194 LAY SERMONS, ESSAYS, AND REVIEWS. [x. 



occurrence of Orthoccras 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, 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 JVIacrurous 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 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 



