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 uncertain. 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 diffusion of species upon the 
earth's surface is cpute 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 inferred from such facts, and the 
