Abstract
The author has investigated the sex ratio at birth or the secondary sex ratio in relation to the father's age. Depending on various observations, he suggests that there is a “sexual age” in man; that sexually young men give a surplus of girls over boys and vice versa. In other words, the activity of the X- and Y-sperms or their relative ability to fertilize is influenced by the “sexual age” He affirms that this hypothesis can be applicable to horses also.
