Argues that homosexuality is not a disease or a sin, but perfectly natural, and that the strict line of differentiation between men and women has been overemphasised. This book contends that male, as well as female, homosexuality results from a crossing of the male and female generative principles during the first stages of foetal development.

Turning to the science of embryology, Ulrichs contends that male, as well as female, homosexuality results from a crossing of the male and female generative principles during the first crucial stages of foetal development. Thus, homosexual men are essentially 'male' in body, 'female' in desire - crucially different from heterosexual men. Homosexuality (and, with that hermaphroditism and bisexuality) is the work of nature, hence innate and unavoidable.