30 Geological Extinction and | [ January, 
of evolution, and one cannot in these days feel satisfied that the 
solution of the problem lies in the theory of natural selection, 
which accounts for the preservation of species rather than their 
origin or extinction. Mr. Darwin having been the means of 
bringing many naturalists to believe in the theory of descent, they 
are not to lie supinely on their backs, resting securely on the 
dogma of natural selection and cease from all attempts to inves- 
tigate the cause of evolution, to cease building a foundation for 
the working of natural selection. On the contrary the search for 
the causes of the transformation of species will be carried on with 
more energy, thoroughness and success. The clews which we 
already have discovered will be followed up, and finally we shall, 
by means of observation in the field and experiment in the labor- 
atory, wrest from nature the secrets of life and its origin, and of 
the phenomena of death and extinction, not only of species but of 
orders. 
Extinction may be both slow and rapid, the causes of each be- 
ing primarily dependent on slow or rapid changes in the environ- 
ment. The object of this article is to endeavor to show that the 
extinction of species is intimately connected with geological 
changes. It is a meagre sketch or outline of the more salient 
facts and laws which appear to us to immediately bear upon this 
intricate and difficult subject. 
_ In Paleozoic times the climate of the globe was far more uni- — 
form than now. The continents of the present day were then of — 
much less extent, in fact archipelagoes rather than continents; 
the land-surfaces were of moderate height, and mountain ranges _ 
smaller and lower. While the land masses or embryo continents — 
of the eastern and western hemispheres were more or less inde- — 
pendent centers of evolutionary creation, there was on the whole — 
a great uniformity of plant and animal life. Species were per 2 
more cosmopolitan than now. 
When at the end of the Coal period the process of ~ontingel a 
making and mountain-building became greatly accelerated, result- 
ing in that stupendous crisis in American geological history, the _ 
upheaval of the Appalachian mountain system, there must have 
been some degree of differentiation or setting aside of portions of : 1 
the then continent into distinct areas or basins bounded by moun- 4 
tains, inland seas and rivers, and some slight subdivision into 
local faunas. We know that there was a distinct coal basin, or : : 
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