288 L. Agassiz on the Primitive Diversity 
reduce or expand the scale applied to the investigations of 
different authors, when using them for the same purposes, 
exactly in the same manner as thermometric observations 
made with the scales of Reaumur, or Celsius, or Fahrenheit, 
are reduced to the same standard, before being compared. 
In the second place, species must be referred to genera cir- 
cumscribed within the same limits, before they can fairly be 
compared, or at least lead tc trustworthy general results. As 
long as certain bivalve shells of the carboniferous and oolitic 
series were referred to the genus Unio, it could appear that 
the family of Naiades began its existence at a very early pe- 
riod ; but since the oolitie species of this kind have been as- 
certained to differ essentially from our freshwater shells, and 
to constitute by themselves a natural genus more closely allied 
to Crassatella than to Unio, nobody thinks any longer of look- 
ing for Unios in marine deposits. As long as certain fossil 
fishes of the Zechstein and Lias were referred to the genera 
Esox and Cyprinus, the families of which these genera are 
the types could be supposed to have extended their range 
far beyond the tertiary formations, before which, however, i 
no one of their representatives is to be found. Before the 
Spatangoids were divided into natural genera, the genus Spa- 
tangus was mentioned among the fossils of the oolitic as well 
as the cretaceous and tertiary formations ; now it is restricted 
to the last among the fossils, and found also among the living. 
I do not believe that a single genuine species of Gorgonia is 
found among the fossil Polypi, and yet that genus appears in 
the lists of fossils from the paleeozoic period to the present 
time. 
Since it is not my intention to enter here upon a special 
criticism of the innumerable errors of this kind still to be — 
found in even modern lists of fossils, I shall not multiply my 
examples. These may be sufficient to show how important a : 
correct generic identification of the fossils may be in the esti- 
mation of the order of sucession of organized beings; and I 
cannot but lament the utter want of consideration evinced 
even by many distinguished paleontologists in this respect, 
who seem to think that the knowledge of species is sufficient 
in itself to a proper appreciation of the order of creation, and | g 
