x.] PERSISTENT TYPES OF LIFE. 223 



certain increase of complexity in the newer genera. 

 Here, however, one is met at once with the occurence 

 of Orthoceras and Baculitcs at the two ends of the 

 scries, and of the fact that one of the simplest genera, 

 Nautilus, is that which now exists. 



The Crinoidca, 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 con- 

 dition. But then, on careful consideration of the facts, 

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

 the palaeozoic 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 ex- 

 hibiting a gradual passage from a more generalized to a 

 more specialized 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 peclicellariae of the former are marks 

 of at least as great differentiation as the petaloid ambu- 

 lacra and semitoe of the latter. 



Once more, the prevalence of Macrurous before Bra- 

 chyurous 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 Podoph- 

 thalmia depart as far in one direction from the common 

 type of Podophthalmia, or from any embryonic condition 



