GEOLOGY AND EVOLUTION. 197 
simple muscle which binds them together, are the characteristics. They are 
found now living in nearly all parts of shoal ocean, outside of the Frigid Zones. 
But a short notice of a simple species will better illustrate the stable nature of this 
genus. Take Ostrea Congesta. It lived through nearly the whole of the Creta- 
ceous Age. It occurs on both sides of the Rocky Mountains over wide areas. 
It is collected in Kansas over ten thousand square miles, and I have seen fifty thou- 
sand on a square acre. While there is in shape as great a variety in any one 
thousand, as in the living Virginia oyster, still no greater variance can be traced 
in specimens from the lowest to the highest geological horizon, or from the beds 
in Texas, New Mexico, or Nebraska. Where so little change occurs, and such 
constancy of species is seen, what support is there for Lamarck’s conjecture that 
man may have sprung from an oyster? Yet oysters have been known to vary, at 
least in size, by varying circumstances. A most interesting and remarkable illus- 
tration, on a large scale, of the effect of a change of circumstances, in controlling 
the vitality of Mollusks, has taken place in the north of Europe. 
When man in the old stone age first dwelt on the shores of the Baltic Sea, it 
was a large open bay of the ocean, covering nearly twice as many square miles 
as at present. It opened into the Arctic ocean on the north, Norway and 
Sweden being an island. Denmark was then represented by a few low islands, 
and the salt waters of the ocean had free access. The Baltic was then in reality 
a part of the ocean, and animals which could live in one, existed, in the 
other. But early in man’s history this part of Europe began very slowly to rise, 
till its pre ent position is about two hundred feet higher than when it began to 
ascend. ‘This elevation united the islands of Denmark, forming that peninsula, 
and joined it to the continent. ‘The southern shores of Norway and Sweden 
encroached on the outlet, and the Baltic became a land-locked sea, smaller and 
less deep than before. Formerly, as now, it received the drainage of that por- 
tion of Europe; and those large rivers, Oder, Vistula, Duna, and others, carried 
so much fresh water into it, that animals which required water of the full ocean 
saltness, suffered, and finally disappeared. As the geographical change was very 
gradual, so also was the change in the Mollusks. The common oyster (ostrea 
edulis), now so abundant in the ocean on the shores of the North Sea, flourished 
equally well, before this elevation, on what is now the shores of the Baltic. By 
the quantity of fresh water poured into this inland sea, these oysters became 
stunted and dwarfed to one-third of their natural size, then diminished in num- 
bers and became locally extinct. The whole history of this interesting change in 
animal life can be studied in the remains of the ‘‘kitchen middens,” or ‘‘ refuse 
heaps,” on the coast of Sweden. ‘There the old pre-historic inhabitants at first 
ate the full sized oyster, and continued to use it as an article of food till it disap- 
peared, near the close of the elevation, and then were obliged to procure a sub- 
stitute. This process of local extermination occupied many thousands of years. 
During all this dwarfing in the oyster, it lost none of its characteristics as a 
species, and showed no tendency to diverge into any other variety, but simply 
