[Footnote 73: See the beautiful discussions of this point in Hering's _Memory as a General Function of Organised Matter_ (1870), Chicago, The Open Court Publishing Co., 1887. Compare also Dubois, _Ueber die Uebung_, Berlin, 1881.] [Footnote 74: Spencer, _The Principles of Psychology_. London, 1872.] [Footnote 75: See the article _The Velocity of Light_, page 63.] [Footnote 76: I am well aware that the endeavor to confine oneself in natural research to _facts_ is often censured as an exaggerated fear of metaphysical spooks. But I would observe, that, judged by the mischief which they have wrought, the metaphysical, of all spooks, are the least fabulous. It is not to be denied that many forms of thought were not originally acquired by the individual, but were antecedently formed, or rather prepared for, in the development of the species, in some such way as Spencer, Haeckel, Hering, and others have supposed, and as I myself have hinted on various occasions.] [Footnote 77: Compare, for example, _Schiller, Zerstreute Betrachtungen über verschiedene ästhetische Gegenstände_.] [Footnote 78: We must not be deceived in imagining that the happiness of other people is not a very considerable and essential portion of our own. It is common capital, which cannot be created by the individual, and which does not perish with him. The formal and material limitation of the _ego_ is necessary and sufficient only for the crudest practical objects, and cannot subsist in a broad conception. Humanity in its entirety may be likened to a polyp-plant. The material and organic bonds of individual union have, indeed, been severed; they would only have impeded freedom of movement and evolution. But the ultimate aim, the psychical connexion of the whole, has been attained in a much higher degree through the richer development thus made possible.] [Footnote 79: C. E. von Baer, the subsequent opponent of Darwin and Haeckel, has discussed in two beautiful addresses (_Das allgemeinste Gesetz der Natur in aller Entwickelung_, and _Welche Auffassung der lebenden Natur ist die richtige, und wie ist diese Auffassung auf die Entomologie anzuwenden_?) the narrowness of the view which regards an animal in its existing state as finished and complete, instead of conceiving it as a phase in the series of evolutionary forms and regarding the species itself as a phase of the development of the animal world in general.] ON THE PRINCIPLE OF COMPARISON IN PHYSICS.[80]