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It is the aim of this book to picture the early life of our continent, to tell something of the fishes that once swam about its shores, the reptiles that splashed through the swamps, and the mammals that long ago roamed the Western plains. Books and museums and zoological gardens have made us familiar with the animals of the present, but we are apt to have rather vague ideas of the animals of the past. Whirling through New Jersey, the traveler notes in the morning paper that a mastodon has just been unearthed near New Brunswick, and may wonder how the landscape would look with herds of mastodons in place of the familiar cattle. The geologist might say that this was the case only a short time ago, while long before the same land was occupied by huge, unfamiliar reptiles. The life of our continent has indeed changed, and some of these changes have taken place in our own day and almost under our very eyes. The bison has been practically swept out of existence, and his doom was sealed when the white man first settled here. But other and different bison had lived and died before this one came upon the scene, and these in turn took the place of other creatures. For there has been a constant change among the animals of this country, and they have been very different at different periods. Could one see examples of them in some great zoological garden, it might be imagined not only that they had been brought together from very remote regions, but that some of them had come from quite another world. Museums have done a great deal of late years to make us acquainted with these extremely ancient animals, but much of the knowledge thus acquired is fragmentary in every sense of the word. It is one of the purposes of this book to clothe such fragments, so far as may be, with flesh and blood, and make the reader acquainted with some of these early animals of the continent. For these objects are not merely so many specimens of fossil shells and pieces of petrified bones: they represent the life of their day, and this life was quite as real as that now about us.

There are several ways in which this ancient natural history might be written: one would be to start with the animals found in the lowest rocks and mention the various species or groups of animals which occur in the formations as we come upward. This, it has been thought, would give a somewhat mixed and disconnected view of the life of our continent, and would result, moreover, in the frequent repetition of names and the unavoidable scattering of information. Still another method would be to tell the history of each group as a whole; of its origin, rise, period of supremacy, and final decline; and this has many things to recommend it. But the plan finally adopted has been to treat the history of the past by periods, and endeavor to sketch the characteristic or more striking features of the life of well-marked epochs; to tell something of the habits, appearance, and relationships of the more conspicuous animals. In doing this, an effort has been made to call attention to some of the causes that are believed to have brought about the marked changes that have taken place in the life of our continent and of the world generally, as well as to impart some of the varied information that has been obtained from the study of fossils. Something, too, has been said of the localities where are to be found the fossils from which the life of the past has been reconstructed, and the methods followed in reproducing the appearance of these animals and interpreting their habits from a study of their bones.

The preference has naturally been given to the larger animals. As a rule, not only do we know them better, but they are likely to be more impressive and interesting than their small associates. Just as to-day the larger animals give us our clearest impressions of the differences between the animals of different parts of the earth, so do they mark most plainly the distinctions between the life of the present and that of the past.

For many reasons this book is not confined entirely to the animals of this country, but deals to some extent with those of other parts of the world as well. We know how the history of the United States has more or less to do with that of England, Holland, and France; and just so the story of the past can not be told without referring to that of other regions than ours. No animal nor any group of animals stands by itself; each and all have somewhat in common with others.

Nowadays the animals of different parts of the earth are so unlike, that its surface may be mapped into regions each distinguished by particular kinds of animals: Africa has its antelopes, North America its deer and bison, South America its monkeys with prehensile tails. But it was not always so: far back in the past, when conditions appear to have been much the same throughout the world, many animals were similar. More than this: the so-called solid earth is not so stable as it seems, and there have been times in the past when lands now separated by the sea have been so joined that there was an interchange of animals. So it frequently happens that where some pages of our own history are lacking we can make good the loss by borrowing a few pages from that of our neighbors.

Finally, it may be said that the author has tried to tell the story of the past life of our continent in as plain language as possible; and while the use of some scientific names and technical terms has been unavoidable, it is hoped that these may not be found so formidable as many suppose.