I have to thank Mr. Francis Galton, D.C.L., F.R.S. and Mr. F. Howard Collins for valuable assistance generously rendered for the sake of one whom all who knew him held dear. For he was, if I may echo the words of Huxley, "a friend endeared to me, as to so many others, by his kindly nature, and justly valued by all his colleagues for his powers of investigation and his zeal for the advancement of science." C. LLOYD MORGAN. BRISTOL, _May 1897_. CONTENTS CHAPTER I. ISOLATION 1 CHAPTER II. ISOLATION (_continued_) 28 CHAPTER III. PHYSIOLOGICAL SELECTION 41 CHAPTER IV. EVIDENCES OF PHYSIOLOGICAL SELECTION 62 CHAPTER V. FURTHER EVIDENCES OF PHYSIOLOGICAL SELECTION 81 CHAPTER VI. A BRIEF HISTORY OF ISOLATION AS A FACTOR IN ORGANIC EVOLUTION 101 GENERAL CONCLUSIONS 144 APPENDIX A. MR. GULICK'S CRITICISM OF MR. WALLACE'S VIEWS ON PHYSIOLOGICAL SELECTION 151 APPENDIX B. AN EXAMINATION BY MR. FLETCHER MOULTON OF MR. WALLACE'S CALCULATION TOUCHING THE POSSIBILITY OF PHYSIOLOGICAL SELECTION EVER ACTING ALONE 157 APPENDIX C. SOME EXTRACTS FROM THE AUTHOR'S NOTE-BOOKS 169 _ISOLATION_ CHAPTER I. ISOLATION. This treatise will now draw to a close by considering what, in my opinion, is one of the most important principles that are concerned in the process of organic evolution--namely, Isolation. I say in _my_ opinion such is the case, because, although the importance of isolation is more or less recognized by every naturalist, I know of only one other who has perceived all that the principle involves. This naturalist is the Rev. J. Gulick, and to his essays on the subject I attribute a higher value than to any other work in the field of Darwinian thought since the date of Darwin's death[1]. For it is now my matured conviction that a new point of departure has here been taken in the philosophy of Darwinism, and one which opens up new territories for scientific exploration of an endlessly wide and varied character. Indeed I believe, with Mr. Gulick, that in the principle of Isolation we have a principle so fundamental and so universal, that even the great principle of Natural Selection lies less deep, and pervades a region of smaller extent. Equalled only in its importance by the two basal principles of Heredity and Variation, this principle of Isolation constitutes the third pillar of a tripod on which is reared the whole superstructure of organic evolution. [1] It will be remembered that I regard Weismann's theory of heredity, with all its deductive consequences, as still _sub judice_.