214 HEROES OF SCIENCE. 



throughout all time, but it becomes sea in those 

 parts where it was land, and again it becomes land 

 where it was sea ; and there is reason for thinking 

 that these changes take place according to a certain 

 system and within a certain period. Everything 

 changes in the course of time." 



There was a great geographer called Strabo, about 

 whose life little is known, except that he travelled 

 far and wide in Europe and North Africa, and wrote 

 largely on the earth. He did not content himself 

 with simple geography, however, for he entered into 

 a discussion, which was a very common one in those 

 days, as it has been since, concerning the nature of 

 the fossil shells which are found in strata or layers 

 of the earth remote from the sea. He attributed 

 the collecting of the shells where they are found 

 to the former subsidence of the land, and not to 

 the rising of the sea. It is not, he said, because 

 the lands covered by seas were originally at different 

 altitudes, that the waters have risen or subsided or 

 receded from some parts and inundated others, but 

 the reason is, that the same land is sometimes 

 raised up and sometimes depressed, so that it either 

 overflows or returns to its own place again. We 

 must therefore ascribe the cause to the ground, 

 either to that ground which is under the sea or to 

 that which becomes flooded by it, but rather to that 

 which lies beneath the sea, for this is more moveable, 

 and on account of its humidity can be altered with 

 greater celerity. This philosopher clearly laid down 

 the law that the general level of the sea has re- 



