26 INTRODUCTION. 



such as the Cephalopoda, which abounded in the Cretaceous 

 seas, but which seem to have no representative in the abyssal 

 mud of the Atlantic. 



x DOCTRINE OF COLONIES. It only remains in this connec- 



tion to consider very briefly the doctrine of " colonies," laid 

 down by M. Barrande, the eminent Bohemian palaeontologist. 

 It has been laid down as a law that when once a species dis- 

 appears it never again makes its appearance in the geological 

 record. This is unquestionably 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 pointed out 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 Upper and Lower Silurian Rocks of Bohemia are 

 characterised by very distinct assemblages of fossils. It is 

 found, however, that the Lower Silurian 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 containing 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 



