Every class is constituted and held together by a concept or idea expressed in its definition. Every arrangement of ideas is itself an idea. Consequently, every classification whatever is governed by an idea, however loose and incongruous it may be. A natural classification, that is to say, a birth-al classification, is a classification whose governing idea coincides with the idea which determines the things classified to exist. An idea, so far as it has any relation to life, is a possible purpose. [—] Should there be no human purpose, there may, nevertheless, be an evolutionary agency that acts like a purpose, or there may be a principle similar to such agency except that it is related, not to a temporal, but to a logical sequence of results.