TRANSFORMATION OF ORGANS OF ANIMALS 55 



It is so-called owing to the remarkable length of 

 its tarsus (fig. 27), a very exceptional formation, 

 for the elongation of an animal's foot is usually 

 effected by means of a lengthened metatarsal. 

 This abnormal formation in the foot of the 

 tarsius can, however, be accounted for. Like 

 most of the Primates, the tarsius is adapted to 

 an arboreal life, and, among other characters, it 

 exhibits an opposable great toe. When on the 

 ground, however, instead of running on two or 

 four legs, it progresses by leaping ; and this 

 may also be noticed with some of the lemurs 

 of Senegal (Galago). The tarsius has become 

 adapted to leaping by the lengthening of the 

 hind limbs, the thigh, leg and foot being equally 

 elongated. The foot, however, being of fan-like 

 structure, it is clear that if all the toes were 

 elongated, the feet would overlap one another 

 in the middle line, and therefore with most 

 animals that leap the elongation of the foot 

 involves a reduction in size of the outer and 

 inner toes to prevent overlapping. The tarsius, 

 however, has remained arboreal, although it has 

 taken to leaping, and it therefore requires a well- 

 developed foot with an opposable great toe in order 

 to obtain a firm grip on branches. To avoid over- 

 lapping of the feet, the length has been augmented 

 at the ankle, which, not being shaped like a fan, 

 does not consequently gain in length. 



A more detailed examination would doubtless 



