Paleontological Speculations. — Gratacap. 231 
ilarity in the fauna of the Marcellus and Genessee despite 
specific or generic contrasts and the diversity of those in the 
latter may be regarded as the result of changes induced in the 
former during some period of sequestration and absence. 
Where the Alarcellus fauna disappeared to it is impossible 
at present to determine, but its reappearance as the Genessee 
is not an unnatural supposition. The lithological resemblance 
of the ]\Jarcellus and Genessee is striking and would seem to 
represent similar or identical sediments. The fauna in each 
is marked by cephalopodous features and while the lamelli- 
branchs of the Marcellus which remained within it from the 
Upper Helderberg seas are absent in the Genessee, the brach- 
iopodous occupants of both are unmistakably related, and the 
ichthyic fauna of both suggest still further a biological affinity 
or descent. 
If the specific changes evinced in the Chemung bivalves 
are traceable to the confinement of the Hamilton in limited 
zones of development and changed environment, upon con- 
tinental elevation and its (the fauna's) recession to the mar- 
gins of the deeper seas, then it is logical to reverse the deduc- 
tion in direction and find in the expanded (?) Genessee fauna 
a growth and change produced by the retreat of the Marcellus 
to the interior edges of the continent upon the invasion of the 
deeper waters of the Hamilton. 
In basins, bays, and inundated half land-locked emargin- 
ations, the Marcellus sea-life, and especially its ammonoid 
contents, developed in congested centers, and prepared the 
new efflux of species and genera later preserved in the Gen- 
essee shales. 
These suggestions point to the seeming efficacy of circum- 
scribed areas in the development of the species ; that a certain 
amount of biological pressure brings about interactions, per- 
haps not always referable to struggle or natural selection, 
which modify form and create species.. And yet aside from 
natural selection what influence in the proximity of species 
or individuals could possibly dift'erentiate them? Is not such 
an expression as "biological pressure" both ambiguous and 
childish? It might more properly be assumed that an excess 
of nutrition, perfect thermal conditions, purity of water, erad- 
ication or absence of enemies brought about the highest var- 
