1903.] TRANSPOSITION OF MAMMALIAN TESTES. 325 
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beyond the surface of the body-wall, besides being a necessary 
result of more massive organs and the incidence of more powerful 
forces, is in itself a further means of preservation. In physical 
character, the testes, as already mentioned, are definite bodies of 
concentrated form, suspended dorsally in the body-cavity by the 
thin mesorchial membrane. Also investigation shows that, with 
the exception of the heart, the testes are the densest organs in 
the vertebrate body, and hence, as they possess appreciable volume, 
their mass is also considerable. Moreover, the testes (and ovaries) 
are distinctly separated from the remainder of the body, this 
separation (lending additional facility to transposition) arising 
out of the fact that the gonads are from their very nature indi- 
vidualised, reproduction essentially consisting of the separating 
off of a portion of the organism; and hence the body primawily 
serves as a mere carrier of these organs, which, unlike the 
kidneys e.g., bear’ no relation to the economy of the animal. 
From this it follows that the transposition of the testes is of no 
concern to the rest of the organism, since the process can cause 
no derangement of function in other portions of the body; and 
hence the testes, differmg from all other organs in this respect, 
do not, under ordinary conditions, require to be retained in their 
pr imitive position by the special development of fixative structures. 
Thus, as regards their definiteness and concentration of form, 
their means of suspension, their superior density, and their 
structural and physiological separateness from the rest of the 
organism, the testes fully conform to the above-specified con- 
ditions favourable to transposition. The transposition of the 
testes occurring under conditions which permit the descent of 
these organs alone (and the testes alone have descended), and a 
cause capable of effecting this transposition solely existing in the 
Mammalha (in which group descent has alone occurred), it is 
probable that the latter phenomenon is the cause of the former. I 
hold that, in the majority of the Mammalia, the testis attachment 
has thr oughout the history of the race been constantly subjected 
to severe “strains consequent on the character and conditions of 
mammalian locomotion, and that on account of the resulting 
disruption or distension of the mesorchium, the testis has migrated 
in a postero-ventral line (7. e. in an opposite direction to the 
forward and upward accelerations imparted), coming to lie at the 
terminal periphery of the body-wall and forming the scrotal pro- 
trusion. Just as when a man runs, a weight in his coat-pocket 
will periodically “drag” and ultimately wear a hole in the lining 
by constant distension, so the testis of mammals has responded to 
like forces resulting in “ descent.” 
I now proceed to consider the genital organs of the Mammalia 
as a whole, the general conformation of which amply confirms 
the foregoing conclusion, tending to show that the testes have, 
in every case, reacted in a degree proportional to the forces 
concerned. Indeed, the general correspondence between situation 
of the testes and grade of impulsiveness displayed in the various 
