162 
ELEMENTS OF GEOLOGY. 
man; and we have thus the concurrent testimony of several 
classes of geological facts to the vast antiquity of the human 
race. In the first place, the disappearance of a great variety 
of species of wild animals from every part of a wide conti¬ 
nent must have required a vast period for its accomplish¬ 
ment ; yet this took place while man existed upon the earth, 
and was completed before that early period when the Dan¬ 
ish shell-mounds were formed or the oldest of the Swiss lake- 
dwellings constructed. Secondly, the deepening and widen¬ 
ing of valleys, indicated by the position of the river gravels 
at various heights, implies an amount of change of which 
that which has occurred during the historical period forms a 
scarcely perceptible part. Thirdly, the change in the course 
of rivers which once flowed through caves now removed 
from any line of drainage, and the formation of solid floors 
of stalagmite, must have required a great lapse of time. 
Lastly, ages must have been required to change the climate 
of wide regions to such an extent as completely to alter the ge¬ 
ographical distribution of many mammalia as well as land and 
fresh-water shells. The 3000 or 4000 years of the historical 
period does not furnish us with any appreciable measure for 
calculating the number of centuries which would suffice for 
such a series of changes, which are by no means of a local char¬ 
acter, but have operated over a considerable part of Europe. 
Eelative Longevity of Species in the Mammalia and Testacea. 
—I called attention in 1830"^ to the fact, which had not at 
that time attracted notice, that the association in the Post¬ 
pliocene deposits of shells,"exclusively of living species, with 
many extinct quadrupeds betokened a longevity of species 
in the testacea far exceeding that in the mammalia. Subse¬ 
quent researches seem to show that this greater duration of 
the same specific forms in the class mollusca is dependent on 
a still more general law, namely, that the lower the grade of 
animals, or the greater the-simplicity of their structure, the 
more persistent are they in general in their specific charac¬ 
ters throughout vast periods of time, l^ot only have the in- 
vertebrata, as shown by geological data, altered at a less rap¬ 
id rate than the vertebrata, but if we take one of the classes 
of the former, as for example the mollusca, we find those of 
more simple structure to have varied at a slower rate than 
those of a higher and more complex organization; the brach- 
iopoda, for example, more slowly than the lamellibranchiate 
bivalves, while the latter have been more persistent than the 
univalves, whether gasteropoda or cephalopoda. In like man¬ 
ner the specific identity of the characters of the foraminifera, 
* Principles of Geology, 1st ed., vol. iii., p. 140. 
