488 HISTORICAL GEOLOGY. 



Crustaceans, and other inferior species, a place in the Cambrian would prop- 

 erly be made for it, unless the beds were proved to be Huronian by evidence 

 that they had been formed before the epoch of mountain-making which closed 

 Archaean time. Mere divergence to this extent from the Lower Cambrian 

 in life would not be sufficient to require separation from it. 



Progress throtigh the appearance and disapp)earance of sp)ecies. — This 

 feature in the world's biological progress is well illustrated in Walcott's 

 reports. Of the many species of Trilobites from the Lower Cambrian, very 

 few are known to occur in the Middle Cambrian ; and few of those of the 

 Middle, in the Upper. According to the facts thus far gathered, it may 

 seem that events passed with a rush ; that exterminations and renewals 

 followed one another at short intervals. But the thickness of the rocks 

 proves that the three divisions of the period Avere immensely long. There 

 may have been many successive faunas in each. It is quite certain, judging 

 from the teachings of the geological past, that the abrupt breaks are gener- 

 ally, if not always, breaks in the record, not breaks in the succession of 

 species. 



The total number of ascertained species from the American Lower Cam- 

 brian is stated to be less than 200. The number 200, though large, con- 

 sidering the remoteness of the period, is very small compared with that of 

 the marine invertebrates of existing American seas. There are reasons for 

 its being so small ; for (1) only a small part of the rocks has been examined ; 

 (2) hardly a tenth of the deposits made in the Lower Cambrian would have 

 escaped the destroying action of denuding agencies; and (3), in any case^ 

 only a small part of any fauna is likely to become fossilized. The num- 

 ber of species known from the Middle Cambrian is much smaller than 

 that from the Lower. This is not evidence of fewer species at one time than 

 another in the fauna of the world. It may be proof that the conditions were 

 unfavorable over the regions geologically studied for the preservation of 

 their remains. These unfavorable conditions may have been due to tem- 

 porary changes of water level that made densely brackish seas over large 

 parts of the continental surface, or as great fresh-water seas ; or to other 

 local circumstances not now discoverable. The absence of Lamellibranchs 

 in the Middle Cambrian, although present in both the Lower and Upper,, 

 means the absence offos.nls from the rocks, not of species from the faunas. 



Progress in Cambrian life after the Lower Cambrian. — This progress 

 is strongly marked. In the Upper Cambrian, Brachiopods are of more genera ;. 

 Conularia is added to the Pteropods ; Gastropods are of normal size, and 

 those with spiral shells are multiplied; and Crustaceans are advanced to the 

 grade of non-multiplicate Hymenocarids ; and before the epoch ended there 

 were true Crinoids and Star-fishes in the seas ; Trilobites had appeared of 

 the genus Asaphus ; Ceratiocarid Crustaceans were in the waters ; and be- 

 sides these, Cephalopods, the higher Mollusks, were represented by species- 

 of Orthoceras and Cyrtoceras, the straight form of Orthoceras apparently 

 preceding the curved form of Cyrtoceras. . 



