Science, whose high aim it is to investigate Nature, to under stand her secret workings, and thus to win for man the mastery of Nature, must set out with the conviction that Nature is intelligible, comprehensible, and conquerable. In the domain of biological science the problem of heredity occupies a position of great importance, one full of interest to every student of life. For the serious thinker who has not only looked backwards and studied the past of the human race but is inspired by ideals and desires for its future good, the subject of heredity provides an inspiring theme for contemplation and study. The development of our knowledge and the history of human endeavours to reach a complete understanding of the phenomena and conditions of heredity form one of the most interesting chapters in human evolution. Theories of heredity, like theories regarding other phenomena of life, have been expressed in three sets of terms: theological, metaphysical, and scientific. It required no skilled observation of early man to see that in the act of fecundation the male furnished the seminal substance, whereas the female seemed to furnish nothing except the receptacle or “mould,” in the form of the womb, within which the fótus was formed. Thus, what was more natural than to suppose that heredity was solely paternal, that the male element was the germ or seed, and the female organs the soil, in which, by some mysterious process, growth and development of the germ took place. This view of heredity has been expounded in the Manava Dharma-Sastra, one of the ancient sacred books of the Hindus (Delage, L'hérédité, 1903, p. 380). The same view, more or less modified according to the prevailing state of knowledge, was current among the ancient Greeks (Eristratos, Diogenes, and others). Galen and the school of philosophers of Alexandria also upheld the doctrine of the paternal factor of heredity, and thus constituted themselves the school of the Spermatists. Spermatist views prevailed for many centuries, and when towards the close of the seventeenth century Leeuwenhoeck discovered the presence of spermatozoa by the aid of the microscope, the spermatists had a season of rejoicing. Hartsoeker (1694) supposed that within the spermatozoon there was a little being, a human being, in miniature, with all its parts and organs complete, and figured a spermatozoon (highly magnified, of course) in which the little “homunculus” is to be seen seated within the “head” of the former with its arms and legs folded together in small compass, somewhat like a fcetus in utero. The theory of the spermatists was not destined to remain in undisputed possession of the field. The rival school of Harvey in the sixteenth century taught that the semen or sperm did not fertilise the ovum nor even enter the womb, but that it fertilised the entire constitution of the mother by a sort of contagion which rendered her capable of acting as the stimulus of development for the ova in the uterus, and Descartes, in the early part of the seventeenth century, entertained the same views. The ovists now claimed that all the organs of the future being already existed, preformed in miniature, in the ovum, as opposed to the spermatists, who claimed the same preformed structure for the spermatozoon. To the ovists, therefore, the act of fecundation was only an impulse or stimulus to development communicated by the male element to the ovum; the male contributed nothing material in forming the parts and organs of the fótus which existed, preformed in the ovum, so that the child was the product of the mother alone. Among the upholders of the ovist theory, in the eighteenth century were Malpighi, Haller, Bonnet, and Spallanzani. Difficulties, however, arose over both these theories of exclusive inheritance, for the ovists could not explain how the offspring sometimes resembled the father rather than the mother, and the spermatists could not account for cases of close resemblance between the mother and offspring, while neither could, again, account for cases of the mixed or blended resemblance of the offspring to both parents. The theory of preformation gradually lost its interest and its vitality, and received its death-blow at the hands of Wolff (1759), who, not only by theoretical arguments but by indisputable facts as to the nature and process of development of the hen's egg, demonstrated the baselessness of the fancies of the pre-formationists, whether of the spermatic or ovarian school. Finally, there gradually grew up in the nineteenth century the modem view that the male and female (germ and sperm) cells of the respective parents contributed in equal, or nearly equal, proportions to the constitution of the embryo, and that the environment and nourishment of the fertilised ovum during its growth and evolution in the womb was a third factor of importance, especially in the case of those animals which went through a long period of intra-uterine growth and evolution, as in the case of man and the higher mammals.