Speculations about geological time vary enormously.[7] Estimates of the age of the oldest rocks by geologists and astronomers starting from different standpoints have varied between 1,600,000,000, and 25,000,000. The lowest estimate was made by Lord Kelvin in 1867. Professor Huxley guessed at 400,000,000 years. There is a summary of views and the grounds upon which the estimates have been made in Osborn’s Origin and Evolution of Life; he inclines to the moderate total of 100,000,000. It must be clearly understood by the reader how sketchy and provisional all these time estimates are. They rest nearly always upon theoretical assumptions of the slenderest kind. That the period of time has been vast, that it is to be counted by scores and possibly by hundreds of millions of years, is the utmost that can be said with certainty in the matter. It is quite open to the reader to divide every number in the appended time diagram by ten or multiply it by two; no one can gainsay him. Of the relative amount of time as between one age and another we have, however, stronger evidence; if the reader cuts down the 800,000,000 we have given here to 400,000,000, then he must reduce the 40,000,000 of the Cainozoic to 20,000,000. And be it noted that whatever the total sum may be, most geologists are in agreement that half or more than half of the whole of geological time had passed before life had developed to the Later Palæozoic level. The reader reading quickly through these opening chapters may be apt to think of them as a mere swift prelude of preparation to the apparently much longer history that follows, but in reality that subsequent history is longer only because it is more detailed and more interesting to us. It looms larger in perspective. For ages that stagger the imagination this earth spun hot and lifeless, and again for ages of equal vastness it held no life above the level of the animalculæ in a drop of ditch-water.
Not only is Space from the point of view of life and humanity empty, but Time is empty also. Life is like a little glow, scarcely kindled yet, in these void immensities.
III
NATURAL SELECTION AND THE CHANGES OF SPECIES
NOW here it will be well to put plainly certain general facts about this new thing, life, that was creeping in the shallow waters and intertidal muds of the early Palæozoic period, and which is perhaps confined to our planet alone in all the immensity of space.
Life differs from all things whatever that are without life in certain general aspects. There are the most wonderful differences among living things to-day, but all living things past and present agree in possessing a certain power of growth, all living things take nourishment, all living things move about as they feed and grow, though the movement may be no more than the spread of roots through the soil, or of branches in the air. Moreover, living things reproduce; they give rise to other living things, either by growing and then dividing or by means of seeds or spores or eggs or other ways of producing young. Reproduction is a characteristic of life.
No living thing goes on living forever. There seems to be a limit of growth for every kind of living thing. Among very small and simple living things, such as that microscopic blob of living matter the Amæba, an individual may grow and then divide completely into two new individuals, which again may divide in their turn. Many other microscopic creatures live actively for a time, grow, and then become quiet and inactive, enclose themselves in an outer covering and break up wholly into a number of still smaller things, spores, which are released and scattered and again grow into the likeness of their parent. Among more complex creatures the reproduction is not usually such simple division, though division does occur even in the case of many creatures big enough to be visible to the unassisted eye. But the rule with almost all larger beings is that the individual grows up to a certain limit of size. Then, before it becomes unwieldy, its growth declines and stops. As it reaches its full size it matures, it begins to produce young, which are either born alive or hatched from eggs. But all of its body does not produce young. Only a special part does that. After the individual has lived and produced offspring for some time, it ages and dies. It does so by a sort of necessity. There is a practical limit to its life as well as to its growth. These things are as true of plants as they are of animals. And they are not true of things that do not live. Non-living things, such as crystals, grow, but they have no set limits of growth or size, they do not move of their own accord and there is no stir within them. Crystals once formed may last unchanged for millions of years. There is no reproduction for any non-living thing.
This growth and dying and reproduction of living things leads to some very wonderful consequences. The young which a living thing produces are either directly, or after some intermediate stages and changes (such as the changes of a caterpillar and butterfly), like the parent living thing. But they are never exactly like it or like each other. There is always a slight difference, which we speak of as individuality. A thousand butterflies this year may produce two or three thousand next year; these latter will look to us almost exactly like their predecessors, but each one will have just that slight difference. It is hard for us to see individuality in butterflies because we do not observe them very closely, but it is easy for us to see it in men. All the men and women in the world now are descended from the men and women of A.D. 1800, but not one of us now is exactly the same as one of that vanished generation. And what is true of men and butterflies is true of every sort of living thing, of plants as of animals. Every species changes all its individualities in each generation. That is as true of all the minute creatures that swarmed and reproduced and died in the Archæozoic and Proterozoic seas, as it is of men to-day.
Every species of living things is continually dying and being born again, as a multitude of fresh individuals.