ost 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 zooelogists as to the use of special terms,
arising from their individual differences in appreciating, structural
features; but I believe it to be, nevertheless, true, that general 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
vanish from the scientific nomenclature of this department also, and there
will be no hesitation as to the use of words for which we shall then have
a positive, definite meaning.

Although the fivefold division of Werner, by which he separated the rocks
into Primitive, Transition, Secondary, Alluvial, and Volcanic, proved to
be based on a partial misapprehension of the nature of the earth-crust,
yet it led to their subsequent division into the three great groups now
known as the Primary, or Palaeozoic, as they are sometimes called, because
here are found the first organic remains, the Secondary, and the Tertiary.
I have said in a previous article that the general unity of character
prevailing throughout these three divisions, so that, taken from the
broadest point of view, each one seems a unit in time, justifies the
application to them of that term, _Age_, by which we distinguish in human
history those periods marked throughout by one prevailing tendency;--as we
say the age of Egyptian or Greek or Roman civilization,--the age of stone
or iron or bronze. I believe that this division of geological history into
these great sections or chapters is founded upon a recognition of the
general features by which they are characterized.

Passing over the time when the first stratified deposits were accumulated
under a universal ocean in which neither an