Thus neither the degree of organization nor the relationship alone determines the strength of the regenerative capacity. And as nearly related species may behave quite differently in this respect, so also do the different parts of one and the same animal; and here, too, the strength of the capacity seems to depend on the more frequent or rarer injury of the relevant part and on its importance in the maintenance of life. Let us take a few examples. Parts which, in the natural life of the animal, are never injured, show in many cases no power of regeneration. This is so in regard to the internal parts of the newt, whose regenerative capacity is otherwise so high. I cut half or nearly the whole of a lung away from newts anæsthetized with ether; the wound closed, _but no renewal of the organ took place_. The same thing happened when a piece of the spermatic duct or of the oviduct was cut away. It is true that the kidney enlarges in higher animals when a piece has been cut out, by the proliferation of the remaining tissues, but that is a mere physiological substitution, evoked by the increased functional stimulus, due to the accumulation in the blood of the constituents of the urine. Such substitution depends on the growth of parts already existing, and it occurs in man when one kidney is removed, for the other, as is well known, may then grow to double its normal size. This is mere hypertrophy of the part that is left, it is not regeneration in the morphological sense, and it is not comparable to the re-formation of a cut-off leg in the salamander, or of a head in the worm, where the growth is not a mere increase of the remaining stump, but a new formation. It would be regeneration if a new kidney developed from the remnants of the kidney-tissue, or, in the liver, if new lobes grew in place of those which were cut off. But neither of these things happens, and, as far as I am aware, nothing of the kind has ever been observed, nothing more than new formation of liver-cells through increase of existing ones; that, however, is not regeneration in the morphological sense. I have referred to the slight power of regeneration possessed by the blind Proteus in regard to its legs or tail, and I connected this with the absence of enemies in its thinly peopled cave-habitat. But the same animal can regenerate its gills when these are bitten off, and this is probably associated with the habit that Proteus has, in common with other newts with external gills, of nibbling at its neighbour's gills. Thus, the power of regenerating the gills was retained even when the animals migrated to the quiet caves of Krain, and were thus secured from the attacks of other enemies.