Hi PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



Devonian, and a Carboniferous fauna might all exist in different areas 

 at the same time. Experience, as yet, affords no clue to the 

 rate of geographical transmission of groups of genera and species ; 

 but, in the case of a large fauna, such as that of the Caradoc or Bala 

 beds, I incline to think that the enormous thickness of the formation 

 represents a range in time probably vast enough within its own 

 limits to allow (where conditions were good) of transmission, with or 

 without some amount of modification, over very large areas, so that a 

 strong family likeness would exist in distant regions, and some of the 

 itrata in supposed contemporaneous formations would run so many 

 chances of being actual equivalents that the likelihood that none of 

 them are so is reduced to a minimum. Further, if the idea put by 

 Professor Huxley be just, it appears to me that in the piles of forma- 

 tions built up in Britain, on the Continent, and in America, the 

 chances are overwhelmingly strong that in each or in some one area 

 there might be a recurrent fauna, — which is not the case. To this I 

 attach great weight. 



Furthermore, many considerations, partly stated, lead me to suspect 

 that we must look to the lapse of time unrepresented by strata, 

 as the chief cause, or, rather, as the necessary accompaniment, of the 

 influences that produced the great difference in species between any 

 two formations one of which lies unconformably on the other, 

 whether we adopt the old view of gradual extinction and replace- 

 ment by special creation, or Mr. Darwin's more philosophical argu- 

 ment of descent with modification. In other words, believing that the 

 causes that produced physical changes were much the same in former 

 times as now, both in kind and intensity (speaking generally, when 

 spread over long epochs), then the upheaving, contorting, and disloca- 

 tion of the strata, and the vast denudations they underwent before re- 

 submergence, generally represents a period of time longer than that 

 occupied respectively by the deposition of the formation disturbed, or 

 of that which overlies it unconformably. 



In the present state of knowledge these things cannot be proved, 

 but we may strongly suspect them to be probably true ; and if they 

 are so, then it follows that the periods of time stratigraphically un- 

 represented during the Palaeozoic epoch were much longer than those 

 of which the various formations of that epoch bear witness ; and I throw 

 out these suggestions in the hope that as data accumulate, and thought 

 is expended, a true solution of the question may be arrived at. 



It was my intention when I began this Address to have gone over 

 the evidence of the same kind that applies to the Secondary and 

 Tertiary formations, some of the problems to be solved there present- 

 ing difficulties that do not occur in the Palaeozoic rocks of Britain. 

 Time, however, will not permit me to do so ; but, if I find that what I 

 have now said is not unacceptable, I may return to the subject on a 

 future occasion. 



