8 PRINCIPLES OF PALEONTOLOGY. 



a greater or less extent, the lineal descendant of the group which 

 immediately preceded it in point of time, and is more or less 

 fully concerned with giving origin to the group which immedi- 

 ately follows it. That this law of " evolution " has prevailed 

 to a great extent is quite certain ; but it does not meet all the 

 exigencies of the case, and it is probable that its action has been 

 supplemented by some still unknown law of a different character. 



We shall have to consider the question of geological "con- 

 tinuity" again. In the meanwhile, it is sufficient to state that 

 this doctrine is now almost universally accepted as the basis of 

 all inquiries, both in the domain of geology and that of palaeon- 

 tology. The advocates of continuity possess one of immense 

 advantage over those who believe in violent and revolutionary 

 convulsions, that they call into play only agencies of which we 

 have actual knowledge. We know that certain forces are now 

 at work, producing certain modifications in the present condi- 

 tion of the globe ; and we know 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 destructive and restorative, of which we 

 have, and can have, no direct knowledge. They endow the 

 whirlwind and the earthquake, 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 over- 

 looked by the advocates of continuity and uniformity. Catas- 

 trophism 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 present, 

 and they carry down the period of this excessive action to the 

 commencement of the present terrestrial order. The Uniformi- 

 tarians, in effect, deny this proposition, at any rate as regards 

 any period of the earth's history of which we have actual cog- 

 nizance. If, however, the " nebular hypothesis " of the origin 

 of the universe be well founded as is generally admitted 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 



