310 



UNIFORMITY OF CHANGE 



[Ch. XIV. 



mud 



gypseous 



far west/ The waters of the Uruguay, says Darwin, draining 

 a granitic country, are clear and "black, those of the Parana, 



red.* 



mud 



is of a clayey hue, that of the Chenab, on the other hand, is 

 reddish, that of the Sutlej is more pale.f The same causes 

 which make these several rivers, sometimes situated at no 

 great distance the one from the other, to differ greatly in the 

 character of their sediment, will make the waters draining 

 the same country at different epochs, especially before and 

 after great revolutions in physical geography, to be entirely 

 dissimilar. It is scarcely necessary to add, that marine cur- 

 rents will be affected in an analogous maimer in consequence 

 of the formation of new shoals, the emergence of new islands, 

 the subsidence of others, the gradual waste of neighbouring 

 coasts, the growth of new deltas, the increase of coral reefs, 

 volcanic eruptions, and other changes. 



Uniformity of change considered, secondly, in reference to the 

 living creation. — Secondly, in regard to the vicissitudes of the 

 living creation, all are agreed that the successive groups of 

 sedimentary strata found in the earth's crust are not only 

 dissimilar in mineral composition for reasons above alluded 

 to, but are likewise distinguishable from each other by theii 



organic remains. 



study and comparison of the various groups 

 chronological order, is this : that at successive periods, distinct 

 tribes of animals and plants have inhabited the land and 

 waters, and that the organic types of the newer formations 



The general inference drawn from 



arranged in 



more 



& 



more 



If we then turn to the present state of 



animate 



contra 



in a state of continual flux — that there are many causes in 

 action which tend to the extinction of species, and which 



are conclusive 

 durability. 



There are also causes which give rise to new varieties and 



against 



the doctrine of their unlimited 



* Darwin's Journal, p. 163, 2nd. 

 ed. p. 139. 



f Jonrn. Key. Geograph. Soc., vol 

 iii. p. 142. 





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