282 FORMS OF LIFE CHANGING, [CHAP. XL 



fossils are concerned, occurred during the periods when the bed of 

 the sea was either stationary or rising, and likewise when sediment 

 was not thrown down quickly enough to embed and preserve 

 organic remains. During these long and blank intervals 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 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 invariably been the case, and that large areas have in- 

 variably been affected by the same movements. When two forma- 

 tions have been deposited 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 Europe. Mr. Prest- 

 wich, in his admirable Memoirs on the eocene deposits 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 of the species belong- 

 ing to the same genera, yet the species themselves differ in a 

 manner very difficult to account for considering the proximity of 

 the two areas, unless, indeed, it be assumed that an isthmus 

 separated two seas inhabited by distinct, but contemporaneous, 

 faunas. Lyell has made similar observations on some of the later 

 tertiary formations. Barrande, also, shows that there is a striking 

 general parallelism in the successive Silurian deposits of Bohemia 

 and Scandinavia; nevertheless he finds a surprising amount of 

 difference in the species. If the several formations in these regions 

 have not been deposited during the same exact periods, a forma- 

 tion in one region often corresponding with a blank interval in the 

 other, and if in both regions the species have gone on slowly 

 changing during the accumulation of the several formations and 

 during the long intervals of time between them ; in this case the 

 several formations in the two regions could be arranged in the 

 same order, in accordance with the general succession of the forms 

 of life, and the order would falsely appear to be strictly parallel ; 

 nevertheless the species would not be all the same in the apparently 

 corresponding stages in the two regions. 



