In order to illustrate what is meant by this law, the following examples may be given. Food and water bring back the organism to its normal condition. The absence of food in the body leads to hunger, and this to the taking in of more food; or, in other words, the need of food leads to the search for food, or at least to the taking in of food. The sexual desire, or the need to reproduce, brings about the condition of the animal that leads to reproduction. A defect in the valves of the heart leads to the enlargement of the right or the left ventricle. The removal of one kidney leads to the hypertrophy and increased function of the other. And although not explicitly stated by Pflüger in this place, we may add to this list the removal of a part of an animal, that leads to the regeneration of that part. Pflüger further states that we are making no subtle distinction when we point out that these phenomena, if looked at from the point of view of purposeful acts, appear to have a teleological side. In reply to this it may be stated, however, that in certain cases of regeneration it can be shown that the result is entirely useless, or even injurious to the organism; hence the teleological nature of the process is entirely lost sight of, and we are the more ready to accept a simple causal explanation of the phenomena. The best example of this that I can give is the development of a tail at the anterior end of a posterior piece of an earthworm. This process is not an occasional one, but is constant. An example of an apparently useful result, so far as the individual’s well-being is concerned, but entirely useless from the point of view of the continuance of the species, is found in the development, in the earthworm, of a new head after the removal of the anterior end, including the reproductive region. New reproductive organs are not formed, and, although, in virtue of the regeneration of a new head, the individual is capable of carrying on its existence, yet the race of earthworms is not thereby benefited. The production of two tails in lizards, or of two or more lenses in the eyes of newts, are examples of the regeneration of superfluous structures. If, however, it is claimed that in the large majority of cases the process of regeneration is for the welfare of the individual, and for the race also, this must be admitted, and it is this fact which has made a deep impression on the minds of many biologists.