232 



vonian, Carboniferous, Permian, and Triassic, and with occa- 

 sional interruptions up to the Hving species ; there is an unbroken 

 succession of linyulce .up to the Jurassic strata ; they are not 

 found in the oolite, in the seven beds above the lias ; in the low- 

 est cretacean (neocomian) they appear again, then there is an 

 interruption until the Tertiary epoch. About forty species of fos- 

 sil Imgulce are found in these beds ; only seven species of living 

 Ungulce were known to exist until he had recently added an 

 eighth (X. Ravenelli, ^g-) fi'om South Carolina, the first found 

 on the American side of the Atlantic basin ; when the shell gapes, 

 one valve moves over the other, a circumstance rar^e in the brachio- 

 pods. He thought the persistence of this form through so exten- 

 sive a period, the last no more perfect than the first, was a fatal 

 objection to the theory of gradual development. 



Prof. Rogers admitted that the persistency of lingula, and 

 other similar cases that might be adduced, were formidable ob- 

 jections to this theory ; but he thought that Darwin would meet 

 such objections by the fact that the vital characters of some ani- 

 mals fit them for resisting change and extinction better than more 

 plastic natures ; from our knowledge of domesticated animals we 

 find that dogs have changed very much, and that cats have 

 changed hardly at all ; some have great energy of resistance, and 

 some very little. He adduced several cases of interruption, like 

 those in Bohemia illustrated by Barrande, and in middle Tennes- 

 see by Mr. Satford, which he explained by migrations to and 

 from a given region. On the coast of Virginia and Maryland 

 there is an extensive oyster-bed, but which has not been contin- 

 uous through all time ; at one time the oysters disappear, and 

 clams make their appearance ; the latter disappear, and oysters 

 reappear ; these he regarded as instances of emigration and remi- 

 grations over great spaces. Similar facts in Bohemia, embracing 

 strata of many thousands of years' duration, show^ evidences of re- 

 introduction of forms from below, colonization, and remigration. 

 In middle Tennessee, we have the Black Hiver limestone over 

 the Potsdam sandstone ; over this slaty limestones with Trenton 

 fossils ; after several hundred feet of thickness, this is succeeded 

 by the Black River fossils again. He thought these evidences of 

 migration, and not in the least degree of sudden creation without 



