Naturally the fusion of three separate series of essays into one series, has made needful a general re-arrangement. Whether to follow the order of time or the order of subjects was a question which presented itself; and, as neither alternative promised satisfactory results, I eventually decided to compromise--to follow partly the one order and partly the other. The first volume is made up of essays in which the idea of evolution, general or special, is dominant. In the second volume essays dealing with philosophical questions, with abstract and concrete science, and with aesthetics, are brought together; but though all of them are tacitly evolutionary, their evolutionism is an incidental rather than a necessary trait. The ethical, political, and social essays composing the third volume, though mostly written from the evolution point of view, have for their more immediate purposes the enunciation of doctrines which are directly practical in their bearings. Meanwhile, within each volume the essays are arranged in order of time: not indeed strictly, but so far as consists with the requirements of sub-classing. Beyond the essays included in these three volumes, there remain several which I have not thought it well to include--in some cases because of their personal character, in other cases because of their relative unimportance, and in yet other cases because they would scarcely be understood in the absence of the arguments to which they are replies. But for the convenience of any who may wish to find them, I append their titles and places of publication. These are as follows:--"Retrogressive Religion," in _The Nineteenth Century_ for July 1884; "Last Words about Agnosticism and the Religion of Humanity," in _The Nineteenth Century_ for November 1884; a note to Prof. Cairns' Critique on the _Study of Sociology_, in _The Fortnightly Review_, for February 1875; "A Short Rejoinder" [to Mr. J. F. McLennan], _Fortnightly Review_, June 1877; "Prof. Goldwin Smith as a Critic," _Contemporary Review_, March 1882; "A Rejoinder to M. de Laveleye," _Contemporary Review_, April 1885. LONDON, _December, 1890_. CONTENTS OF VOL. I. PAGE THE DEVELOPMENT HYPOTHESIS 1 PROGRESS: ITS LAW AND CAUSE 8 TRANSCENDENTAL PHYSIOLOGY 63 THE NEBULAR HYPOTHESIS 108 ILLOGICAL GEOLOGY 192 BAIN ON THE EMOTIONS AND THE WILL 241 THE SOCIAL ORGANISM 265 THE ORIGIN OF ANIMAL WORSHIP 308 MORALS AND MORAL SENTIMENTS 331 THE COMPARATIVE PSYCHOLOGY OF MAN 351