Chap. VII.] THEOKY OF NATURAL SELECTION, 301 



mata, which he considers as " essentially similar/' having 

 been developed through natural selection in widely dis- 

 tinct divisions of the animal kingdom. But, as far as 

 structure is concerned, I can see no similarity between 

 tridactyle pedicellarise and avicularia. The latter re- 

 semble somewhat more closely the chelse or pincers of 

 Crustaceans; and Mr. Mivart might have adduced with 

 equal appropriateness this resemblance as a special diffi- 

 culty; or even their resemblance to the head and beak of 

 a bird. The avicularia are believed by Mr. Busk, Dr. 

 Smitt, and Dr. Nitsche — naturalists who have carefully 

 studied this group — ^to be homologous with the zooids 

 and their cells which compose the zoophyte; the move- 

 able lip or lid of the cell corresponding with the lower 

 and moveable mandible of the avicularium. Mr. Busk, 

 however, does not know of any gradations now existing 

 between a zooid and an avicularium. It is therefore 

 impossible to conjecture by what serviceable gradations 

 the one could have been converted into the other:. but it 

 by no means follows from this that such gradations 

 have not existed. 



As the chelae of Crustaceans resemble in some degree 

 the avicularia of Polyzoa, both serving as pincers, it 

 may be worth while to show that with the former a long 

 series of serviceable gradations still exists. In the first 

 and simplest stage, the terminal segment of a limb shuts 

 down either on the square summit of the broad penulti- 

 mate segment, or against one whole side; and is thus 

 enabled to catch hold of an object; but the limb still 

 serves as an organ of locomotion. We next find one 

 corner of the broad penultimate segment slightly promi- 

 nent, sometimes furnished with irregular teeth; and 

 against these the terminal segment shuts down. By an 



