Not only a direct consideration of the matter, but all analogy, goes to show that in the earlier and simpler stages must be sought the key to all subsequent intricacies.  The time was when the anatomy and physiology of the human being were studied by themselves—­when the adult man was analysed and the relations of parts and of functions investigated, without reference either to the relations exhibited in the embryo or to the homologous relations existing in other creatures.  Now, however, it has become manifest that no true conceptions, no true generalisations, are possible under such conditions.  Anatomists and physiologists now find that the real natures of organs and tissues can be ascertained only by tracing their early evolution; and that the affinities between existing genera can be satisfactorily made out only by examining the fossil genera to which they are allied.  Well, is it not clear that the like must be true concerning all things that undergo development?  Is not science a growth?  Has not science, too, its embryology?  And must not the neglect of its embryology lead to a misunderstanding of the principles of its evolution and of its existing organisation?

There are a priori reasons, therefore, for doubting the truth of all philosophies of the sciences which tacitly proceed upon the common notion that scientific knowledge and ordinary knowledge are separate; instead of commencing, as they should, by affiliating the one upon the other, and showing how it gradually came to be distinguishable from the other.  We may expect to find their generalisations essentially artificial; and we shall not be deceived.  Some illustrations of this may here be fitly introduced, by way of preliminary to a brief sketch of the genesis of science from the point of view indicated.  And we cannot more readily find such illustrations than by glancing at a few of the various classifications of the sciences that have from time to time been proposed.  To consider all of them would take too much space:  we must content ourselves with some of the latest.

* * * * *

Commencing with those which may be soonest disposed of, let us notice first the arrangement propounded by Oken.  An abstract of it runs thus:—­

     Part I. MATHESIS.—­Pneumatogeny:  Primary Art, Primary
               Consciousness, God, Primary Rest, Time, Polarity, Motion,
               Man, Space, Point.  Line, Surface, Globe,
               Rotation.—­Hylogeny:  Gravity, Matter, Ether, Heavenly
               Bodies, Light, Heat, Fire.

        (He explains that MATHESIS is the doctrine of the whole;
        Pneumatogeny being the doctrine of immaterial totalities, and
        Hylogeny that of material totalities.)

     Part II.  ONTOLOGY.—­Cosmogeny:  Rest, Centre, Motion, Line,
               Planets, Form, Planetary System, Comets.—­Stoechiogeny: 
               Condensation, Simple Matter, Elements, Air, Water,
               Earth—­Stoechiology:  Functions of the Elements, etc.,
               etc.—­Kingdoms of Nature:  Individuals.