THROUGHOUT THE WORLD. 333 



as we do find, a less strict degree of parallelism in the suc- 

 cession of the productions of the land than with those of 

 the sea. 



Thus, as it seems to me, the parallel, and, taken in a large 

 sense, simultaneous, succession of the same forms of life 

 throughout the world, accords well with the principle of new 

 species having been formed by dominant species spreading 

 widely and varying ; the new species thus produced being 

 themselves dominant, owing to their having had some ad- 

 vantage over their already dominant parents, as well as over 

 other species, and again spreading, varying, and producing 

 new forms. The old forms which are beaten and which yield 

 their places to the new and victorious forms, will generally 

 be allied in groups, from inheriting some inferiority in com- 

 mon ; and, therefore, as new and improved groups spread 

 throughout the world, old 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 that blank 

 intervals of vast duration, as far as 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 invari- 

 ably been affected by the same movements. When two for- 

 mations 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 th§ Q$\$V fof 

 wodi^cation, extinction, and immigration^ 



