HOMOLOGIES. 205 



ral law which nevertheless controls and includes 

 them all. 



I wish that I could take as the illustration of 

 this statement animals with whose structure the 

 least scientific of my readers might be presumed 

 to be familiar ; but such a comparison of the 

 Vertebrates, showing the identity and relation of 

 structural elements throughout the Branch, or 

 even in any one of its Classes, would be too ex- 

 tensive and complicated, and I must resort to the 

 Radiates, that branch of the Animal Kingdom 

 which, though less generally known, has the sim- 

 plest structural elements. 



I will take, then, for the further illustration of 

 my subject, the Radiates, and especially the class 

 of Echinoderms, Star-Fishes, Sea-Urchins, and 

 the like, both in the fossil and the living types ; 

 and though some special description of these ani- 

 mals is absolutely essential, I will beg my readers 

 to remember that the general idea, and not its 

 special manifestations, is the thing I am aiming 

 at, and that, if we analyze the special parts char- 

 acteristic of these different groups, it is only that 

 we may resolve them back again into the struc- 

 tural plan that includes them all. 



I have already in a previous article named the 

 different Orders of this Class in their relative 

 rank, and have compared the standing of the liv- 

 ing ones, according to the greater or less compli- 



