332 MR. W. WOODLAND ON THE [ Apr. 21, 
progression, is shown on comparison of two such forms as Triton 
and the Frog (see diagram below, p. 334). If, as we admit, anuran 
progression involves on a small scale those impulse s to which we 
attribute the descent of the mammalian testes’, why have not 
these organs migrated more than they have in this instance? In 
the first place, it may be contended that the total impulsiveness of 
a frog, though quite appreciable, is yet of small value as compared 
with that of a mammal of the same dimensions, say the Common 
Mouse. Secondly, the physiological and strucbur: al relations 
obtaining between the testes and the kidneys afford sufficient 
grounds for the special retention of the former, and, moreover, 
the firm attachment of the testis to the kidney is shielded from the 
abruptness of incident strains by the slight mobility of the latter 
organ. Finally, in addition to the above considerations, it is 
probable that the descent of the testes would here be prohibited 
in consequence of the depression of the hinder portion of the 
trunk and resulting proximity to the external surface. However, 
that such a cause for retention might be operative, it would 
obviously necessitate transposition to an extent that enabled the 
organs to experience adverse influences, and it is questionable as 
to whether they have travelled so far. 
In the typical Aves, as in Mammalia, the expenditure of energy 
is very great, but in the former, as also in Pisces, aquatic 
Amphibia, &ec., the bionomic conditions negative impulsiveness of 
locomotion, and their organisation is not subject to the kinetic 
influences which affect terrestrial animals. 
One noteworthy subject is the large increase in volume of the 
testes during the brief courting-season: thus Owen provides a 
figure illustrating the periodic enlargement of these organs in the 
Common Sparrow, showing that their size ranges from that of a 
pin’s head to that of a fair-sized marble, half an inch in diameter; 
and this same periodic increase of volume, though not so marked, 
occurs in sundry orders of the Mammalia, e. g. in Rodents and 
Insectivores. It is easily comprehensible that if an animal’s 
activity, however impulsive, were largely restricted to the inter- 
breeding periods, the testes, possessing small mass during that 
time, would exhibit less tendency to displacement; but as yet it 
is not possible to decide whether this factor has any significance. 
Connected with a consideration of the position of the testes in 
birds, is the important case involved by the habits of the 
Ostriches and their allies. The considerable speed attained by 
these birds, comparable indeed with that of the swiftest mammals, 
must involve a certain amount of impulsiveness ; and such being 
the case, why have not the testes responded by descent ? For the 
situation of the testes in Ostriches is normally avian, being “‘ placed 
above and a little external to the kidneys,” which latter structures 
are “elongated, flattened, glandular masses...... lying deeply 
seated, and extending from the posterior edge of the diaphragm 
1 “The creat Pull-Frog may clear six feet at a leap, and repeat them so rapidly 
as to escape a pursuer, unless chased at a great distance from the water” (Owen). 
