The writer of this book is of the opinion that one may very well understand Haeckel without being bound to consider everything else as nonsense which does not flow directly from Haeckel’s own presentments and premises. The author is further of the opinion that Haeckel cannot be understood by attacking him with “fire and sword,” but by trying to grasp what he has done for science. Least of all does he hold those opponents of Haeckel to be in the right, against whom he has in his book, _Haeckel and his Opponents_, sought to defend the great naturalist; for surely, the fact of his having gone beyond Haeckel’s premises by placing the spiritual conception of the world side by side with the merely natural one conceived by Haeckel, need be no reason for assuming that he was of one mind with the latter’s opponents. Any one taking the trouble to look at the matter in the right light must see that the writer’s recent books are in perfect accord with those of an earlier date. But the author can also conceive of a critic who in general and offhand looks upon the presentations of this book as the out-pourings of a fantasy run wild or as dreamy thought-pictures. Yet all that can be said in this respect is contained in the book itself, and it is explicitly shown that sane and earnest thought not only can but _must_ be the touch-stone of all the facts presented. Only one who submits what is here advanced to logical and adequate examination, such as is applied to the facts of natural science, will be in a position to decide for himself how much reason has to say in the matter.