[1] To this general statement Wolff forms a remarkable exception, for though without any clear knowledge of what we call cells he had very distinct notions on the relations of growth and development. The difficulties of understanding the formation of the individual from the structureless germ led anatomists at one time to accept the view "according to which the embryo preexisted, even though invisible, in the ovum, and the changes which took place during incubation consisted not in a formation of parts, but in a growth, _i.e._ in an expansion with concomitant changes of the already existing germ." Great as is the interest attaching to the simple and isolated life histories of individual organisms, this interest has been increased tenfold by the generalizations of Mr Charles Darwin. It has long been recognized that the embryos and larvæ of the higher forms of each group pass, in the course of their development, through a series of stages in which they more or less completely resemble the lower forms of the group[2]. This remarkable phenomenon receives its explanation on Mr Darwin's theory of descent. There are, according to this theory, two guiding, and in a certain sense antagonistic principles which have rendered possible the present order of the organic world. These are known as the laws of heredity and variation. The first of these laws asserts that the characters of an organism at all stages of its existence are reproduced in its descendants at corresponding stages. The second of these laws asserts that offspring never exactly resemble their parents. By the common action of these two principles continuous variation from a parent type becomes a possibility, since every acquired variation has a tendency to be inherited. [2] Von Baer who is often stated to have established the above generalization really maintained a somewhat different view. He held (_Ueber Entwickelungsgeschichte d. Thiere_, p. 224) that the embryos of higher forms never resembled the adult stages of lower forms but merely the embryos of such forms. Von Baer was mistaken in thus absolutely limiting the generalization, but his statement is much more nearly true than a definite statement of the exact similarity of the embryos of higher forms to the adults of lower ones.