1903.] TRANSPOSITION OF MAMMALIAN TESTES. 323 
limbs make contact with the earth, when, as already descriped, 
the entire portion of the trunk is suddenly upheaved with con- 
siderable force. If the horizontal velocity could be reckoned as 
constant, the only accelerations which the trunk would acquire 
would be in a vertical line; but since the horizontal velocity is a 
quantity which, owing to the resistance of the air, is continually 
decreasing, the upward acceleration suddenly imparted to the 
trunk at each contact of the limbs with the earth has to be com- 
pounded with a sudden forward acceleration in order to maintain 
the pace, and hence the resultant acceleration which the half of 
the body acquires at every upheaval is in an wpward and forward 
direction. It is most essential to recognise this fact that the body 
as a whole has periodically imparted to it large accelerations in 
these two directions. As is obvious, the upward and horizontal 
velocities due to these accelerations become respectively neutralised 
by gravity and atmospheric resistance at the middle and end of 
the interval which exists between successive contacts of the limbs 
with the earth. From the instant when the maximum elevation 
of the half of the trunk is attained, 7.e. when the upward velocity 
is completely neutralised, until contact again occurs, the body 
continues to acquire a downward acceleration due to gravity. 
But on contact taking place, the large downward v elocity due to 
this acceleration is suddenly destroyed, and as suddenly exchanged 
for an equal upward velocity, in the brief instant of time occupied 
by the impact of limbs and earth. Whence it is evident that the 
degree of upward acceleration imparted to the half of the body 
during the sudden loss and gain of downward and upward velocities 
must be considerable. It is noteworthy that whereas each forward 
acceleration, due to either pair of limbs, is imparted to the body 
as a whole, each upward acceleration is solely imparted to that 
portion of the trunk to which the pair of limbs causing the 
elevation belongs; and hence the attachments of those organs 
situated in the median portion of the trunk are evidently not so 
subject to downward strains as in the case of organs placed more 
terminally. Thus it is important to notice, with regard to what 
follows, that the primitive pre-mammalian portion of the testes is 
decidedly posterior (see diagram, p. 334), and that in consequence 
the motion of the hind portion of the body is the factor of most 
importance in the consideration of the present problem. 
From the foregoing, it is sufticiently manifest that at each 
elevation of the body in mammals a considerable force is imparted 
to the organisation asa whole (and by necessary implication to its 
constituent parts) inan upward and forward directior ras to 
be proved. If the constituent parts of the body be cousidureah it 
will be evident that such a force works differential effects among 
them. For these organs, largely differing among themselves in 
regard to mass, and the same force being incident on all, it follows 
that the individual reactions are e different, and hence there exists 
a tendency to segregation of those organs of greater mass fromm 
those of less. Moreover, organs differ inter se in their relation 
