Therefore the inappropriate reaction of the margin of the iris to an artificial double stimulus affords additional reason for regarding regeneration as an adaptive phenomenon. If it were the outcome of an adaptive force it could never be inappropriate; and if it were the operation of a general and primary power of the organism it would be exhibited by the nearly-related frog as well as by the newt. But, in the frog, extraction of the lens gives rise only to a sac-like proliferation of the cells of the iris margin, which form no transparent lens, but an opaque cluster of cells, which destroys vision altogether. It appears, therefore, that the frog no longer requires the power its ancestors possessed of regenerating a lost lens. LECTURE XXI REGENERATION (_continued_) Phyletic origin of the regenerative capacity--The liberating stimuli of regeneration--Production of extra heads and tails in Planarians (Voigt)--Regeneration in the Starfish--Atavistic regeneration in Insects and Crustaceans--Progressive regeneration--Regeneration has its roots in the differentiation of organisms--The nuclear substance of unicellular organisms is the first organ for regeneration--The ultimate roots of regeneration. In the previous lecture we have considered many different forms of regeneration, and have recognized them as adaptive phenomena; we have now to inquire how such regeneration-adaptations have arisen, and this is a very difficult question even in general, while in particular cases it is often quite unanswerable at present. In regard to the case last discussed, the regeneration of the lens in the eye of Triton, our hypotheses would require to reach back to the time of the primitive vertebrates with an unpaired eye, for the lens of the paired vertebrate eye, from Mammals down to the lowest Fishes, does not arise in embryonic development from the retinal cells, but always from the corneal epithelium, as the elaborate researches of Rabl have recently shown. It is true that the unpaired parietal eye of some reptiles forms its lens from the cells of the retinal layer, but it would be difficult to demonstrate the possibility of a genetic connexion between it and paired eyes, and in the meantime we must refrain from elaborating a hypothesis as to the origin of the marvellous faculty the retinal cells possess of transforming themselves into lens-fibres. But it is easier to form some sort of picture of the origin and adaptation of the faculty of regeneration in general.