8 PROGRESS : ITS LAW AND CAUSE. 



ing been so imperfectly explored that a new series of strata 

 has been added within these four years, — it is manifestly 

 impossible for us to say with any certainty what creatures 

 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 sedimentary 

 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 worthlessness of the assumption 

 that we have discovered the earliest, or anything like the 

 earliest, organic remains, is becoming equally clear. That 

 the oldest known sedimentary rocks 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 manifest that the title, Palmozoic^ as applied to the 

 earliest known fossiliferous strata, involves ?i ijetitio princi' 

 pli y 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. On neither side, therefore, is the evi- 

 dence conclusive. Nevertheless we cannot but think that, 

 scanty as they are, the facts, taken altogether, tend to show 

 both that the more heterogeneous organisms have been 

 evolved in the later geologic periods, and that Life in 

 general has been more heterogeneously manifested as time 

 has advanced. Let us cite, in illustration, the one case of 



