One of the most difficult questions with which the biologist has to deal is the meaning of the adaptation of organisms to their environment. Pflüger, in an article entitled “The Teleological Mechanics of Living Nature,” has drawn attention to the teleological character, or purposefulness, of certain processes in the living organism. “There has been found only one general point of view, which if not absolute, yet is the rule, to account for the eternal transformations of energy in the living body. Only those combinations of causes take place that are as favorable as possible for the welfare of the animal. This holds true even when entirely new conditions are artificially introduced into the living organism. What is more remarkable than that, even in the highly organized mammal, there should be a regeneration of the bile duct after its removal, or that after a large piece of a nerve has been extirpated by a severe operation it should be again renewed?... What is more surprising than that the organism should become accustomed to the most diverse kinds of organic and inorganic poisons?... And, finally, there are a number of facts that make good the law that changes appear to be governed by no other principle than the purpose of making certain the existence of the organism.” Pflüger’s teleological law of causality is that “the cause of every need of a living being is at the same time the cause of the fulfilment of the need.” Pflüger explains that the word “cause” is here intentionally chosen in order to bring out the necessary, lawful connection in which the cause of each need stands in relation to the fulfilment of that need. He adds that it would have been more correct, but less pointed, to have said “motive” or “inducement” instead of “cause.”