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Show 1 9 0 3 . ] TRANSPOSITION OF MAMMALIAN TESTES. 33 3 to the anterior extremity of the pelvic cavity " (Macalister). It is obvious that non-descent cannot be ascribed to the serious risk of injury that externality would involve (for, unlike as in the mammals, the position of the proportionately thin legs and the absence of a tail ensure no protection from external influences), since such an explanation is negatived by the present position of the testes. However, if the several characteristics of the mammalian gallop be called to mind, it is easy to afford an explanation of this apparently anomalous instance of the non-transposition of the testes. For, as I shall show, the bipedal progression of the Ostrich does not involve impulsive upheavals of the body to anything like the same extent as in the gallop of a mammal. The great upward impulses associated with the upheavals of the mammalian body are in large part due to the elongation of the trunk, for the depression of one half of the body is largely accelerated by the impetus imparted to it 011 the sudden elevation of the other half. That is, the upheaval of one half of the body rotates the whole length of the trunk about its centre of gravity, and so causes the other half to descend with greater acceleration than it would do under the sole influence of gravity. Hence the marked intensity of the contacts of the limbs with the earth, which have not only to destroy the downward velocity due to gravity, and the additional downward velocity due to this lever-like action of the trunk as a whole, but have also to impart an upward velocity equal in amount to the two it has neutralised. In the Ostrich, on the other hand, there is 110 elongated trunk possessing two pairs of limbs, and hence there can be none of that lever-action just described. Moreover, the individual elevations of the body are small compared with those of mammals, owing to the alternate action of the limbs (c f the Kangaroo, in which the action of the limbs is not alternate, and in which the elevations are consequently great). And lastly, the gliding motion adopted by the Ostrich in running is yet another factor tending to diminish the intensity of the involved concussions. Hence for these three reasons the individual impulsive accelerations imparted to the body of the Ostrich are very small compared with those which exist in the case of mammals ; and therefore it can easily be understood how it is that, despite the terrestrial locomotion and the perpendicular limbs, the testes have not descended in this instance. It is also possible that the posterior extension of the sternum and ribs in the strutliious trunk, leading to tight packing of the viscera, so serves as a support for the testes (and other organs), but whether this is the case can only be determined by referring to the facts. In conclusion, it will not be out of place to here give a brief summary of the entire argument concerning the cause of the descent of the testes, since it will show how strong is the probability of the theory advanced. We have seen that in mammals alone there exists a mechanical cause competent to effect transposition. If any doubt remains in the mind of the reader as to |