Introduction--The Bladderworts or Utriculariæ--Pitcher-plants, Nepenthes--The Toothwort, Lathræa--The Butterwort, Pinguicula--The Sundew, Drosera--The Flytrap--Aldrovandia--Conclusions. THAT the principle of selection dominates, to a large extent at least, all the structural characters of plants, and moulds these in direct relation to the prospects of greater success which may be offered in the vicissitudes of the life-conditions of a single species or group of species, is nowhere more apparent than in the case of the so-called 'insectivorous' or 'carnivorous' plants. Here again it was Charles Darwin who led the way, for while many plants had long been known on the sticky leaves of which insects were often caught and killed, it had occurred to no one to regard this as of any special use for the plant, much less to look on the peculiar dispositions of such leaves as especially determined for this purpose. Darwin was the first to show that there is no small number of plants--we now know about 500--which secure only a portion of their nutritive material by the usual method of assimilation, and gain another and smaller portion by dissolving and utilizing animal protoplasm, especially nitrogenous muscle substance. The correctness of this interpretation was at first disputed, but Darwin showed that pieces of muscle, or any nitrogenous organic substance, were really dissolved by the relevant parts of the plant, and were afterwards absorbed. It can therefore no longer be doubted that the remarkable contrivances by which animals are laid hold of by plants--are in a certain sense caught and killed--have arisen with reference to this particular end; or, to speak less metaphorically, that existing structural and functional peculiarities in a plant which caused animals to be held fast were of advantage to the nutrition of the plant, and were therefore augmented and perfected by natural selection. That this was possible is obvious from the number of insectivorous plants which now live upon the earth, and that these processes of selection ran their courses quite independently of one another, and even that they started from different parts of the plant, is shown by the diversity of the contrivances which occur in plants of several different families. A few of these I wish to discuss in some detail. [Illustration: FIG. 23. _Utricularia grafiana_, after Kerner. _A_, a plant in its natural position, floating in the water. _FA_, traps. _B_, a trap enlarged four times. _sz_, suctorial cells. _kl_, valve, which closes the entrance to the trap. _C_, suctorial cells on the internal wall of the trap, enlarged 250 times.]