192 THE LAW. 



likely depend on forces purely astronomical, are questions 

 that lie beyond the scope of the present Sketch. Enough 

 for our purpose to have indicated the probability of such 

 recurrences, and derive therefrom the conclusion that the 

 conditions of life have been very much the same through all 

 geological periods successively varying in different areas, 

 but never presenting, any more than they do now, a uni- 

 versal similitude, and that least of all through the influ- 

 ence of the earth's internal temperature. 



[Introduction of New Life-form?.] 



As each geological epoch is characterised by its own pe- 

 culiar plants and animals, the question naturally arises, 

 Whether these are independent creations, or whether there 

 is in nature some law of development by which, during the 

 lapse of ages and under the change of physical conditions, 

 the lower may not be developed into the higher species, 

 and the simpler into the more complex? On this topic 

 much has been said and written, but after all, geology is 

 not in a position to solve the problem of vital gradation 

 and progress. It cannot tell, for instance, why trilobites 

 should have flourished so profusely during the silurian 

 epoch and died out before the deposition of the oolite ; why 

 chambered cephalopods, like the ammonite, should have 

 come to their meridian, as it were, during the liassic era; 

 reptilian life during the oolite and chalk ; or why mamma- 

 lian development should have been reserved to the tertiary 

 and current epochs. It cannot explain why the palaeo- 

 therium should not continue to inhabit the same forest 

 with the tapir of South America, or the ichthyosaurus 

 gambol in the same waters with the alligator of the Ama- 

 zon. It can discover no physical condition in the oolitic 



