2 THE GEOLOGICAL HISTORY OF PLANTS. 



the more eminent workers in the subject. Now, in the 

 evening of his days, he thinks it right to endeavour to 

 place before the world a summary of facts and of his own 

 matured conclusions — feeling, however, that nothing can 

 be final in this matter ; and that he can only hope to 

 sketch the present aspect of the subject, and to point the 

 way to new developments, which must go on long after 

 he shall have passed away. 



The subject is one which has the disadvantage of pre- 

 supposing some knowledge of the geological history of 

 the earth, and of the classification and structures of mod- 

 em plants ; and in order that all who may please to read 

 the following pages may be placed, as nearly as possible, 

 on the same level, this introductory chapter will be de- 

 voted to a short statement of the general facts of geological 

 chronology, and of the natural divisions of the vegetable 

 kingdom in their relations to that chronology. 



The crust of the earth, as we somewhat modestly term 

 that portion of its outer shell which is open to our obser- 

 vation, consists of many beds of rock superimposed on 

 each other, and which must have been deposited succes- 

 sively, beginning with the lowest. This is proved by the 

 structure of the beds bhemselves, by the markings on 

 their surfaces, and by the remains of animals and plants 

 , which they contain ; all these appearances indicating that 

 each successive bed must have been the surface before it 

 was covered by the next. 



As these beds of rock were mostly formed under water, 

 and of material derived from the waste of land, they are 

 not universal, but occur in those places where there were 

 extensive areas of water receiving detritus from the land. 

 Further, as the distinction of land and water arises prima- 

 rily from the shrinkage of the mass of the earth, and 

 from the consequent collapse of the crust in some places 

 and ridging of it up in others, it follows that there have, 

 from the earliest geological periods, been deep ocean- 



