and Number of Animals in Geological Times. 291 
studied long after systematic zoology had made considerable 
progress ; but even to this day the limits of the faune are no- 
where circumscribed with any kind of precision, the prin- 
ciples upon which they might be determined are in many re- 
spects questionable, and a large number of animals are daily 
described without any reference to their natural distribution 
upon the earth; though much has already been done since 
Buffon, to place this branch of our knowledge upon a better 
foundation, and especially to ascertain the laws regulating 
the geographical distribution of certain classes and families 
considered isolately. The point which requires now particu- 
lar attention, is the combination of these different types 
within definite regions, and their common circumscription 
within natural zoological provinces. This study would be 
particularly important with reference to the comparison of 
the local faunz of former geological periods with those of the 
present creation. But since the latter even are comparatively 
little known, we must be satisfied to wait for the time when 
thorough comparisons shall be possible between the local 
faune of each and all geological periods inter se, and with 
those of other periods. 
In closing this digression, I may sum up my criticism upon 
paleontological investigations, by saying that any generaliza- 
tion respecting the succession of organized beings which is 
not based upon materials in which the synchronism and suc- 
cession of species and their geographical distribution is not 
duly considered, and in which the identification of species is 
not made with reference to sound zoological principles, with 
due regard to the equal limitation of genera, and also with 
reference to our improved classifications in zoology, is not fit 
to be trusted. All species taken into consideration should 
undergo a revision with reference to their chronology, their 
topography and their zoology, and in the last point of view 
the range and natural limitation as well as identity of the 
_ Species, their generic affinities and their zoological classifica- 
_ tion should be equally tested. 
Returning now to the main subject of this paper, I have 
further to say that the very fact that certain stratified rocks, 
even among the oldest formations, are almost entirely made 
t™ 2 
