of the Quaternary period was characterized by a crust movement up to the present condition of things.

What is true of the instability of the North American Continent is true of all the continents of the globe. They have all grown from small beginnings to their present huge proportions, and are now undergoing slow but irresistible changes. When these facts are held in mind, one may form a faint conception of the colossal changes that have taken place throughout the sweep of bygone ages. Environment means a complexity of conditions almost infinite in their number and character, and almost infinite in their variations.

SECTION IV.
TRANSMUTATION OF LIVING FORMS.

TRANSMUTATION OF LIVING FORMS.

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.