54 INTRODUCTION. 



disappeared in one region, is found in deposits of a later age 

 in another area. The above-mentioned law, therefore, can 

 obviously only hold good of cases in which a species has 

 definitely and finally become extinct; and this implies an 

 amount of knowledge on our part which we seldom or never 

 possess. M. Barrande, however, has pointed out that there 

 are other cases in which groups or species peculiar to one set 

 of beds may appear in a temporary and sporadic manner in a 

 much earlier set of beds, the two deposits thus characterised 

 being separated by beds containing fossils peculiar to the 

 earlier and older series. Thus, the Upper and Lower Silurian 

 rocks of Bohemia are characterised by very distinct assem- 

 blages of fossils. It is found, however, that the Lower Silu- 

 rian rocks contain in places a group of fossils characteristic 

 of the Upper Silurian series. The beds containing this 

 " colony " of Upper Silurian forms are succeeded by strata 

 filled with Lower Silurian fossils ; and it is only after several 

 alternations of this kind that the Upper Silurian fauna comes 

 in definitely and generally. These temporary appearances of 

 a later fauna in the midst of an older fauna are termed by 

 M. Barrande " colonies," and he explains their occurrence as 

 follows : If we suppose the seas of the Bohemian area to 

 have been peopled with Lower Silurian animals at a time 

 when other portions of Europe were covered by a sea contain- 

 ing Upper Silurian animals, and suppose the former area to 

 have been shut off from the latter by a land-barrier, we can 

 readily understand how the " colonies " were produced. If, 

 from any cause, a channel of communication were opened 

 between the Bohemian area and the general area of Northern 

 Europe, an immigration of species would take place from the 

 latter into the former area. The Upper Silurian species of 

 the latter area would thus be imported, in greater or less 

 numbers, into the midst of the general Lower Silurian fauna 

 of Bohemia, and would be preserved in the Lower Silurian 

 rocks. If, however, the channel of communication were 

 speedily closed, so that the new-comers could not be con- 

 stantly reinforced by fresh immigrants, the " colonial " species 

 would die out, and the general Lower Silurian fauna would 

 again reign supreme. A reopening of the channel of com- 



