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Show 1 9 0 3 . ] TRANSPOSITION OF MAMMALIAN TESTES. 3 3 5 to the intensity of the forces concerned, and have thus supplied the inductive evidence required to verify the conclusion arrived at on a priori grounds. It has also been pointed out that transposition of the testes can, from the nature of the function of these organs, the mode of emission of their genital products, and the absence of adverse external influences, neither cause derangements in the individual economy nor impair the fertility of the race ; and that in consequence of a similar change of position in the case of other organs inevitably entailing one or other of these penalties, the testes alone have descended. Moreover, the superior density, the disposition of the attachment, and the appropriateness both as regards volume and definiteness and concentration of form for change of position, appertaining to the testes, lend greater additional facilities to their transposition than in the case of other organs not so characterised. Hence we can not only show why the testes have descended, but also why they alone have thus responded to the incident forces-other organs, for one or more of the reasons supplied, requiring to be maintained in position by the special retentive structures developed to that end. Combining with these facts the additional evidence derived from a study of mammalian anatomy, showing that impulsive locomotion is not alone responsible for the descent of the testes, but that it has also been the prime cause of the evolution of the diaphragm, the metanephric kidney, and other minor anatomical features, we see still more clearly " how strong is the probability of the theory advanced." Other Illustrations of the Relationship between Visceral Conformation and Impulsiveness : the Evolution of the Diaphragm and the Metanephric Kidney. Although differing from the reproductive elements of the body as regards their economic relations to the rest of the organism, the non-reproductive organs, possessing like material properties, may be expected to portray similar structural characteristics in relation to locomotor impulsiveness. A consideration of such will not only prove of utility as affording indirect confirmation of the foregoing, but will provide opportunity for tendering similar explanations in the case of other anatomical features. The constant interrelations of position obtaining between the various organs in the vertebrate body are in every case easily accounted for, either on mechanical or physiological grounds, and it is needless to discuss every organ from these points of view; it will suffice if we treat of two or three structural phenomena, the interpretation of which is not quite so self-evident or well known. The complete descent of the testes and the presence of a fully-developed diaphragm both being common characteristics of the Mammalia, it is possible that the cause to which the former is attributable may also suffice to explain the evolution of the latter, P r o c . Z o o l . S o c .-1903, V o l . I. No. XXII. 22 |
