"COLONIES." 6 1 



Bohemian palaeontologist. It has been laid down as a law that when 

 once a species disappears it never again makes its appearance in the 

 geological record. This is almost certainly true, so long as we 

 remember that it can only apply to cases in which a species has 

 entirely and totally disappeared from the earth, and that it is often 

 very difficult, or altogether impossible, to obtain evidence as to the 

 exact time at which a given species has thus become actually extinct. 

 There are plenty of cases in which a species seemingly disappears in 

 a particular set of rocks, to reappear in some higher and later set of 

 rocks in the same region, whilst its remains are wanting in all the 

 intermediate deposits of the area. It also often occurs that a species, 

 having 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 

 endeavoured to prove that there are other cases in which groups of 

 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 Ordovician and Silurian rocks of 

 Bohemia are characterised by very distinct assemblages of fossils. 

 According to M. Barrande, however, the Ordovician rocks contain in 

 places a group of fossils characteristic of the Silurian series. The 

 beds containing this " colony " of Silurian forms are succeeded by 

 strata filled with Ordovician fossils ; and it is only after several 

 alternations of this kind that the 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 Ordovician animals at 

 a time when other portions of Europe were covered by a sea con- 

 taining Silurian animals, and suppose the former area to have been 

 shut off from the latter by a land-barrier, we can readily understand 

 how " colonies " might be 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 Silurian species 

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

 into the midst of the general Ordovician fauna of Bohemia, and 

 would be preserved in the Ordovician rocks. If, however, the 

 channel of communication were speedily closed, so that the new- 

 comers could not be constantly reinforced by fresh immigrants, the 

 " colonial " species would die out, and the general Ordovician fauna 

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



