The book "Genetic and operational evidence concerning secondary sex characteristics" by the famous author Thomas Gantt Morgan is a fundamental study devoted to the evolution and inheritance of secondary sex characteristics. In this book, the author examines the influence of genetic factors and operational evidence on the formation of sexual characteristics in various species. The key concepts and principles underlying the study of the heredity of sexual characteristics and their development in nature are highlighted. The work was pioneering in the genetics of sex characteristics and their relationship to evolution, and became the basis for further research in this field. The book is of great importance to scientists interested in the evolutionary biology and genetics of sex.