But in the domain of the development of the organic world have we not quite analogous proofs of the efficacy of known forces? Is not the _variability_ of all types of forms a fact? and must not this under the action of natural selection and heredity lead to permanent changes? Has not the problem of explaining the subserviency of all organic form to law as a _result_ without invoking its aid as a _principle_ been thus successfully solved? It is true that we have not directly observed the process of natural selection from beginning to end; neither has anybody directly observed the mode in which the heat of the animal body is generated by the processes of combustion going on in the blood and in the tissues; nevertheless, this is believed as a certainty, and a “vital force” is not invoked. Now the above-mentioned Darwinian principles of transmutation are certainly not simple forces of nature like those underlying the development of the individual, _i.e._ chemico-physical forces, and it cannot be said _à priori_ whether in one of these principles--perhaps in variability or in correlation--there may not lie concealed a metaphysical principle in addition to the physical forces. In fact it has lately been asserted by Edward von Hartmann[286] that the theory of selection is not a mechanical explanation, since it combines forces which are only partly mechanical and in part directive. It must therefore be next investigated whether this assertion is tenable. I. ARE THE PRINCIPLES OF THE SELECTION THEORY MECHANICAL? Edward von Hartmann may justly claim that his views should be considered and tested by naturalists.[287] He would be correctly classed with those philosophers who have approached this question with a many-sided scientific preparation. It can nevertheless be perceived in his case how difficult, and indeed how impossible, it is to estimate the true value of the facts furnished by the investigation of nature, when we attempt to take up only the results themselves, without being practised in the methods by which these are reached, _i.e._ without being completely at home in one of the scientific subjects concerned through one’s own investigations. It appears to me that the denial of the purely mechanical value of the Darwinian factors of transformation arises in most part from an erroneous classification of the scientific facts with which we have to deal. There can certainly be no mistake that the entire philosophical conception of the universe, as laid down by Von Hartmann in his “Philosophy of the Unconscious,” is unfavourable to an unprejudiced estimate of scientific facts and to their mechanical valuation. Variability, heredity, and above all correlation, would not be regarded by Von Hartmann as purely mechanical principles, but he would therein assume a metaphysical directive principle.