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.
All natural classification is […] essentially, we may almost say, an attempt to find out the true genesis of the objects classified. But by genesis must be understood, not the efficient action which produces the whole by producing the parts, but the final action which produces the parts because they are needed to make the whole. Genesis is production from ideas. It may be difficult to understand how this is true in the biological world, though there is proof enough that it is so. But in regard to science it is a proposition easily enough intelligible. A science is defined by its problem; and its problem is clearly formulated on the basis of abstracter science. This is all I intended to say here concerning classification, in general.
All classification, whether artificial or natural, is the arrangement of objects according to ideas. A natural classification is the arrangement of them according to those ideas from which their existence results. No greater merit can a taxonomist have than that of having his eyes open to the ideas in nature; no more deplorable blindness can afflict him than that of not seeing that there are ideas in nature which determine the existence of objects. The definitions of Agassiz will, at least, do us the service of directing our attention to the supreme importance of bearing in mind the final cause of objects in finding out their own natural classifications.