In this volume, the product of decades of study and research, the world's foremost geneticist surveys the major developments in what is emerging as the most important single area of scientific inquiry in the twentieth century: biological theory of evolution in particular. Noting that the theory of evolution in biology is more than a century old, Dr. Dobzhansky points out that it is nevertheless only in recent times that our knowledge of its physical basis as well as our understanding of its dynamics has progressed greatly. Yet, he notes, new problems have replaced the older ones at the forefront of scientific inquiry, problems which require entirely new approaches. It is to these manifold and exciting new questions that the author brings a lifetime of experience. Throughout, his goal is to create not a summary of all the available literature, nor a professional book written only for the scientific community, but rather a presentation of basic ideas, accompanied by the indispensable references which would enable interested readers to pursue the matter further. The book has been purposely kept short to enable it to be read as a whole, and above all, it has been written in a manner which will hold the attention as well as inform both the general reader and the professionally concerned scientist.