PERSISTENT TYPES OF LIFE 391 



the septal sutures exhibiting a certain increase of complexity 

 in the newer genera. Here, however, one is met at once 

 with the occurrence of Orthoceras and Baculites at the two 

 ends of the series, and of the fact that one of the simplest 

 genera, Nautilus, is that which now exists. 



The Crinoidea, in the abundance of stalked forms in the 

 ancient formations as compared with their present rarity, 

 seem to present us with a fair case of modification from a 

 more embryonic towards a less embryonic condition. But 

 then, on careful consideration of the facts, the objection 

 arises that the stalk, calyx, and arms of the paleozoic 

 Crinoid are exceedingly different from the corresponding 

 organs of a larval Comatula ; and it might with perfect 

 justice be argued that Actinocrinus and Eucalyptocrinus, 

 for example, depart to the full as widely, in one direction, 

 from the stalked embryo of Comatula, as Comatula itself 

 does in the other. 



The Echinidea, again, are frequently quoted as exhibiting 

 a gradual passage from a more generalized to a more 

 specialized type, seeing that the elongated, or oval, Spa- 

 tangoids 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 pedicellariae of the former are marks of at least as 

 great differentiation as the petaloid ambulacra and semitae 

 of the latter. 



Once more, the prevalence of Macrurous before Brach- 

 yurous Podophthalmia is, apparently, a fair piece of 

 evidence in favour of progressive modification 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 Podoph- 

 thalmia, 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 apear to me to have a 

 foundation less open to criticism than these ; and if this 

 be so, no careful reasoner would, I think, be inclined to 

 lay very great stress upon them. Among the Vertebrata, 



