306 FORMS OF LIFE CHANGING Chap. X. 



havinfi^ been formed by dominant species spreading widely and 

 varying; the new species thus produced being themselves dom- 

 inant, owing to their having had some advantage over their 

 already dominant parents, as well as over other species, ana 

 again spreading, varying, and producing new forms. The old 

 forms which are beaten and Avhicli yield their places to the new 

 and victorious forms, will generally be allied in groups, from 

 inheriting some inferiority in common ; and therefore, as new 

 and imjjroved groups spread throughout the world, old groups 

 disappear from the world ; and the succession of forms every- 

 where tends to corres])ond 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 that blank intervals of vast dura- 

 tion, as far as fossils are concerned, occurred during the periods 

 when the bed of the sea was cither stationary or rising, and 

 likewise when sediment was not thrown down quiclcly enough 

 to embed and preserve organic remains. During these long 

 and blank intervals I suppose that the inhabitants of each re- 

 gion underwent a considerable amount of modification and ex- 

 tinction, and that there was much migration from other parts 

 of the world. As we have reason to believe that 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 invari- 

 ably been the ease, and that large areas have invariably been 

 affected by the same movements. Wlien two formations have 

 been deposited in two regions during nearly, but not exactly, 

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

 plained in the foregoing paragraphs, the same general succes- 

 sion in the forms of life ; but the species would not exactly 

 correspond ; for there will have been a little jnore time in the 

 one region than in the other for modification, extinction, and 

 immigration. 



I suspect that cases of this nature occur in Europe. Mr. 

 Prestwich, in his admirable memoirs on the eocene dejiosits of 

 England and France, is able to draw a close general parallelism 

 between the successive stages in the two countries ; but when 

 he compares certain stages in England with those in France, 

 although he finds in both a curious accordance in the numbers 



