348 THE LAW 0F EVOLUTION CONTINUED. 



ones, and whether the Earth's present Flora and Fauna are 

 more heterogeneous than the Flora and Fauna of the past, — 

 we find the evidence so fragmentary, that every conclusion 

 is open to dispute. Two-thirds of the Earth's surface being 

 covered by water; a great part of the exposed land being in- 

 accessible to, or untra veiled by, the geologist; the greater 

 part of the remainder having been scarcely more than 

 glanced at; and even the most familiar portions, as England, 

 having been so imperfectly explored, that a new series of 

 strata has been added within these few years, — it is mani- 

 festly impossible for us to say with any certainty what crea- 

 tures have, and what have not, existed at any particular 

 period. Considering the perishable nature of many of the 

 lower organic forms, the metamorphosis of many sedimen- 

 tary strata, and the gaps that occur among the rest, we shall 

 see further reason for distrusting our deductions. On the 

 one hand, the repeated discovery of vertebrate remains in 

 strata previously supposed to contain none, — of reptiles 

 where only fish were thought to exist, — of mammals where 

 it was believed there were no creatures higher than rep- 

 tiles; renders it daily more manifest how small is the value 

 of negative evidence. On the other hand, the worthless- 

 ness of the assumption that we have discovered the earliest, 

 or anything like the earliest, organic remains, is becoming 

 equally clear. That the oldest known aqueous formations 

 have been greatly changed by igneous action, and that still 

 older ones have been totally transformed by it, is becoming 

 undeniable. And the fact that sedimentary strata earlier 

 than any we know, have been melted up, being admitted, it 

 must also be admitted that we cannot say how far back in 

 time this destruction of sedimentary strata has been going 

 on. Thus it is manifested that the title Palceozoic, as applied 

 to the earliest known fossiliferous strata, involves apetitio 

 principii ; and that, for aught we know to the contrary, only 

 the last few chapters of the Earth's biological history may 

 have come down to us. 



