Ernst Haeckel (1834-1919) was a German biologist and philosopher, who helped popularize Charles Darwin's theory of evolution, and also wrote books such as The Evolution of Man. [NOTE: page numbers refer to the 394-page 1900 hardcover edition.]
He wrote in the Preface to this 1900 book, "The present study of the monistic philosophy is intended for thoughtful readers ... who are united in an honest search for the truth... The studies of these 'world-riddles' which I offer in the present work cannot reasonably claim to give a perfect solution to them; they merely offer to a wide circle of readers a critical inquiry into the problem, and seek to answer the question as to how nearly we have approached the solution at the present day... The one point that I can claim ... is that my Monistic Philosophy is sincere from beginning to end---it is the complete expression of the conviction that has come to me, after many years of ardent research into Nature and unceasing reflection, as to the true basis of its phenomena... I am fully convinced that this ... [philosophy] will receive no important addition and suffer no substantial modification during the brief spell of life that remains to me." (Pg. v-vii)
He asserts that the notion of the freedom of the will "is not an object for critical, scientific inquiry at all, for it is a pure dogma, based on an illusion, and has no real existence." (Pg. 16) He adds, "No cosmic problem is solved, or even advanced, by the cerebral function we call emotion. And the same must be said of the so-called 'revelation,' and of the 'truths of faith' which it is supposed to communicate; they are based entirely on a deception, consciously or unconsciously..." (Pg. 18) Still later, he contends that "Any impartial scholar who is acquainted with geological calculations of time... must admit that the crude notion of an eternal life is not a COMFORT, but a fearful MENACE, to the best of men. Only want of clear judgment and consecutive thought can dispute it." (Pg. 207-208)
He states, "The substantial similarity in outer form and inner structure which characterizes the embryo of man and other vertebrates in this early stage of development is an embryological fact of the first importance; from it, by the fundamental law of biogeny, we may draw the most momentous conclusions." (Pg. 65)
He concedes, "According to the amphitheists, the world is ruled by TWO different gods, a good and an evil principle, God and the Devil. They are engaged in a perpetual struggle... Amphitheism is undoubtedly the most rational of all forms of belief in God, and the one which is least incompatible with a scientific view of the world. Hence we find it elaborated in many ancient peoples thousands of years before Christ." (Pg. 278) But he concludes, "Pantheism teaches that God and the world are one. The idea of God is identical with that of nature of substance... in pantheism God... is everywhere identical with nature itself, and is operative WITHIN the world as 'force' or 'energy.' The latter view alone is compatible with ... the law of substance. It follows necessarily that pantheism is the world-system of the modern scientist." (Pg. 288-289) He summarizes, "The modern man... needs no special church, no narrow, enclosed portion of space... his church is commensurate with the whole of glorious nature." (Pg. 345)
Haeckel's view is hardly as "epochal" as it once seemed to many; but for those interested in speculative "scientific" philosophies, his book remains of great interest.