For this reason I do not propose to make any further reply to the opponents of _The Riddle of the Universe_, and I am still less disposed to take up the personal attacks which some of my critics have thought fit to make on me. In the course of this controversy I have grown painfully familiar with the means with which it is sought to silence the detested free-thinker--misrepresentation, sophistry, calumny, and denunciation. "Critical" philosophers of the modern Kantist school vie in this with orthodox theologians. What I have said in this connection of the theologian Loofs, of Halle, the philologist Dennert, of Godesberg, and the metaphysician Paulsen, of Berlin, in the appendix to the cheap German edition of the _Riddle_, applies equally to many other opponents of the same type. These heated partisans may continue to attack and calumniate my person as they will; they will not hurt the sacred cause of truth in which I labor. Much more interesting to me than these attacks were the innumerable letters which I have received from thoughtful readers of the _Riddle_ during the last five years, and particularly since the appearance of a popular edition. Of these I have already received more than five thousand. At first I conscientiously replied to each of these correspondents, but I had at length to content myself with sending a printed slip with the intimation that my time and strength did not permit me to make an adequate reply. However, though this correspondence was very exacting, it afforded a very welcome proof of the lively sympathy of a large number of readers with the aim of the monistic philosophy, and a very interesting insight into the mental attitude of the most varied classes of readers. I especially noticed that the same remarks and questions occurred in many of these five thousand letters, very often expressed in the same terms. Most of the inquiries related to biological questions, which I had cursorily and inadequately touched both in _The Riddle of the Universe_ and _The History of Creation_. The natural desire to remedy these deficiencies of my earlier writings and give a general reply to my interrogators was the immediate cause of the writing of the present work on _The Wonders of Life_. I was confirmed in this design by the circumstance that another scientist, the botanist Johannes Reinke, of Kiel, had published two works in which he had treated the general problems of natural philosophy, especially of biology, from a purely dualistic and teleological point of view; these works were his _Die Welt als That_ (1899) and _Einleitung in die theoretische Biologie_ (1902). As both these works are well written and present the principles of dualism and teleology with admirable consistency--as far as this is possible--it seemed to me that it was desirable to give a thorough exposition of my own monistic and causative system.