Periman Formation of Texas. 123 
cause both its Coal-measure and Triassic age can be even more 
readily proved, in an ex parte way, by special selections from its 
fossils, than its Permian age. And yet the sum of all the evidence 
is in favor of the latter. 
The following paragraph from the work of Professors Wm. M. 
Fontaine and I. C. White tersely states^ the principle which ought 
to govern the investigator in these cases, although it was written 
only with reference to the Permian character of the flora which they 
were then investigating. 
*'It is good evidence that we have to deal with a more recent for- 
mation, when we find it to show a decadence of old forms, and an 
introduction of new ones, destined to reach their culmination at a 
later period. Thus if we find, in a series of rocks, plants charac- 
teristic of the Carboniferous formation, and perceive that these die 
out^and disappear, we should not conclude from their mere pres- 
ence that the age of the strata is Carboniferous, but rather that it 
is Permian. So also the finding of genera and species, even iden- 
tical with those of the Trias or Jurassic, would not necessarily 
imply a Triassic or Jurassic age. If we find them to be exceed- 
ingly rare, their presence is rather indicative of a formation older 
than the Trias or the Jurassic. It is only by taking into considera- 
tion all the above named characters and other points which maybe 
presented by the entire body of specimens, that we can determine 
the nature of the evidence offered by the life of a formation. It 
will not suffice to say arbitrarily that this or that feature is without 
value as evidence. Circumstances might reverse the normal relative 
weight of evidence from the several sources, and give preponder- 
ating weight to what would, if unaffected by them, have slight 
value." 
Besides the observance of this principle, the investigator should 
remember the entire improbability that distinguishing types could 
have been simultaneously introduced in all parts of the world ; and 
the no less evident fact that certain types in different parts of the 
world long survived their extinction in other parts. He should al- 
so bear in mind the now evident fact that the rate of pro- 
gressive development of vertebrate, invertebrate and plant life 
respectively has not been uniform in all parts of the world. It 
therefore ought not to be expected that precisely the same associa- 
st .Virginia and S. W. Pennsylvania. 
