Sachs has advocated a view which has many points of similarity to that of Bonnet, although, in reality, it is not a theory of pre-formation at all, but one of pure epigenesis. His idea rests on the view that the form of a plant, or of an animal, is the expression of the kind of material of which it is composed. Any change in its material leads to a corresponding change in the form of the new parts. Sachs holds that the idea of many morphologists, that there is for each organism a specific form that tends to express itself, and which controls the development of the organism, is a metaphysical idea that has no ground in science. For instance, Sachs thinks that the flower buds of a plant develop, not because of some innate, mystical force that causes the plant to complete its typical form, but because some substance is made in the leaves which, being carried into the growing region, becomes there a part of the material of that region, and from this new material a flower is formed. Simple and clear as this hypothesis appears to be at first sight, it will be found on more careful examination that it fails to account for some of the most characteristic phenomena of development and of regeneration. It may be granted at the outset that the presence of certain substances may undoubtedly influence the kind of growth of a new part; but, on the other hand, one of the most characteristic things of the organism is that it asserts its specific nature within quite a wide range of change, and, on the whole, resists the influence of other kinds of substances than those connected with its ordinary life. While Sachs looked no farther than the material substratum, and supposed that any change in this altered the form, there is, at present, sufficient evidence to show that it is the _structure_ of the material that determines the most important changes that take place in it. This means, if we attempt to divest the statement of its somewhat metaphysical appearance, that the material of the organism is not simply a mixture of different kinds of materials, but a special kind of substance that has a definite structure of its own. This structure may, of course, be changed, but only by the addition of materials that the structure can take up as a part of itself. If the material does not become a part of the structure or organization, it is without effect on the form.[130] My meaning can, perhaps, best be illustrated by the method of regeneration of the tail of the fish from an oblique cut-surface. The growth of the new part is not determined by the kind or by the amount of the new material that is brought to the growing part, for, if it were, the new part would grow at an equal rate at every point; but the growth of the new part is regulated by the form of the tail of each particular kind of fish. The structure of the new part controls the growth of the material of the new part, and not the reverse. The only interpretation that can