=New Discoveries in the Field of Heredity.--=Writing in 1899, one of America's well-known zoologists asserts that, "It is easier to weigh an invisible planet than to measure the force of heredity in a single grain of corn." And yet only two or three years later we find another prominent naturalist saying regarding heredity that, "The experiments which led to this advance in knowledge are worthy to rank with those that laid the foundation of the atomic laws of chemistry." Again, "The breeding pen is to us what the test-tube is to the chemist--an instrument whereby we examine the nature of our organisms and determine empirically their genetic properties." Here is a decided contrast of statement and yet both were justifiable at the time of utterance. For even at the writing of the first statement the investigations were in progress which, together with the rediscovery of certain older work, were to transfer our knowledge of heredity from the realm of speculation to that of experiment and disclose certain definite principles of genetic transmission. Through a knowledge of these principles in fact, the shifting of certain characters is reducible to a series of definitely predictable proportions and the skilled breeder may proceed to the building up of new and permanent combinations of desirable characters according to mathematical ratios and, what is of equal importance, he can secure the elimination of undesirable qualities. While there are many limitations in the application of these principles and while new facts and modifications are constantly being discovered concerning them, nevertheless they represent the first approximations to definite laws of hereditary transmission that we have ever been able to make, and the practical fact confronts us that whatever our theoretical interpretations may be, the principles are so definite that through their application important improvements of crops and domesticated animals have already actually been secured and one may confidently expect still others to follow. =Mendel.--=The principles involved are called the Mendelian principles after their discoverer, Gregor Johann Mendel, abbot of a monastery at Brünn, Austria. After eight years of patient experimenting in his cloister garden with plants, chiefly edible peas, he published his results and conclusions in 1866, in the _Proceedings of the Natural History Society of Brünn_. While known to a few botanists of that day, the full importance of the contribution was not recognized, and in the excitement of the post-Darwinian controversy, the facts were lost sight of and ultimately forgotten.