The two chief factors which play a part in the origin of species are heredity and variation, and until we know more about the laws which govern these factors we can not hope to arrive at any satisfactory criteria by which we can estimate the importance of the data accumulated for us by comparative anatomists and embryologists. Signs are not wanting that this view is beginning to be appreciated. The publication of ‘Materials for the Study of Variation,’ by Mr. Bateson, a few years ago, shows that there exists a small but active school of workers in this field; and the recent congress on hybridization, held in London under the auspices of the Royal Horticultural Society, is evidence that in America, on the Continent and in Great Britain one of the most important sides of heredity is being minutely and extensively explored. Prof. Cossar Ewart’s experiments, which we shall attempt to summarize, deal with heredity and cognate matters, and, although they are so far from complete that the results hitherto obtained can not be regarded as final, they mark an important stage in the history of the subject. Five years ago Professor Ewart began to collect material for the study of the embryology of the horse, about which, owing to the costliness of the necessary investigations, very little is at present known. At the same time he determined to inquire into certain theories of heredity which have for centuries influenced the breeders of horses and cattle, and the belief in which has played a large part in the production of our more highly bred domestic animals. Foremost among these is the view widely held among breeders that a sire influences all the later progeny of a dam which has once produced a foal to him. This belief in the ‘infection of the germ,’ or ‘throwing-back’ to a previous sire, is probably an old one--possibly as old as the similar faith in maternal impressions which led Jacob to place peeled wands before the cattle and sheep of his father-in-law Laban. The phenomenon has recently been endowed with a new name--Telegony. Since the publication of Lord Morton’s letter to Dr. W. H. Wollaston, President of the Royal Society, in 1820, it has attracted the attention not only of practical breeders, but of theoretical men of science. The supporters of telegony, when pressed by opponents, having almost always fallen back on Lord Morton’s mare, it will be well to recall the chief incidents in the history of this classic animal.