Most of the following essays have appeared in the pages of the Quarterly Review, and I am greatly indebted to the editor and to the proprietor of that periodical for permission to reprint them. The article on 'The Infinite Torment of Flies' is an address I delivered before the British Association at Pretoria in 1905, and the eighth essay appeared in Science Progress. As far as possible I have tried to avoid the use of long words, and thus escape the censure of recent critics in the Times; but I fear I have not altogether succeeded, and my excuse must be that with new discoveries new conceptions arise, and these conceptions require new names, or we cannot talk or write about them with any precision. The essay dealing with zebras and hybrids was the first to be written, and appeared before the rediscovery of Mendel's remarkable work, and must be regarded as a pre-Mendelian contribution to a subject which has recently, in connexion with the Deceased Wife's Sister Bill, again aroused attention. Had it been written later the language and the attitude taken would have been modified by recent research. In the inquiry into the aims and finance of Cambridge Universitythe only essay which does not deal with{viii} questions of economic zoologyI have had the great advantage of the collaboration of Mr. H. A. Roberts, the Secretary of the Cambridge University Association. But for his help I fear I should have lost my way in the intricate mazes of the University accounts. For the care he has taken in making the Index, I owe thanks to Mr. G. W. Webb, of the University Library. A. E. S. Christ's College, Cambridge. March 10, 1908.