8 PRINCIPLES OF PALZONTOLOGY. 
of all inquiries, both in the domain of geology and that of 
paleontology. The advocates of continuity possess one im- 
mense advantage over those who believe in violent and revo- 
lutionary convulsions, that they call into play only agencies 
of which we have actual knowledge. We now that certain 
forces are now at work, producing certain modifications in the 
present condition of the globe; and we £zow that these forces 
are capable of producing the vastest of the changes which 
geology brings under our consideration, provided we assign a 
time proportionately vast for their operation. On the other 
hand, the advocates of catastrophism, to make good their 
views, are compelled to invoke forces and actions, both de- 
structive and restorative, of which we have, and can have, no 
direct knowledge. They endow the whirlwind and the earth- 
quake, the central fire and the rain from heaven, with powers 
as mighty as ever imagined in fable, and they build up the 
fragments of a repeatedly shattered world by the intervention 
of an intermittently active creative power. 
It should not be forgotten, however, that from one point of 
view there is a truth in catastrophism which is sometimes 
overlooked by the advocates of continuity and uniformity. 
Catastrophism has, as its essential feature, the proposition that 
the known and existing forces of the earth at one time acted 
with much greater intensity and violence than they do at pre- 
sent, and they carry down the »period of this excessive action 
to the commencement of the present terrestrial order. The 
Uniformitarians, in effect, deny this proposition, at any rate as 
regards any period of the earth’s history of which we have 
actual cognisance. If, however, the “nebular hypothesis” of 
the origin of the universe be well founded—as is generally ad- 
mitted—then, beyond question, the earth is a gradually cooling 
body, which has at one time been very much hotter than it is 
at present. There has been a time, therefore, in which the 
igneous forces of the earth, to which we owe the phenomena of 
earthquakes and volcanoes, must have been far more intensely 
active than we can conceive of from anything that we can see 
at the present day. By the same hypothesis, the sun is a 
cooling body, and must at one time have possessed a much 
higher temperature than it has at present. But increased heat 
of the sun would seriously alter the existing conditions affect- 
ing the evaporation and precipitation of moisture on our earth ; 
and hence the aqueous forces may also have acted at one time 
more powerfully than they do now. The fundamental prin- 
ciple of catastrophism is, therefore, not wholly vicious; and 
we have reason to think that there must have been periods— 
