Among animals there is only one well-studied instance of our first type of adaptive morphological characters. *Salamandra atra*, the black salamander, a species which only inhabits regions at least two thousand feet above sea-level, does not bring forth its young until metamorphosis has taken place. The larvae, however, may be removed from the mother’s body at an earlier stage and forced to complete their development in water. Under these circumstances, as was shown in an excellent memoir by Kammerer,[80] they will change the whole histological type of their gills and skin in order to meet the new functional conditions. The change of the conditions of functioning is very severe here, for whereas the gills had served for nutrition and respiration in the uterus--by a process of endosmosis--they now serve for respiration only, and, of course, are surrounded by quite an abnormal chemical medium. [80] *Arch. Entw. Mech.* 17, 1904. TRUE FUNCTIONAL ADAPTATION[81] [81] Roux, *Gesammelte Abhandlungen*, vol. i. 1895; in particular, *Der Kampf der Teile im Organismus*, Leipzig, 1881. But all other cases of morphological adaptation among animals, and several in the vegetable kingdom too, belong to our second group of these phenomena, which in our analytical discussion we have called adaptations to functional changes that result from the very nature of functioning, and which we shall now call by their ordinary name, “functional adaptation.” It was Roux who first saw the importance of this kind of organic regulation and thought it well to give it a distinguishing name. *By functioning the organisation of organic tissues becomes better adapted for functioning.* These words describe better than any others what happens. It is well known that the muscles get stronger and stronger the more they are used, and that the same holds for glands, for connective tissue, etc. But in these cases only quantitative changes come into account. We meet with functional adaptations of a much more complicated and important kind, when for instance, as shown by Babák,[82] the intestine of tadpoles changes enormously in length and thickness according as they receive animal or vegetable food, being nearly twice as long in the second case. Besides this the so-called mechanical adaptations are of the greatest interest. [82] *Arch. Entw. Mech.* 21, 1906. By a very detailed comparative study Babák was able to prove that it is the plant proteids to which the effect of vegetable food is chiefly due; thus we have an adaptation to digestibility. Mechanical circumstances are only of secondary importance. (See also Yung.)