GEOLOGICAL RELATIONS. 179 



tion it may on the other hand differ very materially 

 from it. It has also been suggested that as vegetable 

 remains have been detected in very ancient formations, 

 it might have been expected that the animals -which feed 

 upon vegetables, and especially the herbivorous land- 

 mollusks would have existed contemporaneously with 

 them, and that their remains should now be found in the 

 same strata ; but that as they do not appear in any of 

 the formations older than the tertiary, and but very 

 sparingly in that, they could not have existed antecedent 

 to, and were far from numerous during the tertiary 

 period. Hence, as a further inference from these infer- 

 ences, it has been stated, that the present time is the 

 period of their greatest numerical development, and 

 that their actual numbers far exceed those of any former 

 era. These conclusions also ought to be received with 

 great caution, for the premises on which they are 

 founded are very uncertam. We have seen that the 

 remains of these animals, by reason of their frail and 

 perishable nature, soon decay, and we must take it for 

 granted that only a small part of their whole number is 

 washed into rivers and carried away by their currents. 

 The deposites which finally receive them can therefore 

 represent but very feebly their former numerical condi- 

 tion, and a very general diflFiision of species upon the 

 earth's surface is quite consistent with the existence and 

 deposition of only a small number of their remains. 

 The condition of the species at particular epochs cannot 

 therefore be correctly mferred from such facts, and the 



