300 GEOLOGICAL CONTEMPORANEITY ix 



embryo of Comatula, as Comatula itself does in 

 the other. 



The Echinidea, again, are frequently quoted as 

 exhibiting a gradual passage from a more genera- 

 lised to a more specialised type, seeing that the 

 elongated, or oval, Spatangoids appear after the 

 spheroidal Echinoids. But here it might be 

 argued, on the other hand, that the spheroidal 

 Echinoids, in reality, depart further from the 

 general plan and from the embryonic form than 

 the elongated Spatangoids do ; and that the 

 peculiar dental apparatus and the pedicellariaB of 

 the former are marks of at least as great differ- 

 entiation as the petaloid ambulacra and semitae of 

 the latter. 



Once more, the prevalence of Macrurous before 

 Brachyurous Podophthalmia is, apparently, a fair 

 piece of evidence in favour of progressive modifi- 

 cation in the same order of Crustacea; and yet 

 the case will not stand much sifting, seeing that 

 the Macrurous Podophthalmia depart as far in one 

 direction from the common type of Podophthalmia, 

 or from any embryonic condition of the Brachyura, 

 as the Brachyura do in the other ; and that the 

 middle terms between Macrura and Brachyura 

 the Anomura are little better represented in the 

 older Mesozoic rocks than the Brachyura are. 



None of the cases of progressive modification 

 which are cited from among the Invertebrata 

 appear to me to have a foundation less open to 



