And here, indeed, let it be pointed out how completely irrelevant is the test of conceivableness as applied to these ultimate physiological actions. For as here, from the un-united ends of the broken bone, there presently arises a rude joint with fit membranes, ligaments, and even synovial fluid, though we are absolutely unable to imagine the process by which the adjacent tissues produce this structure; so there may be from an organ enlarged by function, such reactive effect upon the system at large as eventually to influence the reproductive cells, though we may be absolutely unable to imagine how this can be done.] CHAPTER IX. PHYSIOLOGICAL INTEGRATION IN ANIMALS. § 305. Physiological differentiation and physiological integration, are correlatives that vary together. We have but to recollect the familiar parallel between the division of labour in a society and the physiological division of labour, to see that as fast as the kinds of work performed by the component parts of an organism become more numerous, and as fast as each part becomes more restricted to its own work, so fast must the parts have their actions combined in such ways that no one can go on without the rest and the rest cannot go on without each one. Here our inquiry must be, how the relationship of these two processes is established--what causes the integration to advance _pari passu_ with the differentiation. Though it is manifest, _à priori_, that the mutual dependence of functions must be proportionate to the specialization of functions; yet it remains to find the mode in which the increasing co-ordination is determined. Already, among the Inductions of Biology, this relation between differentiation and integration has been specified and illustrated (§ 59). Before dealing with it deductively, a few further examples, grouped so as to exhibit its several aspects, will be advantageous. § 306. If the lowly-organized _Planaria_ has its body broken up and its gullet detached, this will, for a while, continue to perform its function when called upon, just as though it were in its place: a fragment of the creature’s own body placed in the gullet, will be propelled through it, or swallowed by it. But, as the seeming strangeness of this fact implies, we find no such independent actions of analogous parts in the higher animals. Again, a piece cut out of the disc of a _Medusa_ continues with great persistence repeating those rhythmical contractions which we see in the disc as a whole; and thus proves to us that the contractile function in each portion of the disc, is in great measure independent. But it is not so with the locomotive organs of more differentiated types. When separated from the rest these lose their powers of movement. The only member of a vertebrate animal which continues to act after detachment, is the heart; and the heart has motor powers complete within itself.