EXERCISES. 65 



suggest any definite exercises, further than the mode already 

 pointed out, of self-examination by converting the statements 

 of the previous chapter into questions, and returning the 

 appropriate answers. 



6. The tropical climate of the ancient earth, the changes 

 in the types of animal and vegetable life consequent on 

 its refrigeration, and the nature and effects of modern 

 causes, are, in like manner, facts respecting which no exact 

 tasks can be assigned ; the student will find the most 

 instructive mode of studying these and similar principles, to 

 consist in transferring the heads of them to his note-book, 

 and making them the subject of those remarks and illus- 

 trations which will not fail to suggest themselves in the 

 course of his geological studies. 



