OBJECTIONS A GA INS T E VOL UTION. 1 65 



accentuated. Such being the case, the number of 

 gradational forms will be far less numerous than the 

 forms contained in the species which persist with 

 little or no modifications during long cycles of time. 

 Furthermore, it is now generally admitted that 

 the strata which are richest in fossils were usually, if 

 not always, deposited during eras which were least 

 favorable for the development of transitional forms, 

 that is, during eras when variation and extinction 

 were least rapid. On the theory that natural selec- 

 tion has been the dominant factor in Evolution ; on 

 the theory, namely, that progress has resulted solely, 

 or at least chiefly, in consequence of the accumula- 

 tion of infinitesimal increments, a condition of things 

 must have existed during the formation of fossilifer- 

 ous strata, which it is certain could have obtained 

 only at extremely rare intervals. For, as Darwin 

 points out : " In order to get a perfect gradation be- 

 tween two forms in the upper and lower parts of the 

 same formation, the deposit must have gone on con- 

 tinuously accumulating during a long period suffi- 

 cient for the slow process of modification ; hence 

 the deposit must be a very thick one, and the spe- 

 cies undergoing change must have lived in the same 

 districts throughout the whole time. But we have 

 seen that a thick formation, fossiliferous throughout 

 its entire thickness, can accumulate only during a 

 period of subsidence ; and to keep the depth approxi- 

 mately the same, which is necessary that the same 

 marine species may live on the same space, the sup- 

 ply of sediment must nearly counterbalance the 

 amount of subsidence. But this same movement of 



