The doctrine of development by no means implies, as has sometimes been maintained, a continuous _advancement_ in organization. Although the scale of organic nature has continued to rise as a whole, cases may occasionally occur where a _lower grade_ of organization is better adapted to certain conditions of life. This principle of “degeneration” was recognized by Darwin as early as in the first edition of the “Origin of Species;” it was soon perceived to be applicable to the phenomenon of parasitism, and was first definitely formulated by Dr. Anton Dohrn in 1875. In a lecture delivered before the British Association at Sheffield in 1879, Prof. E. Ray Lankester ascribed to “degeneration” a distinct and well-defined function in the theory of descent. Dr. Weismann’s explanation of the transformation of Axolotl given in the fourth essay of this work, may be regarded as a special contribution to this phase of Darwinism. Whilst refuting the idea held by certain naturalists, that such cases are arguments against the origin of species by the accumulation of minute variations, and prove the possibility of development _per saltum_, the theory here advanced (that _Siredon_ at a former period existed at a higher stage of development as _Amblystoma_, and that the observed cases of metamorphosis are but reversions to this lost higher stage) suggests the question whether there may not still be in existence many other degenerated forms quite unsuspected by naturalists. Many of the opponents of Evolution have from time to time denounced this doctrine as leading to “pure materialism,” a denunciation which may appear somewhat alarming to the uninitiated, but which may not seem fraught with any serious consequences to those who have followed the course of philosophical speculation during the last few years. Those who attack the doctrine on this ground will however do well to consider Prof. Weismann’s views set forth in the last essay in this volume, before hastily assuming that the much dreaded “materialism” is incompatible with any other conception of Nature. The small amount of leisure time which I have been able to devote to the translation of this volume has delayed its completion considerably beyond the anticipated time, and it was with a view to meeting this difficulty that I departed from the original form of the German edition and issued it in parts. Owing to the extremely idiomatic character of the German text, I have throughout endeavoured to preserve only the author’s meaning, regardless of literal translation or of the construction of the original. In some few cases, however, I have intentionally adopted literal translations of certain technical expressions which might, I think, be advantageously introduced into our biological vocabularies. Some alterations have been made in the original text by the author for the present edition, and many new notes have been added. For those bearing my initials I am alone responsible.