This study, first published in 1942, helped to revolutionize evolutionary biology by offering a new approach to taxonomic principles, and correlating the ideas and findings of modern systematics with those of other life disciplines. This book is one of the foundational documents of the "Evolutionary Synthesis". It is the book in which Ernst Mayr pioneered his concept of species based chiefly on such biological factors as interbreeding and reproductive isolation, taking into account ecology, geography and life history. In the introduction to this edition, Mayr reflects on the place of this work in the subsequent history of his field.