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the evolution of those objects. Third, it may be based upon the
abstract forms,- often numerical,- of essential facts
concerning these objects. Fourthly, it may be based upon
considerations largely extraneous to the objects themselves;
usually upon convenience in searching for any given one.
The first kind of classification, to wit, that which is based
upon the ideas of the objects classified, is the sort of classification
with which Plato and Aristotle dealt. It is
elaborated in its application to the animal kingdom in
the Essay on Classification of Louis Agassi, a work
which appeared at an epoch singularly inauspicious for
the general recognition of all its greatness.
There is no room for doubt that in case we know
what ideas have brought objects into existence, they ought
to be classified upon the basis of those ideas; and this is
the case with the sciences. A perfectly parallel remark
is equally indisputable in regard to the evolutionary