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Show 320 MR. W. WOODLAND ON THE [Apr. 21, that the transposition is an inevitable concomitant of some other constant feature of the animal's existence, thus not having arisen in relation to ulterior ends. Again, since the testes consist of ordinary matter possessing inertia and mass, their translation (involving rupture or distension of the mesorchium) implies mechanical force. From both of the foregoing considerations, and from the fact that no other efficient cause is imaginable, it is probable that the displacement results from the reaction on the part of the testes to the incident forces, which, it can be shown, are generated by bodily activity under mammalian conditions of life. The theory here advocated is to the effect that the descent of the testes in the Mammalia has been produced by the action of mechanical strains causing rupture of the mesorchial attachments, such strains being due to the inertia of the organs reacting to the imjimlsiveness involved in the activity of the animals composing the group. Mechanical Aspects of Organisation. Before entering upon a discussion as to the production of the forces above mentioned and the manner in which they have acted, it is well briefly to outline the entire argument from the mechanical standpoint. In the first place, accelerations imparted to the body as a whole generate strains (or stresses) in connection with the attachment or other means of support of every component viscus, and the degree of such accelerations (and therefore of the strains and stresses) is obviously dependent upon the reaction which occurs between the animal's body and the medium in or substratum on which it is supported, being directly proportional to the product of the powers of resistance possessed by the substances constituting the same. Secondly, it must be pointed out that the accelerations to which we refer, and to which alone we attribute any importance, are those involved in the impulses communicated to the body during the actual continuance of locomotion, and which inevitably result from the mode of action of the propelling agency, whatever its nature. The ordinary non-impulsive accelerations involved in the starting of an animal into motion from a state of rest, or its converse do not concern us. In considering, however, the arboreal habits of e. g. Primates, the case is different, accelerations involved in motion from rest here being decidedly impulsive in nature. Now among the Vertebrata, it is evident that fishes, aquatic amphibia, and birds severally exist in media which, owing to their mobility, negative the occurrence of accelerations of high degree, i. e. impulsiveness. Hence, as regards these accelerations, we have to consider only terrestrial animals, which come into contact with a substratum possessing sufficient power of resistance to afford a reaction of marked intensity. Of these terrestrial animals the mammalian group is at once distinguished from terrestrial Amphibia and Ileptilia, both by the high degree of activity which |