L. Agassiz on Animals in Geological Times. 319 
‘The study of the order of succession and gradation of the or- 
ganized beings which have inhabited our globe at different peri- 
ods, presents indeed difficulties of more than one kind. Unhap- 
pily these difficulties have seldom been all considered in their 
natural connection by those who have ventured to consider the 
subject in its whole extent; thus presenting certain results as 
general which would require various qualifications to be true. 
In comparing fossils of one and the same or of different geologi- 
cal formations, it is in reality not enough to ascertain their true 
geological horizon, which we may call the chronological element 
of the enquiry; it is equally important that the differences or re- 
semblances arising from the geographical distribution over the 
wide expanse of the whole surface of the globe, which we may 
call the topographic element of the question, should be also con- 
sidered, for it is already known that within certain limits the 
n separated by long periods of time, and existed upon earth 
under very different physical conditions. This chronological 
confusion is further increased by the too extensive limits fre- 
forming the crust of our globe. For instance, when the creta- 
ceous or the oolitic formations are considered respectively as indi- 
visible natural groups, and the fossils of all their subdivisions are 
enumerated in one single list as the inhabitants of a long period, 
an infinitude of anachronisms are presented to the mind, which 
no special mention of localities can rectify; and until the fossils 
of each of the natural subdivisions of these formations shall have 
been grouped together and compared carefully, as I have attempted 
to do it in my Monographs of the T'rigonie and of the My@ 
of Switzerland and the adjoining countries, or as Al. d’Orbigny 
S done it upon a much larger scale in his Paléontologie Fran- 
goise, no correct ideas can be formed respecting the succession of 
animals and plants characteristic of these long successive periods. 
do not believe there is a single paleontologist, whose opinion is 
Worth having, who can suppose, at this day, that any of the ani- 
) remains of which are buried in the lias, lived sim 
