IN the Nuevo Mundo of Madrid for October 27 it is stated that a mule, belonging to Don Carlos Gimenez, of Argamasella de Calatrava, recently gave birth to a foal. From India, South Africa, and America reports have reached the writer about fertile mules, but in no single instance has the evidence of fertility been altogether satisfactory. In the present case the information thus far submitted is very meagre. Nothing is said about the breeding of the reputed parent of the foal. She may be a she-ass with the conformation of a mule, or a mule in milk which succeeded in stealing a mule foal from a mare. A Przewalsky-horse hybrid bred at Penycuik proved fertile, but all the ass and zebra hybrids experimented with during the last twelve years proved sterile. The male zebra-horse hybrids were sterile because they never succeeded in maturing perfect sperms. The hybrid “Romulus”, e.g., had all the instincts of a pony stallion, and, so far as one could judge with the naked eye, he was capable of getting foals. When, however, a microscopic examination was made, it was ascertained that the sperms were quite or almost tailless—at the most the length of the flagellum was never more than three or four times the diameter of the head, and it was immobile. Why female mules are infertile has not yet been determined.