1 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



steadily, and will shortly disappear. I need not argue it further 

 than to indicate the cause of its deserved decline. 



The modern school of geology, as I have this day argued, more 

 and more leans to the opinion that the series of formations is any- 

 thing but complete, and that many links in the chain of evidence are 

 missing. This belief helped to establish the idea that individual 

 species have died out and been gradually replaced, as slowly as species 

 are now dying out and now coming in, not only in epochs represented 

 by the various known formations, but also during those periods of 

 unrepresented time witnessed by unconformities. This belief equally 

 applies to the old idea of a special creation for each species, or to 

 the views so clearly urged by Darwin ; but it seems to me to have a 

 special significance if we believe, with him, that species acted on by 

 various influences pass slowly from one form into another, and there- 

 fore that partial or complete diversity of fossils in any two forma- 

 tions generally marks a greater or less interval of time. 



If we now, by way of test, pass the Silurian formations in review, it 

 is evident that they were deep-sea deposits, with the exception per- 

 haps of part of the Llandovery beds, which were formed during re- 

 peated oscillations of level. The Upper Llandovery beds especially 

 were, in part, formed so near shore that, in the Longmynd country, 

 they bear almost the characters of a beach. From a consideration of 

 these circumstances the question easily arises, What relation is there 

 between the absolute length of time occupied during the deposition of 

 any one of the greater Silurian formations and the time that elapsed 

 between its close and the commencement of the formation next in 

 succession ? Take, for instance, the Caradoc or Bala beds, in which 

 the comparative uniformity of the fauna from bottom to top shows 

 that the strata were deposited under nearly uniform conditions in a 

 period of slow and steady depression of the sea-bottom. But where 

 such conditions were seriously interrupted, we find either direct 

 evidence of unconformity or strong presumptive evidence of a break 

 of that nature — phenomena implying a great change of physical 

 conditions and a long lapse of time. Supposing, therefore, Mr. 

 Darwin's hypothesis to be correct, it may be asked whether, under 

 the changing conditions coincident with disturbance of strata, there 

 may not have been influences at work that entailed a more rapid 

 development of new species out of old, and of old species into new 

 genera, than those that existed during an epoch when the conditions 

 in a given area remained comparatively unchanged. The notion is 

 simply this : — A change in the relative distribution of sea and land 

 took place, so great perhaps that the creatures that inhabited one 

 area were driven slowly from stage to stage into other latitudes, so 

 that, under circumstances which varied with comparative rapidity, if 

 it so happened that their descendants (mingled probably with species 

 from other stocks) returned into the same area, their forms had 

 changed entirely or in part. Or, in another possible case, frequent 

 oscillation of level produced such frequent changes of condition that, 

 in the end, a like result came about, the intermediate stratigraphical 

 stages being lost. 



