THROUOHOUT THE WOULD. 355 



spreading widely and varying; the new species thus pro- 

 duced being themselves dominant, owing to their having 

 had some advantage over their already dominant pan-rits, 

 as well as over other species, and again spreading, varying! 

 and producing new forms. The old forms wliich are 

 beaten and which yield their places to the new and victori- 

 '^us forms, will generally be allied in groups, from- inherit- 

 ing some inferiority in common; and, therefore, as new 

 and improved groups spread throughout the world, oKl 

 groups disappear from the world; and the succession of 

 forms everywhere tends to correspond both in their first 

 appearance and final disappearance. 



There is one other remark connected with this subject 

 worth making. I have given my reasons for believing 

 that most of our great formations, rich in fossils, were 

 deposited during periods of subsidence ; and tliat blank 

 intervals of vast duration, as far as fossils are concerned, 

 occurred during the periods wlien the bed of the sea was 

 either stationary or rising, and likewise wlien sediment 

 was not thrown down quickly enough to imbed and pre- 

 serve organic remains. During these long and blank inter- 

 vals I suppose that the inhabitants of each region 

 underwent a considerable amount of modification and 

 extinction, and that there was much migration from other 

 parts of the world. As we have reason to believe tliat large 

 areas are affected by the same movement, it is probable 

 that strictly contemporaneous formations have often been 

 accumulated over very wide spaces in the same quarter 

 of the world ; but we are very far from having any 

 right to conclude that this has invariably been the case, 

 and that large areas have invariably been affected by the 

 same movements. When two formations have been depos- 

 ited in two regions during nearly, but not exactly, the 

 same period, we should find in both, from the causes 

 explained in the foregoing paragraphs, the same general 

 succession in the forms of life ; but the species would not 

 exactly correspond ; for there will have been a little more 

 time in the one region than in the other for modification, 

 extinction and immigration. 



I suspect that cases of this nature occur in Eurojie. 

 Mr. Prestwich, in his admirable .Memoirs on the eocene 

 deposits of England and France, is able to draw a close 



