Chjp. VII.] THEORY OF NATURAL SELECTION. 299 



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 pedicellarise. The 

 gradation extends even to the manner in which ordinary 

 spines and the pedicellarise with their supporting cal- 

 careous rods are articulated to the shell. In certain 

 genera of star-fishes, "the very combinations needed to 

 show that the pedicellarise 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 moveable 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 between the arms of 

 the pedicellaria^ and the moveable branches of a spine, is 

 unmistakable. It is generally admitted that the ordinal y 

 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 apparatus. 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, are 

 placed on the summit of a flexible and muscular, though 

 short, stem ; and in this case they probably subserve 

 some additional function besides defence. In the sea- 

 urchins the steps can be followed by which a fixed spine 

 becomes articulated to the shell, and is thus rendered 



