Having thus briefly sketched the doctrines of the sundry Post-Darwinian Schools from a general point of view, I shall endeavour throughout the rest of this treatise to discuss in appropriate detail the questions which have more specially come to the front in the post-Darwinian period. It can scarcely be said that any one of these questions has arisen altogether _de novo_ during this period; for glimmerings, more or less conspicuous, of all are to be met with in the writings of Darwin himself. Nevertheless it is no less true that only after his death have they been lighted up to the full blaze of active discussion[24]. By far the most important of them are those to which the rest of this treatise will be confined. They are four in number, and it is noteworthy that they are all intimately connected with the great question which Darwin spent the best years of his life in contemplating, and which has therefore, in one form or another, occupied the whole of the present chapter--the question as to whether natural selection has been the sole cause, or but the chief cause of modification. [24] It is almost needless to say that besides the works mentioned in this chapter, many others have been added to the literature of Darwinism since Darwin's death. But as none of these profess to contain much that is original, I have not thought it necessary to consider any of them in this merely general review of the period in question. In subsequent chapters, however, allusions will be made to those among them which I deem of most importance. [Since this note was written and printed the following works have been published to which it does not apply: _Animal Life and Intelligence_, by Professor Lloyd Morgan; _The Colours of Animals_, by Professor Poulton; and _Materials for the Study of Variation_, by Mr. Bateson. All these works are of high value and importance. Special reference should also be made to Professor Weismann's Essays.] The four questions above alluded to appertain respectively to Heredity, Utility, Isolation, and Physiological Selection. Of these the first two will form the subject-matter of the present volume, while the last two will be dealt with in the final instalment of _Darwin, and after Darwin_. SECTION I _HEREDITY_ CHAPTER II. CHARACTERS AS HEREDITARY AND ACQUIRED (PRELIMINARY). We will proceed to consider, throughout Section I of the present work, the most important among those sundry questions which have come to the front since the death of Darwin. For it was in the year after this event that Weismann published the first of his numerous essays on the subject of Heredity, and, unquestionably, it has been these essays which have given such prominence to this subject during the last decade.