Page:Popular Science Monthly Volume 77.djvu/82

76 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 characters, 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 previously 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 associate 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 determine 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 themselves that identity of species or identity of faunas can be established.

The fixed characters of species are not only the characters by which