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Show 326 MR. W. WOODLAND ON TI1E [Apr. 21, orders and families of the Mammalia affords such conclusive evidence as to the causal relation subsisting between the two that a systematic review of this evidence is well called for. That the correspondence is not absolute, however-that there exist instances of the concurrence of scrotal testes with sluggish habits -is without doubt mainly, if not wholly, to be explained by phylogenetic considerations. For it must be remembered that once the descent of the testes is inherited, the trait is a constant one (unless acquired sluggishness is able in time to produce effect, which is doubtful), and remains so in successive generations whatever varied habits may be assumed, unless the bionomic aspect so changes that natural selection cancels the " plus variations," so affording an ascendancy to reversionary factors. That such has been the case in several instances will be illustrated below. In the Monotremes the activity (impulsiveness) is of a very low degree. Ornilhorhynchus is " aquatic in its habits, passing most of its time in the water or close to the margin of lakes and streams." The " body is rather long, compact, and almost everywhere of the same thickness. It rests on short, massive legs ............so short that the animal in walking or running actually drags its body along the ground." Echidna is " usually found in rocky districts, and more especially in the mountains............and is mainly of nocturnal habits." It is described as " indolent." The testes in both instances are " abdominal in position throughout life," and afford the only example in mammals of a disposition of these organs anterior to the kidneys. The low status of these animals is well known. In the Marsupials " the testes are always contained in a scrotum, which is suspended by a narrow pedicle to the abdomen in front of the penis." If it is permissible to assume that the ancestor of the marsupials was kangaroo-like, the pre-penial position of the testes may perhaps be attributed to the peculiar mode of locomotion characterising this animal. For the rapid locomotion of a Kangaroo consists of a series of leaps, and such leaps would involve, as in ordinary mammals, a series of antero-dorsal tensions on the suspensory membranes of the testes; but since the bodily depressions are marked by a more prolonged termination, as compared with those involved in the ordinary gallop, the testes would in addition tend to be thrown forward, and these two factors in conjunction have possibly led to the peculiar position of the testes found in marsupials. On no other theory than that of descent from an ancestor characterised by pre-penial testes can the peculiar genital conformation of the variously habited marsupials be correlated with special bionomic conditions; and hence, on this ground alone, it is possible, as implied above, that the ancestor of the Marsupialia was of the type of the Macropodidse, the characteristic saltatory progression originating the pre-penial situation of the scrotum in the manner indicated. The Edentata represent another ancient group. Their activity |