¥903.] TRANSPOSITION OF MAMMALIAN TESTES. 333 
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 
mamumals, 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 mam- 
malian 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 any- 
thing like the same extent as in the gallop of a mammal. The 
great upward impulses associated with the upheavals of the mam- 
malian body are in large part due to the elongation of the trunk, 
for the depression of one half of the body is lareely accelerated by 
the impetus imparted to it on 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 no elon- 
gated 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 (¢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. 
Tfence for these three reasons the individual impulsive accelera- 
tions 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 struthious 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 proba- 
bility of the theory advanced. We have seen that in mammals 
alone there exists a mechanical cause competent to effect trans- 
position. If any doubt remains in the mind of the reader as to 
