GEOLOGY AND EVOLUTION. 199 
is distinguished from it by the smooth curved line of its partitions (not interlac- 
ing) with the siphon passing through them in the center, or nearly so. These char- 
acteristics the Nautilus retains during its geological life. It is claimed that the 
early species are more embryonic than the later, yet the normal plan is followed 
so closely in the five hundred species, that the real advance in organic structure 
is very slight. Most of the species show merely a diverse relation of parts with- 
out any relative advance in rank. Notwithstanding the range of earth’s condi- 
tions has been so extreme that nearly the whole of the Nautili have disappeared, 
and only three or four species are preserved in our oceans, yet the strange adher- 
ence to the normal structure has been such that they have never diverged to the 
Pteropods or Acephals on the one hand, or developed into a crustacean on the 
other. 
We might multiply facts to show the essential uniformity of Mollusks, but it 
is enough to state that Murchison, in his ‘‘Siluria,” gives the following existing 
genera, viz.: Avicula, Mytilis, Chiton, Natica, Patella, Trochus, Discina, Orbic- 
ula, Lingula, Rhynchonella, and Nautilus, as Silurian, which have continued 
through all geological formations, and are now living in our oceans. ‘The expres- 
sion of ‘‘ all-time genus’’* Nautilus, which Dana has given to one, may be con- 
sidered as applicable to all. 
The examples frequently quoted, of change of species, without apnsidestine 
that the genera to which they belong is permanently inflexible, is a strong argu- 
ment against evolution. Thus, Shaler, in the ‘‘ Geological Report of Kentucky 
for 1876,” has given a very careful and critical memoir on the question of the 
variation of the more flexible or protean species, with tables of measurements 
and illustrations by plates, in addition to detailed remarks. One of his examples 
(Orthis accidentals) shows a variation in the proportion of length to width of 
forty-five per cent. This would be a strong proof of a tendency to outgrow the 
normal characters of the sheli, did we not know that the genus Orthis began at 
the opening of the Silurian and ended with the close of the Carboniferous, cov- 
ering three-fourths of our geological time. This shows that while a species may 
be quite variable within its circle of vitality, that variability may be clearly 
restricted in generic features. 
A similar instance is seen in the fresh water /lanorbis. Hilgendorf has 
described a case, where he collected ten graduated varieties of Planorbis muitt- 
Sormis, from the beds of a deposit in Switzerland. Yet the genus /lanorbis has 
lived from the Jurassic to the present time. ‘The multiform variations of this 
species, like Ovthis occidentalis and many others, did not extend to the generic 
features. 
These cases of a tendency to vary in structure (protean species) are frequently 
quoted in proof of evolution. Although a species is variable, but the genus to 
which it belongs is unchanged during long geological eras, the real evidence must 
be considered as bearing against evolution. 
*Manual, p. 598. 
