lii PROCEEDINGS OP THE GEOLOGICAI, SOCIETY. 



if the known geological record is to be regarded as even any consider- 

 able fragment of the whole, it is inconceivable that any theory of 

 a necessarily progressive development can stand, for the numerous 

 orders and families cited afford no trace of such a process. 



But it is a most remarkable fact, that, while the groups which 

 have been mentioned, and many besides, exhibit no sign of pro- 

 gressive modification, there are others, coexisting with them, under 

 the same conditions, in which more or less distinct indications of 

 such a process seem to be traceable. Among such indications I may 

 remind you of the predominance of Holostome Gasteropoda in the 

 older rocks as compared with that of Siphonostome Gasteropoda in 

 the later. A case less open to the objection of negative evidence, 

 however, is that afforded by the Tetrabranchiate Cephalopoda, the 

 forms of the shells and of 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 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 exhibiting 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 pedicellarise 

 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 Brachyurous 

 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 of the Brachyura, 

 as the Brachyura do in the other ; and that the middle terms be- 

 tween Macrura and Brachyura — the Anomura — are little better re- 

 presented 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 criticism than these ; and if this be so, no careful reasoner would, 

 I think, be inclined to lay very great stress upon them. Among 



