Paleontological Speculations. — Gratacap. 93 
ger forms, and in the Upper Silurian develop into the large 
and internally elaborated Trimerellidae. 
The size and skeletal mass of the inarticulate brachiopods 
reached its maximum in the Upper Silurian. The articulate 
brachiopods show increasing size, and increasing internal 
shelly deposits (loops, hinges, cardinal areas, etc.,) from the 
Cambro-Silurian upward. The Rhynchonellidse, Orthidse, 
Strophomenidae gradually yielded their numerical supremacy, 
while advancing themselves greatly in size. They become di- 
minished elements in the fauna of the upper rocks, as they be- 
come more numerously accompanied by the spiriferoids which 
attain large dimensions in the Devonian and Carboniferous 
and exemplify the extension of interior calcareous appendages 
and hinge growth. In the Carboniferous (lower) the families 
of Orthidae and Stropheodontidae also attain large size. The 
lamellibranchs increased in size continuously to the Devonian, 
and through the latter age became conspicuously developed. 
The gasteropods attained large proportions in the Devonian. 
The cephalopods were, in the straight forms, strongly devel- 
oped quite early in the series and through the coiled genera 
continued their growth and accretion to the Jurassic, while in 
modern seas they have attained huge proportions, but without 
shelly coverings. 
The crinoids were slender, small calyxed, long armed, in 
the Lower Silurian, and through manifold divergences, with 
sporadic and sudden ofifshoots reached the thick, heavily 
plated, short armed, stout pedicelled groups of the Devonian 
and Carboniferous. 
When the separate genera or families are examined this 
progression of size and skeletal mass to certain maximum 
points, followed by an apparent decrescence immediately or 
subsequently introduced, is very striking. Its theoretical inter- 
est lies in the impossibility of referring this universal tendency 
in molar character to natural selection, or any series of influ- 
ences purely external to the organism. 
The considerations which warrant this conclusion are not 
so much drawn from particular instances as from the general 
tendency throughout these animal fossil forms. That size, 
stronger, larger shells, internal shelly partitions, surfaces of 
attachment, hinge definition and interior branchial elaborations 
