Because, if its development be not interfered with, a definite egg necessarily gives rise to a definite kind of animal, a complete identity between the rudiment and the product, between cause and consequence, has been assumed more or less consciously. The conception of the sequence has been as if an organism caused its own development in a closed system of forces, in a kind of organic perpetual motion. It has been overlooked that, in the course of the development, many other conditions must be fulfilled, as without them the product never would come from the rudiment. That the same adults may come from the eggs depends upon the egg-cells, in the ordinary course of events, being in similar conditions of anabolism and katabolism, being affected by gravity, light, temperature, and so forth, in the same way. Thus, when we are attempting to grasp the fundamental nature of the course of organic development, we must not omit the part played by these factors. We may dwell for a moment upon this weighty point, as its significance is commonly misunderstood. The course of each organic development depends in the first place, upon the absorption and metamorphosis of matter. Inorganic matter perpetually is being turned into organic material to serve for the growth and development of the rudiments. Thus, what in one stage of the development is mere inorganic material, and an external condition of the development of the rudiment, in the next stage is become a part of the rudiment. The food-yolk of an egg, for instance, like the oxygen of the atmosphere, appears, in its relation to the material of the rudiments, to be something supplied from outside, an external condition of the development; yet it is continually passing into the rudiments and altering them, even though the alteration may be purely quantitative. From this follows the very simple inference that during the course of an organic development external matter is always being changed into internal matter, or that the rudiments are continually growing and changing at the expense of the surroundings.