Such a theory was very simple, but it explained nothing except the bare production of offspring. It gave no clue to their endless variations, nor to the fact that they often resembled the father more than the mother. According to this theory the offspring should resemble the mother, as the complete individual is formed by her and should be in her image. Leeuwenhock, one of the early microscopists, by the aid of his lenses, opened a new world to mankind, and discovered the sperm cells to be active, living, moving elements, and he gave a death-blow to the belief that the perfect organism exists in the ovum; but he went to the opposite extreme, and maintained that it exists in the male cell and that it is only fed and developed by the female. Even today we find in a vague way both these theories held by educated persons. We are indebted to Harvey in the early part of the eighteenth century for advocating the view held by Aristotle, now known as _Epigenesis_, and combatting the view of growth from a miniature, but already perfectly formed animal, to a visible one. Epigenesis consists in the successive differentiation from the relatively homogeneous elements as found in the egg, to the complicated parts and structure as seen in the offspring. According to Huxley, this work of Harvey alone would have entitled him to recognition as one of the founders of biological science, had he not immortalized himself as the discoverer of the circulation of the blood. Not long after Harvey's publication, Casper Frederick Wolf established the theory of epigenesis upon a firm foundation, where it still remains. The doctrine of _epigenesis_ has very much complicated the whole question of heredity. No wonder even so great a mind as that of Darwin exclaimed, "The whole subject is wonderful." How can an egg, which in structure is comparatively simple, an aggregation of cells, not one of which bears the slightest resemblance to any organ in the body, develop into the perfect individual? How can this egg, formed in special organs, develop other organs than those like the ones in which it was formed? How can sexual cells develop brain cells, with their wonderful modes of action? We cannot explain the philosophy of heredity without being able to answer these questions; but difficult as is the problem, our biologists have made various attempts at an explanation. I cannot go over all the various speculations, but only those most intimately connected with the subject will be mentioned.