ca. XL] TWO OBJECTIONS CONSIDERED. 39 



great as that to which we had reached with the Silurian 

 rocks ; and it is now generally admitted that even the 

 Lnuventian strata are modem compared with the beginnings 

 of life upon our globe. 



But this is not all. Along with the immensely long 

 geologic rhythms, which have thus entailed the periodic 

 metamorphosis of strata, there have been going on minor 

 rhythms, resulting in the alternate deposit and denudation of 

 fossil-bearing strata. Each of the sedimentary strata nott 

 surviving was deposited during an epoch of subsidence, and 

 since its elevation to its present position has been more or 

 less denuded. Now it is only during epochs of subsidence 

 that permanent fossil-bearing strata can be deposited. During 

 epochs of elevation the newly-formed sedimentary deposit is 

 rapidly disintegrated by the action of coast-waves ; and even 

 those thin deposits which are made during an epoch of sub 

 sidence are in the next-recurring epoch of elevation soon 

 worn away. It is thus only the thicker strata deposited 

 during an epocji of subsidence which have preserved for our 

 inspection a few specimens of the organisms living at the 

 time when they were deposited. 



But in close juxtaposition to this comes the remarkable 

 fact that the most rapid variation among specific forms must 

 take place during epochs of elevation. For since the only 

 variations preserved by natural selection are those which 

 bring the organism into closer adaptation to its environment ; 

 and since in most cases the organic environment of any group 

 of organisms, comprising its enemies, competitors, and prey, 

 is a much more important factor of change than its inorganic 

 environment, comprising climate and soil; it follows that 

 those periods during which groups of organisms, hitherto 

 isolated, are gradually brought into contact with one another, 

 must be the periods most favourable for specific change. The 

 most rapid variation, attended by the greatest frequency of 

 transitional forms, will therefore occur during those epochs of 



