§ 175. The division of Morphology from Physiology, is one which may be tolerably-well preserved so long as we do not carry our inquiries beyond the empirical generalizations of their respective phenomena; but it is one which becomes in great measure nominal, when the phenomena are to be rationally interpreted. It would be possible, after analyzing our Solar System, to set down certain general truths respecting the sizes and distances of its primary and secondary members, omitting all mention of their motions; and it would be possible to set down certain other general truths respecting their motions, without specifying their dimensions or positions, further than as greater or less, nearer or more remote. But on seeking to account for these general truths, arrived at by induction, we find ourselves obliged to consider simultaneously the relative sizes and places of the masses, and the relative amounts and directions of their motions. Similarly with organisms. Though we may frame sundry comprehensive propositions respecting the arrangements of their organs, considered as so many inert parts; and though we may establish several wide conclusions respecting the separate and combined actions of their organs, without knowing anything definite respecting the forms and positions of these organs; yet we cannot reach such a rationale of the facts as the hypothesis of Evolution aims at, without contemplating structures and functions in their mutual relations. Everywhere structures in great measure determine functions; and everywhere functions are incessantly modifying structures. In Nature the two are inseparable co-operators; and Science can give no true interpretation of Nature without keeping their co-operation constantly in view. An account of organic evolution, in its more special aspects, must be essentially an account of the interactions of structures and functions, as perpetually altered by changes of conditions. Hence, when treating apart Morphological Development and Physiological Development, all we can do is to direct our attention mainly to the one or to the other, as the case may be. In dealing with the facts of structure, we must consider the facts of function only in such general way as is needful to explain the facts of structure; and conversely when dealing with the facts of function.