The edition presents a collection of Baer's most important papers on comparative embryology, anthropology, ecology and evolution for the first time in an English translation. Besides it, they will display the opposing views of Baer and Darwin on evolution as one of the great controversies in the history of science. They do it with compelling examples from embryology, botany, evolution, ecology, and anthropology in an exceptionally clear, concise and, nevertheless, poetic diction. This first volume portrays Baer as the father of embryology, or, as Darwin emphasized once, "the most important zoologist of this [19th] century". Baer discovered the mammalian egg and the notochord, described the five primary brain vesicles, and studied the functions of the extra-embryonic membranes. The experimental (observational) studies on the mammalian egg and comparative embryology will demonstrate his aim to discover the natural system of all animal classes, which, according to Baer, relies upon the integrative mechanisms of development. Furthermore, he formulated the germ-layer theory and the law of corresponding stages, or serial homologies in the development of embryos, which put a preliminary end to the preformism-epigenesis debate of the 18th century. The second section presents Baer's reasoning about evolution and its relations to physical anthropology and comparative embryology. This volume informs about his ideas on race, species, and the intricate net of type and generation, besides his critical argument concerning the homology between vertebrates and invertebrates. Modern evolutionary thought has gained from Baer's ideas partly via recitations by Darwin, who honoured and used Baer's ideas as central arguments in his work The Descent of Man.