76 THE POPULAR SCIENCE MONTHLY 
fauna; not only have I found it necessary to establish identity of the 
species in the recurrent zones with those of the initial zones, but it is 
essential to show that the faunas as a whole are the same. 
To put this in another form of statement we must establish the fact 
that not only the individual species have retained their specific char- 
acters, but the further fact that the equilibrium of adjustment to each 
other in the faunal community has not been changed, in order to prove 
that the recurrent fauna is the direct successor of a fauna represented 
in the rocks at a lower horizon. 
This has led to such distinction as rare and dominant species of the 
fauna, and only as some such comparative frequency of the species in 
the faunal combination is apparent can we be sure that we are not 
considering an accidentally accumulated sample of a general fauna. 
The presence of occasional associated species belonging to the 
normal fauna of the formation in which the recurrent zone appears is 
not antagonistic to the theory, because the theory proposes an invading 
of the territory occupied by the normal fauna, and whatever were the 
causes which brought about the shifting of the fauna they were not 
so completely different as to annihilate all evidence of the fauna previ- 
ously occupying the ground. Hence it is only necessary to find an 
abrupt change of the grand majority of species to make the induction 
that the faunas have shifted their habitat. 
The theory involves the further conception of grand general faunas 
which have their center of habitat and distribution in permanent 
oceanic basins, as distinguished from the special and (in geological 
strata) temporarily expressed faunas such as we are accustomed to as- 
sociate with individual geologic formations. 
In the case before us two such general faunas are in evidence, one 
of which in its dominant characteristics is traced westward into Iowa, 
Idaho and Arizona and up the Mackenzie River valley to the north and 
across the polar regions to Russia and northern Europe. The other 
is traced eastward and southward into central and southern Europe 
and also dominantly into South America. 
Although, with our present knowledge, it is not possible to deter- 
mine in any temporary expression of marine faunas those particular 
species which were derived from one from those derived from the other 
grand source, it is possible to recognize numerous species which belong 
to one center of distribution and others that belong normally to the 
other. 
$10. Interpretation of the Facts.—It is also important to keep our 
heads clear in interpreting the facts. 
It is only by close examination and comparison of the fossils them- 
selves that identity of species or identity of faunas can be established. 
The fixed characters of species are not only the characters by which 
