The same is true of lizards, whose tails are frequently torn or cast off and regenerated. The new structure is, however, as Gadow says, "a sham tail, since neither new centra nor arches [of the vertebra] but only a non-seg mented rod or tutee of fibro-cartilage is pro duced." Gadow (' Amphibia,' 1901) continues: "The regenerated Lail is, however, provided with new muscles and with skin but the scales often differ considerably from those of the normal organ. Boulenger has found that the new -r aberrant scaling is in some cases a reversion to an ancestral form. This, for instance, is the case in Pseudopus, and in the tejoid genus Gymnophthalmus; to a certain extent also in geckos and skinks. On the other hand Lacer Gherrosouride and also Anguida, re produce a caudal scaling true to their type. Injured or broken-off tails are often repro duced double or even trifid; sometimes an ad ditional little tail grows out from an injured spot, anywhere on the side of an injured or mended tail.* Turtles will mend injured plates of their shell, or in some cases are able to reproduce a plate wholly lost.
The physiological process or method by which regeneration is effected through the cells the surface of the injury or amputation is still a matter of theory. The latest statement
of the power of regeneration manifested by the cells concerned is that by Henry Fairfield Osborne ((Origin of Life and Evolution,' 1917), who believes that such "regeneration is attributable to the potentiality of the hereditary chromatin which still resides in the cells of the amputated surfaces." It has been argued by sonic of the stricter followers of Darwin that this power of regeneration is an acquire ment through natural selection, but Morgan shows conclusively that this view is untenable. He speaks of the healing of wounds, which oc curs in all animals, and asserts that its higher manifestation in the restoration of lost limbs is adaptive in character. "The immense useful ness of this power,' Morgan reminds us, "is obvious when it is remembered how exposed most animals are to injuries. By repairing the injury the animal can better carry on its normal functions. Moreover the presence of the wound would give injurious bacteria a ready means of entering the body. In fact, an intact skin is one of the best preventives to the entrance of bacteria.* Consult Morgan, T. H., (Regeneration' (New York 1901).