In the second place, there are no data whatever, which justify the biologist in assigning any, even approximately definite, period of time, either long or short, to the evolution of one species from another by the process of variation and selection. In the ninth of the following essays, I have taken pains to prove that the change of animals has gone on at very different rates in different groups of living beings; that some types have persisted with little change from the paleozoic epoch till now, while others have changed rapidly within the limits of an epoch. In 1862 (see below p. 303, 304) in 1863 (vol. II., p. 461) and again in 1864 (ibid., p. 89-91) I argued, not as a matter of speculation, but, from paleontological facts, the bearing of which I believe, up to that time, had not been shown, that any adequate hypothesis of the causes of evolution must be consistent with progression, stationariness and retrogression, of the same type at different epochs; of different types in the same epoch; and that Darwin's hypothesis fulfilled these conditions. According to that hypothesis, two factors are at work, variation and selection. Next to nothing is known of the causes of the former process; nothing whatever of the time required for the production of a certain amount of deviation from the existing type. And, as respects selection, which operates by extinguishing all but a small minority of variations, we have not the slightest means of estimating the rapidity with which it does its work. All that we are justified in saying is that the rate at which it takes place may vary almost indefinitely. If the famous paint- root of Florida, which kills white pigs but not black ones, were abundant and certain in its action, black pigs might be substituted for white in the course of two or three years. If, on the other hand, it was rare and uncertain in action, the white pigs might linger on for centuries. T.H. HUXLEY. HODESLEA, EASTBOURNE, _April, 1894._ CONTENTS I ON A PIECE OF CHALK [1868] (A Lecture delivered to the working men of Norwich during the meeting of the British Association.) II THE PROBLEMS OF THE DEEP SEA [1878] III ON SOME OF THE RESULTS OF THE EXPEDITION OF H.M.S. "CHALLENGER" [1875] IV YEAST [1871] V ON THE FORMATION OF COAL [1870] (A Lecture delivered at the Philosophical Institute, Bradford.) VI ON THE BORDER TERRITORY BETWEEN THE ANIMAL AND THE VEGETABLE KINGDOMS [1876] (A Friday evening Lecture delivered at the Royal Institution.) VII A LOBSTER; OR, THE STUDY OF ZOOLOGY [1861] (A Lecture delivered at the South Kensington Museum.) VIII BIOGENESIS AND ABIOGENESIS [1870] (The Presidential Address to the Meeting of the British Association for the Advancement of Science at Liverpool.) IX GEOLOGICAL CONTEMPORANEITY AND PERSISTENT TYPES OF LIFE [1862] (Address to the Geological Society on behalf of the President by one of the Secretaries.) X