Chap, ml Theory of Natural Selection. 1 9 1 



We will now turn for a short space to the lower divisions of the- 

 animal kingdom. The Echinodermata (star-fishes, sea-urchins, &c> 

 are furnished with remarkable organs, called pedicellaria?, which 

 consist, when well developed, of a tridactyle forceps— that is, of one- 

 formed of three serrated arms, neatly fitting together and placed on 

 the summit of a flexible stem, moved by muscles. These forceps 

 can seize firmly hold of any object ; and Alexander Agassiz has 

 seen an Echinus or sea-urchin rapidly passing particles of excrement 

 from forceps to forceps down certain lines of its body, in order that 

 its shell should not be fouled. But there is no doubt that besides 

 removing dirt of all kinds, they subserve other functions ; and one 

 of these apparently is defence. 



With respect to these organs, Mr. Mivart, as on so many pre- 

 vious occasions, asks : " What would be the utility of the first 

 rudimentary beginnings of such structures, and how could such 

 incipient buddings have ever preserved the life of a single Echinus ? " 

 He adds, " not even the sudden development of the snapping action 

 could have been beneficial without the freely moveable stalk, nor 

 could the latter have been efficient without the snapping jaws, yet 

 no minute merely indefinite variations could simultaneously evolve 

 these complex co-ordinations of structure ; to deny this seems to do 

 no less than to affirm a startling paradox." Paradoxical as this- 

 may appear to Mr. Mivart, tridactyle forcepses, immovably fixed 

 at the base, but capable of a snapping action, certainly exist on 

 some star-fishes ; and this is intelligible if they serve, at least in 

 part, as a means of defence. Mr. Agassiz, to whose great kindness- 

 I am indebted for much information on the subject, informs me 

 that there are other star-fishes, in which one of the three arms of 

 the forceps is reduced to a support for the other two ; and again, 

 other genera in which the third arm is completely lost. In Echino- 

 neus, the shell is described by M. Perrier as bearing two kinds of 

 pedicellariae, one resembling those of Echinus, and the other those 

 of Spatangus; and such cases are 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 researches and those 

 of Mtiller, that both in star-fishes and sea-urchins the pedicellarise 

 must undoubtedly be looked at as modified spines. This may be 

 inferred from their manner of development in the individual, a& 

 well as from a long and perfect series of gradations in different 

 species and genera, from simple granules to ordinary spines, to- 

 perfect tridactyle pedicellariae. The eradation extends even to 



