DISTRIBUTION OF ORGANISMS. 165 



lopment. Nor is it to be overlooked that the presumed pro- 

 geny exhibit, in their voracious character, and the functions 

 they serve in nature, a perfect family likeness to their an- 

 cestry. The cartilaginous fishes were the chief police for 

 keeping down the redundant life of the Devonian and Carbo- 

 niferous seas, as the cephalopoda had been during the Lower 

 and partly also during the Upper Silurian eras. 



The approach made by the annelides to some of the humbler 

 forms of fish indicates another passage from the invertebrate 

 into the vertebrate animals ; and this passage may have taken 

 place in the Upper Silurian or Devonian era, for annelides 

 are ascertained to have previously existed. Perhaps some of 

 the less destructive of the early cartilagines the Lepidoids 

 were such an inoffensive family have had such an origin. 



It might be suggested, as an inquiry worthy of the attention 

 of zoologists, whether the echinodermal line has not given rise 

 to the more recently developed fish families, those which 

 enter upon the field hi the cretaceous era. If the fistularidae 

 make, as appears, so near an approximation to the lowest 

 bearers of the vertebratal type, it is not easy to see how any 

 preconceived ideas regarding the order of sub-kingdoms to be 

 passed through should stand in the way, especially after so 

 many traces of similar irregularity. The geological history 

 of the animals in question is favourable to the conjecture, for 

 the echinoderms are amongst the most conspicuous and im- 

 portant forms antecedent to the chalk era. Looking, indeed, 

 at the enormous abundance of crinoidea in the carboniferous 

 rocks, one can hardly avoid the idea that this peculiar form 

 was destined for some important ultimate history. It might 

 be suggested that the orders by which the fish class is thus 

 entered are those placed by Cuvier at the bottom of the osseous 

 fishes, the Lophobranchii and Plectognathi, which indicate 

 their nearness to the invertebrate type by many features 

 attaching to some or all of them, as imperfection and slow 

 hardening of the skeleton, deficiency of ribs and fins, low 

 and embryotic forms of mouth, dentition, and gills ; the Lo- 

 phobranchi, moreover, hatching their young in a pouch below 



