The Scopt' of VaJ'Oiitohxjy. — Wdl'inin><. 155 
organisms. In the liistoiy of vertebrates we are all familiar with 
the law of succession: (1) Fish, (2) Amphibians, (3) Keptiles, 
(4) Mammals, and in another line, (3) Reptiles, (4) Reptilian 
Birds, (5) Birds. 
The finding of remains of any one of these groups of animals 
is sufficient evidence that representatives of the lower t3'pe had 
previousl}^ existed. The abundance of reptilian remains is cei'tain 
indication of later age than the Paleozoic; the abundance of 
mammals, of age later than Mesozoic. In the same way the trilo- 
bites are known to be an ancient type, and the decapods a more 
modern type of Crustacea. The Tetracoralla are older than the 
Hexacoralla. And a great number of similar instances can be 
named, where, in a particular class or order of organisms, there 
is a definite succession in the order of their dominance and theo- 
retically it is believed also, in their initiation. A third applica- 
tion of paleontology is made by the comparative study of species 
of a particular genus, or the genera of an order. In each genus 
there is observed to be a period of particular al)undance before or 
after which there is more or less rapid diminution in the number 
of species found. Thus the brachiopods, so abundant all through 
the Paleozoic rocks, present a definite order of sequence in the 
genera and families, relative abundance of species of which, irre- 
spective of specific names, is a reliable indication of geological 
age. The Cambrian is indicated l)y the abundance of its Obolida^ 
and other inarticulate genera, the lower Silurian by abundance of 
Orthida? and Strophomenida?, the upper Silurian by numerous 
genera of Strophomenidae, Pentameridie and Rhynchonellida?, the 
Carboniferous by dominance of the Productida?, the Mesozoic by 
dominance of Terebratulidye and absence of the Paleozoic types. 
So, too, the Cephalopods furnish a scale of families which present 
a natural se(iuence, the Orthoceratidai, the Goniatitidae, the Cerat- 
itida?, the Ammonitid*, and the dominance numerically of these 
several divisions at once testifies to the 1st half of Paleozoic, the 
2d half, the 1st half of Mesozoic, the second half of Mesozoic, 
and absence, or almost total al)sence of each, to the Tertiary and 
Recent. 
Less is known of the succession of species in continuous series 
as indicative of order of time. The famous ease of the classifi- 
cation of the beds of the Lias by its Ammonites is a characteristic 
example. (Jppel, Wright, Buchman and others have studied the 
