The Scope of Paleontology. — Williams. 153 
followed the same law. As among vertebrates, the multirayed 
fin of the fish, the webbed paddle of the enaliosaur, the paw of the 
crawling reptile, the hand of man form a natural sequence of de- 
velopment, the later in each case presupposing the preceding stage. 
In the second case it is well known that there is a natural suc- 
cession of species. This succession points to genetic relationship 
between successive species, and it is an established law of this 
succession that species most like each other occur near together 
in the chronologic order, and species of the same genus, present- 
ing the greatest divergence from each other, are also the more 
widely separated in time. 
In the third case it ig known regarding living organisms that 
they present natural association with each other and in adaptation 
to the various conditions of environment. The law of this asso- 
ciation is expressed by the terms fauna and flora. Paleontology 
teaches that the faunas and floras change, and, as the law of Lyell 
illustrates, that this change is gradual and in a definite order. The 
individualit}' of a fauna can be recognized and can be followed 
out in its successive changes, and these changes are best ex- 
plained by the law of adjustment to environment. 
In the series of modified organs we see the law of orgnn'u- (le- 
vel opmeut^ in the series of successive species of a race, the law of 
hereditary ecohition, and in the composition and changes of sue 
cessive faunas the law of adjustive adaptation to environment. 
It is an expression of these laws that the fossils become such 
delicate tests of the chronological order and the geological condi- 
tions of the past. 
In the lnterj)retation of the divisions of the Tertiary, Lyell ob- 
served the law of succession of a single fauna, and, for a single 
continuous fauna, the gradual accession of new species and ex- 
tinction of old would express a normal law of succession. If, 
however, two faunas are compared, the number of common species 
would depend upon the likeness or difference of the environing 
conditions. 
To apply the Lyellian principle correctly, it is necessary to 
compare the successive faunas of the same province, and for the 
recognition of the province too, it is necessary to consider the 
possible change of climate, or the effect of the shifting of condi- 
tions by elevation or depression of the bottom, or change of re- 
lation of surface of sea to surface of the hind. 
