Page - 247 - in The Origin of Species

THEORY OF NATURAL SELECTION 247 sembling those of Echinus, and the other those of Spatan- gus ; and such cases arc always interesting as affording the means of apparently sudden transitions, through the abortion of one of the two states of an organ. With respect to the steps by which these curious organs have been evolved, Mr. Agassiz infers from his own re- searches and those of Miiller, that both in star-fishes and sea- urchins the pedicellariae must undoubtedly be looked at as modified spines. This may be inferred from their manner of development in the individual, as well as from a long and perfect series of gradations in different species and genera, from simple granules to ordinary spines, to perfect tridactyle pedicellaricT. The gradation extends even to the manner in which ordinary spines and the pedicellariae with their sup- porting calcareous rods are articulated to the shell. In cer- tain genera of star-fishes, "the very combinations needed to show that the pedicellariae are only modified branching spines" may be found. Thus we have fixed spines, with three equi- distant, serrated, moveable branches, articulated to near their bases; and higher up, on the same spine, three other move- able branches. Now when the latter arise from the summit of a spine they form in fact a rude tridactyle pedicellaria, and such may be seen on the same spine together with the three lower branches. In this case the identity in nature be- tween the arms of the pedicellariae and the moveable branches of a spine, is unmistakable. It is generally admitted that the or- dinary spines serve as a protection ; and if so, there can be no reason to doubt that those furnished with serrated and moveable branches likewise serve for the same purpose; and they would thus serve still more effectively as soon as by meeting together they acted as a prehensile or snapping ap- paratus. Thus every gradation, from an ordinary fixed spine to a fixed pedicellaria, would be of service. In certain genera of star-fishes these organs, instead of being fixed or borne on an immovable support, arc placed on the summit of a flexible and muscular, though short, stem; and in this case they probably subserve some additional func- tion besides defence. In the sea-urchins the steps can be fol- lowed by which a fixed spine becomes articulated to the shell, and is thus rendered moveable. I wish I had space here to