1903. ] TRANSPOSITION OF MAMMALIAN TESTES. 339 
urinary function, is evidently a physiological phenomenon, though 
the ultimate structural separation of the two parts must largely 
be attributed to the influence of greater activity of the organism. 
Also the secondary development of the definitive mesonephros is 
indirectly due to increased activity, since the work of the excretion 
resulting from the latter is largely increased. In fact, the whole 
ontogeny of the kidney—the successive developments of the pro-, 
meso-, and metanephros in space and time, the differences obtaining 
between the pro- and mesonephric tubules, the disappearance of 
the pronephros, &c., &e.—can be directly or indirectly attributed 
to the increasing activity of the organism in the course of phylo- 
geny, as a little thought will show. 
With regard to the position of the kidney—a body of consider- 
able mass, and in Amniota similarly disposed to the testes—no 
relation between such and the impulsiveness of the animal being 
traceable, it is evident that some other factor must be involved, 
and enquiry shows that the localisation of the kidney is correlated 
with the total activity of the animal. Excretion heing facilitated 
by proximity to a vigorous plood-supply, we find that in active 
animals the kidney tends to be situated anteriorly, i. e. nearer the 
heart ; on the other hand, close connection with the vent to the 
exterior is similarly advantageous, and in inactive animals, such 
as Reptilia and Urodeles, we find the kidneys situated at the 
posterior extremity of the ccelom. 
The kidneys of mammals are retained in position by a circwm- 
ambient development of areolar tissue, which usually contains 
much fat; support is also contributed by the “tonicity” of the 
muscles of the abdominal wall and by the terminal ribs. 
Tn conclusion, I must acknowledge my indebtedness for details 
vespecting the habits and anatomy of the mammalian orders to 
the following authors and works: Flower and Lydekker, Owen, 
Wiedersheim, Beddard and Gegenbaur, Lydekker’s ‘ Royal 
Natural History,’ ‘Mammals’ by Vogt & Specht, Schmeil’s ‘ Text- 
book of Zoology,’ and Allen’s & Lloyd’s editions of Jardine’s 
‘ Naturalist’s Library.’ 
L also wish to thank Prof. Minchin, Mr. J. T. Cunningham, and 
Mr. H. S. Shelton for kindly reading through the manuscript and 
for making several suggestions and criticisms. 
APPENDIX. 
In respect to the non-descent of the ovary in the Mammalia, it 
is as well to emphasise one factor ensuring its retention. Assum- 
ing that the females of mammals are approximately as active as 
the males, it is evident that if the mammalian ovum had retained 
its yolk (7. e. if the mammalian ovary possessed the size and mass 
of that of the Sauropsida and Monotremes), the retention of the 
ovary within the body-cavity by the development of special liga- 
ments, &c., would have been, if not impossible, yet extremely 
hazardous. And such being the case, it would obviously benefit 
the race if the potency of one of these two factors were decreased. 
