a single family that dates back to the Eocene. The birds of that period were very much like those of to-day, while many families of fishes, and genera even, go back to the Cretaceous. So mammals indicate changes of time and of surrounding conditions much more exactly than other animals. Fresh-water shells, or, better yet, fresh-water fishes, furnish the best testimony as to former land connection between countries now separated by the sea; for, owing to their mode of life, these spread but slowly, and long lapses of time were necessary in order that they might be carried from one region to another.

To apply these facts to the history of our own country, it may be said that fishes still living hint at a former union between North America, Asia, and Europe, while the testimony of fossil mammals is to the effect that Europe and this continent were united just before, or during, the Eocene period. Fossil elephants and mastodons speak of an early connection between Asia and America, while existing animals show that very recently (geologically speaking) Alaska and Siberia were connected by a land bridge in the vicinity of Bering Straits.

As for the testimony of the rocks themselves, thick beds tell of long periods of quiet, when changes in the earth's crust were few and slow, while thin beds of rock speak of frequent changes of level. Fine-grained limestones indicate the presence of lime-secreting creatures such as corals and crinoids, or perhaps of those stony-jointed plants, the so-called nullipores, once counted with the corals, and, like them, aids in reef-building. Fine shales tell of soft mud washed from the adjacent shore and deposited in quiet waters, while coarse-grained sandstones and coarser conglomerates were laid down nearer shore, where the wash of waves and sweep of tides and currents carried away all finer particles, to deposit them farther out at sea.

Such is a general outline of the data available for writing the history of the past, and such the methods by which these data have been interpreted and the scattered parts woven into a connected whole. That many mistakes have been made in doing this is undeniable, nor may we say that all have been corrected. But the same may be said of any history, even of the record of current events, and if errors are pardonable, surely the historian may be forgiven who is writing of events that took place not hundreds, but thousands and millions, of years ago. It must be borne in mind, too, that the student of the past is sadly hampered by what Darwin called the imperfection of the record, the utter lack of anything like a continuous transcript of past life. Very many animals were by their very structure prevented from leaving any vestige of their former presence, and the vast majority of those that could, perished under such conditions that they failed to do so. The greater part of all fossils are inaccessible, for we can only reach those whose ancient burial-places have been laid

bare by the wearing away of overlying rock, or where the edges of strata have been cut through by rivers, or exposed by the mighty thrust of forces that have converted plains into mountains. And even after events like these had laid bare the rocky pages wherein the story of the past is written, the hand of Nature, with the selfsame means, has ruthlessly erased all traces of the record before they had been seen by the eye of man.

There is perhaps no group of animals that illustrates this imperfection of the record so well as birds. There are living to-day not less than 12,000 species, and half of these belong to one group, the Passeres, or perching birds. The ancestors of some of these were living at the time camels, horses, and elephants were among the common animals of North America; but if we go back to the Eocene we find the group represented in our continent by just three specimens, and two of these seem to have been much like modern birds. The Cretaceous has yielded more specimens and more species, but the birds of that day were totally unlike those of the Eocene, for they were birds with teeth, and we can not trace the connection between them. And here the record ceases, so far as North America is concerned, for back of that we have absolutely nothing. And yet birds there were, because our toothed water-fowl represent two groups, one of which had become so specialized for aquatic life that it had lost the power of flight, and almost lost every vestige of wings. The older rocks (Jurassic) of Europe have yielded two birds, besides a single feather, and these differ as widely from our toothed species as do those from the birds of to-day. The wonder is, not that we know so little of the life of the past, but that we know so much.

5. THE STRUGGLE FOR EXISTENCE AND NATURAL SELECTION'

By CHARLES DARWIN

Before entering on the subject of this chapter I must make a few preliminary remarks to show how the struggle for existence bears on natural selection. It has been seen in the last chapter that among organic beings in a state of nature there is some individual variability: indeed, I am not aware that this has ever been disputed. It is immaterial for us whether a multitude of doubtful forms be called species or sub-species or varieties; what rank, for instance, the two or three hundred doubtful forms of British plants are entitled to hold, if the existence of any wellmarked varieties be admitted. But the mere existence of individual variability and of some few well-marked varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in nature. How have all those exquisite adaptations of one part of the organization to another part, and to the conditions of life, and of one organic being to another being, been perfected? We see these beautiful co-adaptations most plainly in the woodpecker and the mistletoe;. and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle which dives through the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the organic world.

Again, it may be asked, how is it that varieties, which I have called incipient species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species? How do those groups of species, which constitute what are called distinct genera and which differ from each other more than do the species of the same genus, arise? All these results, as we shall more fully see in the next chapter, follow from the struggle for life. Owing to this struggle, variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by

Chapter 3, "Struggle for Existence," and summary of Chapter 4, "Natural Selection: or the Survival of the Fittest," of The Origin of Species, by Charles Darwin, first published in 1859.

the offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term natural selection, in order to mark its relation to man's power of selection. But the expression often used by Mr. Herbert Spencer, of the Survival of the Fittest, is more accurate, and is sometimes equally convenient. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man's feeble efforts as the works of Nature are to those of Art.

We will now discuss in a little more detail the struggle for existence. In my future work this subject will be treated, as it well deserves, at greater length. The elder De Candolle and Lyell have largely and philosophically shown that all organic beings are exposed to severe competition. In regard to plants, no one has treated this subject with more spirit and ability than W. Herbert, Dean of Manchester, evidently the result of his great horticultural knowledge. Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult-at least I found it so than constantly to bear this conclusion in mind. Yet unless it be thoroughly ingrained in the mind, the whole economy of nature, with every fact on distribution, rarity, abundance, extinction, and variation, will be dimly seen or quite misunderstood. We behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that, though food may be now superabundant, it is not so at all seasons of each recurring year.

THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE

I should premise that I use this term in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. Two canine animals, in a time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which annually produces a thousand seeds, of which only one on an average comes to maturity, may be more truly said to

struggle with the plants of the same and other kinds which already clothe the ground. The mistletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for, if too many of these parasites grow on the same tree, it languishes and dies. But several seedling mistletoes, growing close together on the same branch, may more truly be said to struggle with each other. As the mistletoe is disseminated by birds, its existence depends on them; and it may metaphorically be said to struggle with other fruit-bearing plants, in tempting the birds to devour and thus disseminate its seeds. In these several senses, which pass into each other, I use for convenience's sake the general term of Struggle for Existence.

GEOMETRICAL RATIO OF INCREASE

A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year; otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.

There is no exception to the rule that every organic being naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in less than a thousand years, there would literally not be standing-room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds-and there is no plant so unproductive as this and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive, descended from the first pair.