390 THE IMPERFECTION OF THE 



parts of this area were very different, and, as a necessary result, cer- 

 tain groups of animals flourished in certain localities, and were absent 

 or but scantily represented in other places. In the deeper parts of 

 the area, we have an abundance of corals, with Crinoids, and at times 

 Foraminifera. In the shallower parts of the area there is, on the 

 other hand, a predominance of forms which affect shallow water. 

 Still, there is no difference in point of time between the deposits of 

 different parts of the area, and in order to obtain a true notion of the 

 Lower Carboniferous fauna, we must add the fossils derived from one 

 portion of the area to those obtained from another. 



In many cases, however, we are acquainted with but one class of 

 deposits belonging to a given period. We may only have the deep- 

 sea deposits of the period, or we may know nothing but its littoral 

 accumulations. In either case it is clear that there is an imperfection 

 of the palseontological record ; for we can not have even a moderately 

 complete record of the marine animals alone of a particular period, 

 unless we have access to a complete series of the deposits laid down 

 in that period. 



IV. Sudden extinction of Animals : — While there can be little 

 doubt but that the changes in animal life indicated by geology were 

 gradually effected, there still remain cases in which individuals seem 

 to have been suddenly destroyed, and others of a more obscure nature 

 in which allied species succeed one another with an inexplicable 

 rapidity. As an example of the first class of cases we may take the 

 great marine Reptiles of the Lias, which often exhibit indications of 

 having met a sudden death, while they shoAV no marks of mechanical 

 injury. It has been suggested by Sir Charles Lyell, with great proba- 

 bility, that the sadden death of marine animals, as in these and similar 

 cases, might be due to the sudden "periodical discharge of large 

 bodies of turbid fresh water into the sea." 



As an example of the second class of cases, we may take the 

 existence in the Lias of zones characterized by particular species of 

 Ammonites. These zones are usually of small thickness, and the 

 Ammonite characterising each is usually confined to that particular 

 horizon ; whilst several of the zones have been found to be j)ersistent 

 over very large areas. As we know of no reason why one species of 

 Ammonite should flourish where another allied species would not, we 

 can not at present account for this sudden disappearance of one species 

 and its seeming immediate replacement by another. 



