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.