106 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
proposition requires us to assume that our second fauna lived con- 
temporaneously with the one immediately underlying it, but in some 
other region separate from the one in which it is recorded Examples 
of such shifting of faunas have occasionally been met with in the 
investigations of deep seas. Professor Verrill/ in his studies of the 
faunas of the Atlantic edge of the New England shores, has pointed 
out a remarkable case of this kind. About 80 miles off Woods Hole 
one season a unique fauna appeared — the tile-fish fauna — with a new 
and abundant set of species, a great proportion of them new and 
representing altogether a new fauna. This fauna afterwards was 
lost sight of, and the dredgers found no traces of it in the region 
where it was first found. The explanation of the sudden appear- 
ance of such a fauna is that the shifting of currents, or some other 
movements of conditions in the ocean, led to the temporary migration 
of the fauna over the banks it occupied, and to its later retreat and 
resumption of its old conditions. 
The tile-fish fauna may belong to the deeper seas under the Gulf 
Stream, or it may be connected with other currents that at present 
we are unfamiliar with. However, this immigration may be taken as 
an example of what has unmistakably taken place over and over 
again in the sea basins whose life records are preserved in the fossils 
of our stratified rocks. Of course the modification of species in 
the course of time would affect such species as lived in a continuous 
series of reproductions for millions of years; such modifications, 
however, might be spoken of as purely evolutional. Paleontology 
gives us evidence of such modifications of a general kind in the 
character of the species of a genus coincident with the passage of 
time; i. e., a young stage, a vigorous middle stage of the life history, 
and a final decadent stage of the life of the genus. Facts of this 
kind may be gathered from the study of faunas which have preserved 
their integrity through a great thickness of sediments in a single 
basin; but the conditions more important to the paleontologist, and 
more necessary to be observed in making correlations, are those 
directly coincident with the movement of faunas from place to place; 
i. e., the shifting of faunas. This shifting of faunas is well illus- 
trated in the history of the latter part of the Paleozoic formations in 
the central basin of North America. The general proposition assumed 
to explain such shifting of the geographical position of faunas and 
their containing formations, as we follow them successively through 
a geological section, is as follows : 
It is assumed, first, that the evolutional process of change is geolog- 
ically very slow in its effects; that so long as the same conditions pre- 
vail with sufficient exactness to prevent the disturbance of the biolog- 
ical equilibrium of a fauna, so long the individual species will retain 
their distinctive characters and relative abundance in a general fauna. 
«Aru. Jour. Sci., 3d series, Vol. XXIV, p. 3(«5. 
