Thank you for sharing. the translator is Josef Maccabe has also great collections of books.
for colored version free here
https://www.sharingebook.com/2020/01/the-riddle-of-universe-at-close-of.html

This book is supposed to contain illustrations but these are missing in this copy. Even the word "illustrations" is crossed out.

During the last quarter of the 19th century many American Protestant leaders promulgated theistic versions of biological evolution. It was only after the First World War that American Christian fundamentalists mounted a concerted attack on the concept of evolution. A present-day reader is surprised to find that some of the attackers believed that the concept came from Germany rather than England. The basis for this mistake was Ernst Haeckel's "The Riddle of the Universe at the Close of the Nineteenth Century," about which the translator, Joseph McCabe, remarked, "No book has been more violently assailed in the religious world."
When Haeckel (1834-1919), soon to be professor of zoology at the University of Jena, read Darwin's "On the Origin of Species" he at once perceived its significance for biology. Moreover, he conceived how the principle of evolution was applicable not just to science but also to philosophy. In "The Riddle of the Universe" he writes that no other scientific idea of his century "has had so profound an influence on the whole structure of human knowledge as Darwin's theory of the natural origin of species."
It was Haeckel's application of the principle of evolution to anthropology and to philosophy that drew the wrath of Christians. Human beings, he said, had evolved and were not a result of special creation and not the apex of existing things. And human beings, like other organisms, were not possessed of immaterial souls that persisted after death.
In the initial chapters of "The Riddle" the author summarizes the histories of the sciences of human anatomy, physiology, and embryology, and of the concept of evolution. (He notes his own discovery of the principle that the development of an embryo recapitulates the evolution of its species.) From this preface he proceeds to the more philosophical part of the book.
He beings by denying the dualism of a material body and an immaterial soul. Psychology--"the scientific study of the soul"--is "a branch of natural science." "Soul" is "a physiological abstraction," a "collective idea of all the psychic functions of protoplasm." Intellection is an evolutionary development of the simple sensitiveness of one-celled organisms. And a human's "psychic life differs from that of the nearest related mammals only in degree, and not in kind." Haeckel calls consciousness "the strong citadel of all mystic and dualistic errors." After discussing various theories, he gives evidence for his own view that consciousness is a physiological activity whose seat is in the cerebral cortex.
The physical "laws" of conservation of matter and conservation of energy are, Haeckel writes, the fundamental rules of the cosmos. He notes the opposition of these physical laws to dualistic philosophy and vitalistic biology.
The author proposes a scientific cosmogony and geogeny in place of belief in extramundane creation. He envisions the spontaneous natural origin of life as a consequence of the chemistry of carbon. The notion of purposefulness in organisms is discussed at length; Haeckel states that evolution explains the presence of functional structures.
Many of Haeckel's specific ideas about biology have not been confirmed. But he was ahead of his time in several respects, as in his chemical hypothesis of the development of life from non-living matter, and his advocacy of a system of classificaiton of living things based upon their evolutionary relations and not just on similarities of structure.
In view of the continued insistence by theologians that the only alternative to supernatural creation is "chance," it is interesting to read Haeckel's remark that in his monistic, mechanical system "there is no such thing as chance."
During the author's extended critique of theism and Christianity, he notes that the religion created by Paul of Tarsus was a mixture of Greek philosophy and Judaism. He corrects the common failure to distinguish religious faith from the scientist's belief--in the form of a hypothesis or theory--in something that is not yet proven, but which--unlike the "supernatural forces and phenomena" assumed by religion--can be proven or disproven. He documents the "warfare" with which Christianity opposes science, which present-day apologists go to great lengths to deny. And he affirms that "the best part of Christian morality" largely coincides with the humanist precepts of virtue based upon "harmony...between self-love and the love of one's neighbor." But with respect to the criteria of truth, and Christianity's contempt for beauty, a monistic "religion" rejects Christianity.