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 



