THE well-devised breeding experiments now in progress at Penycuik under the direction of Prof. J. Cossar Ewart, are, it need scarcely be said, of the highest interest both theoretical and practical. To the general biologist the subject of hybridisation affords a wide field for the investigation of laws of heredity, and especially of such subsidiary factors, whether real or only imaginary, as reversion, prepotency, saturation and telegony; while the question of the sterility of hybrids has important bearings on the general theory of evolution. But besides the purely scientific aspects of the problems which are now being attacked by Prof. Ewart, there is also their practical application, which appeals with much force to the interests of the breeder of stock. It is of course true that the whole history of animals and plants under domestication may be said to provide a body of experiments in these and similar subjects on a very large scale; and it is undoubtedly the case that many of the questions referred to have been already answered, at least provisionally. The experience of many generations of breeders has led to the emergence of certain practical rules, which are seldom if ever disregarded by those whose interests are concerned in the rearing of animals with a definite object. But it still remains doubtful how far the widely-accepted doctrines of fanciers and other breeders rest upon any firm scientific basis; and it is certainly most desirable that precise experiments should be undertaken with the sole object of arriving at the truth in such matters as prepotency, telegony and the effects of inbreeding. It cannot but be to the advantage of breeders if empiric methods founded on vague conjecture and imperfect generalisation can be made to give place to a rational system derived from exact knowledge of facts. Prof. Ewart's design ought therefore to meet with a warm welcome in scientific and practical quarters alike.