112 GENERAL EVOLUTION". 



of acceleration and retardation be true, for in accordance with it, 

 at certain times, Mice does not produce Uhe. 



V. OF EPOCHAL KELATIONS, OE THOSE MEASUEIKG GEOLOGIC 



TIME. 



If it can be shown that groups having the developmental rela- 

 tion above insisted on are contemporaries, and if it can be shown 

 that this relation is identical in kind with that which we regard 

 as measuring the successions of geologic time, we will be led to 

 doubt the existence of any very great interruptions in the course 

 pf this succession throughout geologic time. And if we can show 

 that faunae so related are more or less characteristic of distinct por- 

 tions of the earth's surface, at the present time, we will be led to 

 anticipate that contemporaneous faunse in different regions, during 

 geologic j)eriods also, bore such a relation. If this proposition be 

 true, we are led to the further conclusion, which is at variance 

 with received canons, that identity of faunse proves successional 

 relation in time, instead of synchronism.* That this will ulti- 

 mately be demonstrated appears highly probable to the writer, 

 though, as yet, the evidence is but fragmentary. 



If the relations expressed under the terms homology and het- 

 erology, taken together with the observations on metamorphosis, 

 render it probable that a number of genera have reached their ex- 

 pression-points, or periods of metamorphosis, at near the same 

 time in geologic history, an important point has been gained. 

 If we can render it probable that a change in any organic charac- 

 ter has been nearly simultaneous throughout a large extent of 

 specific forms, the change becomes, on the latter account alone, 

 of higher than generic value, but characteristic of such groups as 

 Marsupialia, Clamatores, Acrodonta, Arcifera, Heterocerca, and 

 the like. 



We have here, also, an important element in the estimation of 

 the value of apparent interruptions in the geological liistory of 

 the life of the globe. These interruptions, it is true, are greater 

 than any such theory as the present can bridge over ; yet such a 

 theory, if true, lessens their importance. They are in any case 

 well accounted for on the theory of the existence of periods of 

 elevation, during which the life of a given region is necessarily 

 almost entirely lost to us, through lack of means of preservation 

 of their remains. 



* This view has been insisted on by Huxley. 



