316 SKETCHES OF C RE AT I OX. 



introduced by Echinoderms, Acalephs, and Protozoans, the 

 two highest and the lowest of the four classes. True coral 

 animals perhaps made their appearance a little later. In 

 the sub-kingdom of Molluscs all the classes stand abreast 

 on their first advent; in that of Articulates, the two lower 

 classes, Crustaceans and Worms, preceded by a long inter- 

 val the Insecteans; and in the sub-kingdom of Vertebrates 

 the classes followed each other in regular gradational suc- 

 cession. Thus we see that, so far as class-groups are con- 

 cerned, it is impossible to reduce the order of succession to 

 any general formula. How is it with the orders of the 

 respective classes ? Among Echinoderms, Cystideans ap- 

 peared before the successively higher Crinideans, Starfishes, 

 and Sea-urchins; among Acalephs, the horny Graptolites 

 ajDpeared before the Coral-builders ; among Protozoans, the 

 Sponges, which ally themselves to Polypi, appeared before 

 the lowest types — always disregarding the mysterious JEo- 

 zoon. On the whole, the order of succession among the 

 groups, based upon relative rank, is, with Radiates, from 

 below upward. With Molluscs we find the straight and 

 simple Orthoceratites preceding the higher Cephalapods; 

 the arcuate and the entire-mouthed Gasteropods leading 

 the higher spiral and flesh-eating families ; the asiphonal 

 Lamellibranchs antedating those with more complete respi- 

 ratory apparatus, and the horny-shelled Lingula and Disti- 

 nct., among Brachiopods, appearing before the stony-shelled 

 and stony-armed Spirifers and Terebratulas. Among the 

 Articulate and Vertebrate classes the gradational succes- 

 sion of the various orders is tolerably perfect. But I must 

 refrain from alluding to specific facts. The following grand 

 generalization rests on a broad survey of data upon which 

 it would be inappropriate, in this place, to enter. 



There is no successional relation between the four sub- 

 kingdoms of animals, nor even between the several classes 



