222 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



are gradual. Let us take the case of the cuscus, one of the marsupials. 

 Herein the fingers of the fore limb, when not grasping a bough, are 

 comparable to those of the human hand. But as soon as a grip is 

 taken of a branch, two fingers curl round one side, and three round 

 the other side of the branch. Moreover, it has a long tail, and the 

 lower half of this is hairless on its under surface, which is very 

 sensitive to touch and can be used as an extra hand in gripping 

 boughs. In the koala, another arboreal marsupial, the fingers of the 

 hand have become permanently separated into a pair turned inward 

 and backward, and three turned outward, while the tail has vanished. 

 In the "anwantibo" (Arctocebus) and the potto, we get a stage fur- 

 ther; in the former the index digit is but slightly developed, while 

 in the potto it is reduced to a mere stump. The last word, so to speak, 

 in this specialization for an arboreal life is found in the sloths, for 

 these live permanently clutching the boughs of trees, suspended, back 

 downward, by means of great curved claws converting the fingers into 

 mere hooks. Finally we have the chameleon, wherein the toes of both 

 fore and hind limb have become apposable, while the tail is pre- 

 hensile. Here, then, are a number of animals belonging to totally 

 different groups, which have, by intensive use, limited to one mode of 

 locomotion, materiall}^ changed the form of their feet. Some have 

 converted the tail into a prehensile organ and others have lost it. In 

 both cases there is a direct association with the habits of the animals 

 concerned and this enhanced function of the tail on the one hand 

 and its loss on the other. 



Now turn to the burrowers. If you were shown a rabbit for the 

 first time and told that it was a burrower, the statement would seem 

 absurd; for there is no indication whatever of such a mode of life in 

 either the fore or hind limbs. And it would seem equally absurd 

 to exhibit a sand martin, or a bee eater, or a kingfisher, and say 

 they were burrowers. For they bear no structural evidence of this 

 mode of activity. The reason is obvious. In none of these cases is 

 burrowing more than an occasional incident in their lives. Tlie 

 rabbit must wander far in search of food, and be able to get swiftly 

 home to its burrow when danger threatens; the martin and the bee 

 eater seek their food in the air, the kingfisher in the water. There- 

 fore flight is essential. But note that in all three the legs are ex- 

 tremely short, for they are never used save to grip branches when 

 they come to rest. Their size bears a direct relation to the intensity 

 of their use. 



Now turn to the mole for a very different aspect of burrowing. 

 This animal, it will be remembered, is one of the shrew tribe, which 

 are also burrowers. But their burrowing does not absorb much of 

 tjieir energies, for they must find their food abroad. Hence, here 



