CONTEMPORANEITY OF STRATA. 53 



to us in point of time than the Silurian, and that we can 

 therefore trace a relationship between certain Cretaceous 

 types and certain living forms that we cannot hope to 

 establish in the case of Silurian fossils. 



It is to be observed, lastly, that certain classes of animals 

 are always likely to flourish in places and times in which 

 favourable conditions are present, wholly irrespective of any 

 genetic connection between successive faunae. Thus the con- 

 ditions present in the deep Atlantic are such as favour the 

 existence of numerous Foraminifera, Sponges, Echinoderms, 

 &c. Similar conditions existed in the seas in which the 

 Chalk was deposited ; and we need not, therefore, be sur- 

 prised at the predominance of similar organisms in the Cre- 

 taceous period. In the same way, there are portions of the 

 Carboniferous Limestone fairly comparable to the Chalk in 

 mineral characters (making due allowance for difference of 

 age), and containing forms of life which may be regarded as 

 representative of the Cretaceous fauna such as Foramini- 

 fera, smooth Terebratulce, Crinoids, and Sea-urchins. The 

 conditions, however, present in the deep Atlantic are not 

 exactly similar to those under which the Chalk was deposited, 

 for there are certain great classes, such as the Cephalopoda, 

 which abounded in the Cretaceous seas, but which seem to 

 have no representatives in the abyssal mud of the Atlantic. 



DOCTHINE 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 de- 

 posits of the area. It also often occurs that a species, having 



