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Show 332 MR. W. WOODLAND ON TIIE [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 impulses to which we attribute the descent of the mammalian testts1, 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 structural 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, &c., 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 interbreeding 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 great Hull-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). " |
