LIFE ON THE EARTH. 25 



as truths generally admitted. Here in Cambridge, 

 I know that these and other great results of the 

 contemplation of nature are well comprehended, 

 since you have all long had the advantage of hearing 

 them from the eloquent lips of the noblest of English 

 Geologists, your pride and my pride Professor 

 Sedgwick. Assuming, then, definite limits and con- 

 ditions for every living form ; a definite local origin, 

 and a longc-ontinued permanence of structure and 

 habits for each species of plant and animal, I may 

 now turn to the appointed purpose of this lecture, 

 and trace the history of the changes of life in the 

 ancient lands and seas. 



TYPES OF LIFE STRUCTURE. 



One of the most arduous of all the enterprises of 

 science is the attempt to classify the variously allied 

 objects of organic and inorganic nature, according 

 to their prevalent structures and qualities to place 

 them in such relations to each other as they really 

 have to represent, in short, the plan of these parts 

 of creation according to the leading ideas of which 

 it is, or seems to us to be, the expression. Not that 

 so great a purpose was at first conceived by Aristotle 

 and his followers, or even by Linnseus, or Cuvier ; it 

 was at first intended to group together things which 

 resembled each other, as crystals herbs trees; 

 residents in the water animals of the land. Pro- 



