AMERICAN 
JOURNAL OF SCIENCE AND ARTS. 
[SECOND SERIES.] 
Arr. XX XIL—The Primitive Diversity and number of Ani- 
mals in Geological Times; by L. Acassiz. 
Tuere is a view generally entertained by naturalists and geol- 
gists that genera and species of animals and plants are greatly 
More numerous at the present age of the world than in any pre- 
vious geological period. This seems to me an entire misconcep- 
tion of the character and diversity of the fossils which have been 
discovered in the different geological formations, and to rest upon 
&stimates which are not made within the same limits, and with 
‘the same standard. Whenever a comparison of the diversity and 
umber of fossils of any geological period has been made wit 
lose of the living animals and plants belonging to the same 
Classes and fainilies, it has been done under the tacit assumption 
Which seems to. me entirely unjustifiable, that the fossils formerly 
inhabiting our globe are known to the same extent as the animals 
Which live at present upon its surface; while it should be well 
_ Mderstood that however accurate our knowledge of fossils may 
be, it has been restricted, for each geological formation, to a few 
circumscribed areas. Comparisons of fossils with the living an- 
‘a Imals ought, therefore, to be limited to geographical districts cor- 
_ “sponding in extent to those in which the fossils occur; or, 
properly, a fossil fauna with all its local peculiarities ought 
!0 be compared with a corr ponding fauna of the present period, _ 
and not with all the animals of the same class living at present 
“Pon the whole surface of the globe. And when this is done 
Stooxp Senies, Vol. XVII, No, 51.—May, 1854. 40 | 
