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Rh our constitutional craving after rationales, has called into use, if not into being, the several theories of adherence to type, of complemental nutrition, of genealogical, yet modified, transmission, and of correlation of growth.

The first of these theories has won with us not a little popularity; its antique dress, striking the eye, diverted the attention from tie utter incongruity which exists between Platonic mysticism and modern science; and, appealing to our reverence for the dreams of our youth, it has lived longer, and made more converts than unassisted by the associations of the Academy it ever could have done. Even now it is fairly in the way of developing out of the larva stage of an Idolon Theatri into an Idolon Fori, a more active, elusive, albeit fragile, Imago. But a few years back, the joint empire of final and formal causes, of confederated ideological and morphological considerations, seemed firmly established in a country delighting in compromise; the legitimacy of the one, and the prescriptive right of the other, placed them, when united, in an apparently unassailable position. The appearance of the theory of complemental nutrition in a deservedly well-known work caused men to accept of a triumvirate of ruling causes. Material causes counted for something as well as final and formal; Wolff's theory could suffice not only for the rationalization of many phenomena which Paley and Oken did explain, but also for the elucidation of some with which their philosophies were incompetent to deal. Mr. Paget's exemplifications of the law of complemental nutrition seem drawn exclusively from a class of cases of what I would call "heterogeneous growth." The evolution of the one structure has rendered possible the evolution of the other, by setting free some residual product which Nature in her economy has worked up into such secondary structure. The perfecting of the plumage contemporaneously with the perfecting of the sexual functions in the pairing bird is one, and may serve as a type of all, of the instances given by Mr. Paget. There is no equality in rank between the two structures, which stand to each other in this relation of complemental nutrition; the one is supported by what the other finds useless, superfluous, or even hurtful; after the production of the one the organism aims and labours, the other is but a "nebenprodukt;" they are heterogeneous in the same sense as the food of the hound and the food of his master, and often in a yet truer sense still.

The instances of Correlated Growths to which I am about to refer, and which from the dissection I shall detail, I hope to elucidate, differ from those classed under the head of Complemental Nutrition, in that both growths draw with equal right, and to an equal extent, upon the same store of nutriment. To the same stock of alimentary matters they stand in the same relation; they share and share alike either as joint consumers or joint elaborators of it. If we may coin a word from but second-hand Greek, and borrow one-half