antelopes, dogs, wolves, and other Ungulata and Carnivora. It is obvious
that when the body is descending to the ground after being hurled upwards
and forwards, the abdominal organs have acquired a rapid movement
downwards and forwards; when the body reaches the ground its movement is
stopped suddenly, while the abdominal organs continue to move. The testes
therefore are violently jerked downwards away from their attachments and
at the same time forward. The check to the forward movement, however, is
momentary, while the body is immediately thrown again upwards and
forwards, which by the law of inertia means that the testes are thrown
still more downwards and backwards. There is no reason to suppose, as Dr.
Woodland suggests, that any rupture of the mesorchium was the usual result
of these strains, but a constant pull or tension was caused in the
direction in which the testes actually move during development. On this
theory we have to consider (1) how such strains could cause a shifting of
the peritoneal attachment, (2) why the testes should be supposed to be
particularly affected more than other abdominal organs. The answer to the
first question is that the strains would cause a growth of the connecting
membrane (mesorchium) at the posterior end, accompanied by an absorption
of it at the anterior end. The answer to the second question is that the
testes are at once the most compact and heaviest organs in the abdomen,
and at the same time the most loosely attached. The latter statement does
not apply to the mesonephros or epididymis which has moved with the
testis, but the latter cannot function without the former, and it may be
supposed that the close attachment of the epididymis to the testis had
come about in the early Mammalia before the change of position was
evolved.

It is evident that the violent shocks of the galloping or leaping movement
do not occur in Birds, Reptiles, or Amphibia. Ostriches run very fast and
do not fly, but their progression is a stride with each foot alternately,