320 MR. W. WOODLAND ON THE y [Avr 25 
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 pro- 
duced 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 impulsiveness 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 com- 
ponent 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 pro- 
portional 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 con- 
tinuance 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, 2. 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 Reptilia, both by the high degree of activity which 
