THE study of heredity is admittedly one of the most difficult branches of science. It has been tackled by some of our greatest modern scientists, including Darwin himself, who put forward a theory of his own. Weismann, Lamarck, and Mendel are also among its greatest exponents. To read some of the views of the writers on heredity one might almost suppose that it was entirely a theoretical matter referring to scientific doctrine. The plain, and so far intelligible, old doctrine that " like produces like " has been so obscured by theories as to the precise way in which ancestral likenesses and qualities are transmitted, that the whole subject has become unduly complicated. The first thing to be kept in mind is this—that confusion and doubt have not arisen because of any uncertainty that like always does and always will produce like under the same conditions. Biologists are all agreed that when we go down to the beginnings of life, to the unicellular organisms, each individual consisting of one cell containing its own protoplasm, and with no relationship to any other cell except by casual contact, those organisms will, under the same temperature and in the same media of air and chemical surroundings, always produce the same kind of cell. But even in those most primitive examples of life, the environment or condition cannot be always the 54same, and minute differences do arise which scientists call "variations." The existence of variations is at the root of the doctrine of evolution. Out of such variations and their progeny have arisen the higher forms of life, and those most fitted to live by special adaptation to their environment. Can such variations be caused by changes of environment? is one of the burning questions of science. It is now held by most biologists that in the case of those low unicellular organisms, that such variations may be caused by changes of environment, and that the variations so produced are capable of transmitting their special characters to their descendants. When the same doctrines are applied to human beings the conditions are found to be so enormously complex that it is doubtful if the laws in regard to heredity which apply to the unicellular organism are also applicable to the human organism which possesses thousands of different forms of cells attached to each other, influencing each other, and together forming a living organism, with a wonderful solidarity in its general working.