LECTURE XVI. 451 



but will have its characters more fixed. It hence is explained 

 that the old species^ the relics of which we find in the alluvium, 

 and the species which existed 5,000 years ago in Egypt, 

 the mummies of which we find in the graves, have not changed 

 since that period, but represent the same types now as then. 

 Just as there are species which in the present creation are 

 scattered over a considerable tract of inhabitable chmates, 

 and which for their acclimatization require only insignificant 

 modifications, so are there types, which during geological 

 changes have remained unaltered till the present period. The 

 genus Lingula has thus been pointed out, as having remained 

 unchanged since the deposition of the oldest Silurian beds to 

 the present period, and as being represented in almost all 

 strata by several species differing but little from each other. 

 Here we find a remarkable constancy, but I cannot look upon 

 this as a proof against Darwin^s theory. The error is con- 

 stantly committed of applying the conditions found in some 

 species to the whole animal kingdom, and forcing it into a 

 strait jacket which the variety in nature does not tolerate. 

 Though the mutability and adaptation to external conditions 

 be a possibility, it is no absolute necessity for all types, as 

 little as the number of these changes, requisite for adaptation, 

 is the same in all types. We know that some species cannot 

 be acclimatized at all, others are easily acclimatized : some 

 cannot undergo the slightest change without perishing, while 

 others experience important alterations before they become 

 adapted to the new conditions. And so, in the history of the 

 earth, there must necessarily exist similar differences, in- 

 asmuch as certain species and types last only short periods 

 and then perish ; others keep pace with the changes of sur- 

 rounding media, and undergo comparatively great alterations. 

 Others, again, require only insignificant modifications for their 

 preservation. 



Although the changes which we now observe in creation are 

 inconsiderable and insignificant, we must not forget that the 

 history of the earth extends over a series of ages of which 

 we have no conception, and that this eternity (for we cannot 

 term it otherwise) comprises an immense series of changes, 



G G 2 



