210 



HABIT AND INTELLIGENCE. 



[chap. 



Imperfec- 

 tion of the 

 geological 

 record. 



Destruc- 

 tion of 

 fossils. 



Denuda- 

 tion. 



Metamor- 

 pliism. 



reply in his chapter on " the Imperfection of the Geological 

 Eecord." On this subject, as on that of Hybridism, it is 

 only necessary for me to state his conclusions in the barest 

 outline. 



Soft-bodied animals, like the naked worms and moUusca, 

 are not preserved at all ; they perish without leaving any 

 record of their existence. The same is mostly true of land 

 animals, though for a different reason. It is only in the 

 rarest cases that land animals can die under such circum- 

 stances as to be buried, and afterwards fossilized. As a rule, 

 it is only the hard parts of aquatic organisms that will be 

 fossilized and preserved ; and when they are so preserved, 

 the older the fossiliferous beds, the less will be the chance 

 of their preservation to our age. " The stir of the forces 

 whence issued the world " i is not quiet yet, and never has 

 been quiet. The deposition of new strata never ceases ; 

 and it must be remembered that, as the quantity of matter 

 in the world is unchangeable, if there is deposition going 

 on in one place, there must be an equivalent amount of 

 denudation somewhere else — that is to say, an equivalent 

 amount of destruction of old strata ; though we habitually 

 forget this, because we see the deposit, and do not see the 

 denudation. 



Besides the effect of denudation in destroying old fossili- 

 ferous beds, there is the effect of metamorphism — that is 

 to say, the chemical effect of heat. It was first pointed 

 out, I believe, by Sir John Herschel,^ that (granting, what 

 is unquestionable, the theory of the earth's central heat) 

 any deposition of strata must raise the temperature of the, 

 strata underneath, exactly in the same way that putting on 

 a warm coat raises the temperature of the skin. Such a 

 rise of temperature, if sufficiently great, will cause a meta- 

 morphic change in the strata so covered, and will destroy 

 their fossils. As metamorphism from this cause, as well as 

 from the intrusion of igneous rocks, is not an exceptional 

 but a normal action, and as the same is true of denudation, 



1 Matthew Arnold. 



2 See his letter to Sir C. Lyell, in the Appendix to Babbage's Ninth 

 Bridgewater Treatise. Darwin has maJe no use; of this argument. 



