= s  - = —=— mas 
and Number of Animals in Geological Times. 289 
that genera are arbitrary divisions established by naturalists 
merely for the sake of facilitating the study of species, as if 
the more general relations of living beings to one another 
were not as definitely regulated in all their degrees by the 
same thinking mind, as the ultimate relations of individuals 
to one another. 
In the third place the natural affinities of genera should be 
ascertained. Unless the genera are referred to the families 
to which they truly belong, unless the rank of these families 
in their respective classes is positively determined, unless the 
peculiarities of structure which characterizes them is taken 
as the foundation of such an arrangement and further corro- 
borated by the mode of development of their respective types 
it would be a hopeless task to attempt to determine the order 
of succession of the fossils in different geological formations. 
Before the Crinoids, which Lamarck placed along the Polyps, 
had been referred to the class of Echinoderms, nobody could 
have understood the beautiful gradation so fully ascertained 
_ now, which may be traced through all geological formations 
among these animals. Before it was ascertained that the 
little animal described by Thompson under the name Penta- 
erinus europeus,as a living Crinoid, for which M. De Blainville 
established the genus Phytocrinus, is in reality the young of 
a Comatula, nobody could have suspected the wonderful re- 
lations which exist between the changes animals now living 
undergo during their growth, and the order of succession of 
entire classes of animals during successive geological ages. 
As long as the natural position of Trilobites remained doubt- 
ful in the animal kingdom, the characters of the prototypes 
of the class of Crustacea could not be appreciated. Who 
does not see how impossible it was for those who classified the 
Trilobites with the Chitons to arrive at any sound results re- 
specting the gradation and order of succession of these ani- 
mals? Whilst now they are beautifully linked to the Mac- 
rura of the Trias, by the gigantic Entomostraca of the De- 
vonian and Carboniferous periods. Again, the knowledge of 
_ the embryology of Crustacea gives us a key to a correct ap- 
_ preciation of the early appearance of the Macrura and the 
late introduction of the Brachyura. The removal of the Bry- 
VOL. LVII. NO. CXIV.—OOTOBER 1854. iz 
