It is the physiological momentum which models the organism, and, by causing its adaptations, has produced its organs by change of function. Gegenbaur illustrates this most important fundamental truth by an excellent example. Suppose that, in an absolutely simple organism, all the parts of its exterior are under the same functional conditions, so that each part of the surface can take in food, and that this is digested, assimilated, in the interior. There is, in this condition, not yet any definite organ. If this organism sinks to the bottom and becomes sessile, this part is excluded from taking in nourishing matter, while the opposite surface alone remains, or becomes more, fit for this function. Thus, a simple variation and adaptation has been produced, and if the same organism continues in this position, its bottom cells will estrange themselves from their original function, while those on the top will convey the food into the interior, where a cavity will be formed, ultimately with a permanent opening, the primitive gut and mouth, both very different from the 'foot.' Thus, by adaptation and variation the organism acquires new functions, organs, features, and it gives up and eventually loses others. Its offspring is like it. Like produces like. This is the principle of _heredity_. Adaptation, when going on generation after generation on the same lines in the same direction, becomes continuous, and has an intensifying, _cumulative_ effect. By always weeding out from a flock of pigeons those birds which possess more dark feathers than the rest, we ultimately produce an entirely white race. We hurry on what Nature does slowly.