212 CLASSICS OF MODERN SCIENCE 

 ciate the gradual manner in which a passage may have taken place 

 from an extinct fauna to that now living, I shall say a few words on 

 the fossils of successive Tertiary periods. When we trace the series 

 of formations from the more ancient to the more modern, it is in 

 these Tertiary deposits that we first meet with assemblages of organic 

 remains having a near analogy to the fauna of certain parts of the 

 globe in our own time. In the Eocene, or oldest subdivisions, some 

 few of the testacea belong to existing species, although almost all of 

 them, and apparently all the associated vertebrata, are now extinct. 

 These Eocene strata are succeeded by a great number of more modern 

 deposits, which depart gradually in the character of their fossils from 

 the Eocene type, and approach more and more to that of the living 

 creation. In the present state of science, it is chiefly by the aid of 

 shells, that we are enabled to arrive at these results, for of all classes 

 the testacea are the most generally diffused in a fossil state, and may 

 be called the medals principally employed by nature in recording the 

 chronology of past events. In the Upper Miocene rocks (No. 5 of 

 the table, p. 135) we begin to find a considerable number, although 

 still a minority, of recent species, intermixed with some fossils com- 

 mon to the preceding, or Eocene, epoch. We then arrive at the PHo- 

 cene strata, in which species now contemporary with man begin to 

 preponderate, and in the newest of which nine-tenths of the fossils 

 agree with species still inhabiting the neighbouring sea. It is in the 

 Post-Tertiary strata, where all the shells agree with species now liv- 

 ing, that we have discovered the first or earliest known remains of 

 man associated with the bones of quadrupeds, some of which are of 

 extinct species. 



In thus passing from the older to the newer members of the Ter- 

 tiary system, we meet with many chasms, but none which separate 

 entirely, by a broad line of demarcation, one state of the organic world 

 from another. There are no signs of an abrupt termination of one 

 fauna and flora, and the starting into life of new and wholly distinct 

 forms. Although we are far from being able to demonstrate geologi- 

 cally an insensible transition from the Eocene to the Miocene, or even 

 from the latter to the recent fauna, yet the more we enlarge and per- 

 fect our general survey, the more nearly do we approximate to such 

 a continuous series, and the more gradually are we conducted from, 

 times when many of the genera and nearly all the species were extinct, 



