THE GROWTH OF CONTINENTS. 121 
names which should have a perfectly definite and 
invariable meaning. As long as zoological no- 
menclature was uncontrolled by any principle, 
the same vagueness and indecision prevailed here 
also. The words Genus, Order, Class, as well as 
those applied to the most comprehensive division 
of all in the animal kingdom, the primary 
branches or types, were used indiscriminately, 
and often allowed to include under one name 
animals differing essentially in their structural 
character. It is only since it has been found 
that all these groups are susceptible of limitation, 
according to distinct categories of structure, that 
our nomenclature has assumed a more precise 
and definite significance. Even now there is still 
some inconsistency among zoologists as to the use 
of special terms, arising from their individual dif- 
ferences in appreciating structural features ; but 
I believe it to be, nevertheless, true, that genera, 
orders, classes, etc., are not merely larger or 
smaller groups of the same kind, but are really 
based upon distinct categories of structure. As 
soon as such a principle is admitted in geology, 
and investigators recognize certain physical and 
organic conditions, more or less general in their 
action, as characteristic of all those chapters in 
geological history designated as Ages, Epochs, 
Periods, Formations, etc., all vagueness will van- 
ish from the scientific nomenclature of this de- 
6 
