THE ORDOVICIAN PERIOD. 345 



abysmal life. The graptolites are mingled more or less freely with the 

 fossils of the shallow- water life. This implies that the open-sea life was 

 borne freely over the epicontinental seas. This is further testimony 

 to the sea-prevalence and the cosmopolitan tendency of the life develop- 

 ment. Pelagic forms in general enjoy great freedom of distribution, 

 and hence their progress is to be presumed to be much the same in all 

 oceanic regions. Even here, absolute equality of progress is not predi- 

 cable, for the ocean-currents form circuits within which they return upon 

 themselves without compassing the whole globe, and without commin- 

 gling very extensively with other waters; and hence even the floating 

 open-sea life is assembled in some measure into vague provinces, and 

 this has probably always been the case. But the Ordovician graptolites 

 are found to be practically identical not only on the European and 

 American sides of the North Atlantic, which might have been dominated 

 by an oceanic circuit of its own, but in the seas of the times as far away 

 as Australia, whose relations are with the Pacific, the Indian, and the 

 Antarctic oceans. The inference is that the range of the graptolite 

 species was ocean- wide. They were par excellence cosmopolitan forms. 

 Lapworth * has shown that the duration of individual species was not 

 very great, geologically speaking, and hence the succession of species was 

 well suited to mark the contemporaneous progress of events in all 

 quarters of the ocean. During the lifetime of the graptolites, which 

 unfortunately was limited to the late Cambrian, Ordovician, and Silu- 

 rian, Lapworth identifies a score of successive zones, each character- 

 ized by a particular species. One of these zones falls in the Cambrian, 

 eight in the Ordovician, and eleven in the Silurian. If these be taken 

 as chronological bench-marks, so to speak, the successive horizons of 

 the different continents may be accurately correlated and the progress 

 of life in the various quarters of the globe referred to a common set of 

 standards. The succession of American faunas will claim our attention 

 later. Our first concern is with the general progress of life. 



1 On the geological Distribution of the Rhabdophora, Ann. Mag. Nat. Hist., 5th 

 Ser., Vol. Ill, pp. 245, 445; Vol. IV, pp. 333, 418; Vol. V, pp. 45, 149, 273, and 358; 

 Vol. VI, pp. 16, 185. On American graptolites, see particularly James Hall, Can. 

 Org. Remains, Dec. II, 1865; 20th Rept. N. Y. State Col. of Nat. Hist., 1868; R. R. 

 Gurley, Jour. Geol., Vol. IV, 1896, pp. 63 and 291, and R. Ruedemann, Mem. No. 7. 

 N. State Mus. 



