TRANSFORMATION OF ORGANS OF ANIMALS 49 



not opposable thumbs. We may presume then 

 that the hind limbs were the first to become 

 adapted to arboreal life, and, since the great toe 

 of man is not opposable, it may be concluded that 

 it ceased to be so when terrestrial life was assumed. 

 This modification is not wholly degenerative, for 

 the functions of the lower limbs are no less im- 

 portant in man than in apes. There are, however, 

 undoubted signs of degeneration in the changes 

 entailed. The muscles which rendered the great 

 toe opposable were probably the first to degenerate, 

 but by referring to a skeleton it may be seen that 

 whereas the great toe and the metatarsals have 

 gained in length, the first phalanges, and still more 

 the other phalanges, are reduced. In some cases 

 the last phalange of the little toe has even dis- 

 appeared altogether. 



(b) Birds. First take the reptile-footed Dino- 

 saurians. These are four-footed beasts, and the 

 hind limbs are plantigrade and five-toed. Counting 

 from the inner side the phalanges of the toes are : 

 2, 3, 4, 5, 4 in number. 



In passing on to the bird-footed Dinosaurians 

 we find that with Hypsilophodon, the foot is func- 

 tionally four-toed. From the first to the fourth 

 toe the phalanges are the same in number as with 

 the reptile-footed Dinosaurians, but the fifth toe is 

 greatly reduced, being merely represented by a 

 pointed metatarsal. In Camptosaurus, the fifth toe 

 has disappeared, the second, third, and fourth are 



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