1903.] ON THE TRANSPOSITION OF MAMMALIAN TESTES, 319 
4. On the Phylogenetic Cause of the Transposition of the 
Testes in Mammalia: with Remarks on the Evolution 
of the Diaphragm and the Metanephric Kidney. By 
W. Wooptanp, University College, London.* 
{Received March 3, 1903.) 
(Text-figure 57.) 
The subject of the descent of the testes in Mammalia, notwith- 
standing its bionomic interest, has hitherto received but little atten- 
tion from. the phylogenetic sti mdpoint; indeed, so far as I know, 
its interpretation has not hitherto been attempted. ‘To exhibit 
the general nature of the problem and the apparent difficulty 
attending its solution, the following paragraph from Mr. Spencer's 
‘Principles of Biology’ (vol. i. p. 573) may be quoted :—“ But 
now let it be confessed that though all phenomena of organic 
evolution must fall within the lines above indicated, there remain 
many unsolved problems. Take as an instance the descent of the 
testes in the Mammalia. Neither direct nor indirect equilibration 
accounts for this. We cannot consider it an adaptive change, 
since there seems no way in which the production of sperm-cells, 
internally carried on in a bird, is made external by adjustment to 
the changed requirements of mammalian life. Nor can we ascribe 
it to survival of the fittest; for it is incredible that any mammal 
was ever advantaged in the str uggle for life by this changed 
position of these organs. Contrariwise, the removal of them 
from a place of safety to a place of danger would seem to be 
negatived by natural selection. Nor can we regard the trans- 
position as a concomitant of re-equilibration; since it can hardly 
be due to some change in the general physiological balance.” I 
agree with Mr. Spencer that the descent of the testes can neither 
have been a change adaptive in nature, nor a result of the 
operation of natural selection; on the other hand, I must dis- 
agree with the statement that the phenomenon cannot be 
regarded “as a concomitant of re-equilibration,” since, as will be 
shown, I attribute it to the direct action of the conditions of life 
found in the Mammalia, 
To ensure due appreciation of the significance of the following 
statements, it will be as well to here indicate the nature of the 
theory about to be advanced. Of possible causes effecting the 
transposition of the testes, there are two categories : («) causes 
which as regards their effects on the organism mediately 
or immediately impart advantage, so supplying the requisite 
material for natural selection ; at (6) causes which in their 
effects on organisation bear no appreciable relation to either 
advantage or disadvantage. Since those of the former category 
are inconceivable in this connection, we are led to conclude 
1 Communicated by Prof. E. A. Mincurn, F.Z,S. 
Proc. Zoot. Soc.—1903, Vou. I. No. X XI, 21 
