Wyman.] 2 t 6 [June 5, 



rical parts, anil we know of no way in which it can be fairly and sat- 

 isfactorily met. 



The thumb and great toe are assumed by most anatomists to be 

 homotypes. First, on account of their relative size. Secondly, because 

 they have similar relative positions in the ordinary attitude of the 

 fore arm. Thirdly, and chiefly, because they have only two phalanges 

 each, while each of the other digits has three or more. 



If the human hand and foot are alone examined, the relative size 

 of the parts in question favors the view that they are homotypes- 

 But this characteristic of size loses its value when they are studied in 

 the lower animals. In the seal the thumb might, as regards its size, 

 be considered the homotype of either the first or the fifth toe, which 

 are the two largest and of equal size. In the walrus the first digit of 

 the hand and the fifth of the foot, or the thumb and little toe, are the 

 largest in their respective Umbs. In the great ant-eater the third digit 

 of the hand is longest, while the fourth is in the foot. If size were 

 the criterion of homology, either of the fingers might in turn become 

 the homotype of either of the toes, for the size of these parts being 

 determined by their physiological adaptations, either may in turn be- 

 come the largest or the smallest in the series. 



The second reason, that based on the fact that they are both on the 

 inside of their respective limbs, loses its force when it is remembered 

 that the parts compare^ are, as it were, in a false position. That but 

 for the rotation of the fore arm in the embryo, the thumb would have 

 been on the outside of the hand, and would consequently have con- 

 formed to the position of the little finger. 



The third argument, derived from the existence of two phalanges in 

 each of the parts, is not so easily disposed of, and forms the greatest diffi- 

 culty in our way. Notwithstanding the wide difference in the physiolog- 

 ical value of these parts in different species of animals, and the conse- 

 quent range of variation in the size of them, the number of phalanges 

 may be said to be almost constant. It is true that in Ichthi/osaurus, Ple- 

 siosaurus, and other marine saurians, the thumb and great toe, like all 

 the other digits, have their phalanges multiplied, and if our comparisons 

 wei'e confined to such animals as these the question of homologies 

 would be easily answered, as it would also in some of the land turtles, 

 where the nmnber of the phalanges in all fingers and toes is reduced 

 to two. If, too, we might apply the saying of Goethe, which holds true 

 in so many instances, viz., " that it is in her monstrosities that nature 

 reveals to us her secrets," we might call to mind an occasional mon- 

 strosity in which the thumb and great toe are each provided with three 

 joints, and thus made to conform with the other digits. Lastly, we 

 might call to mind the fact that in their mode of ossification, the 

 metatarsal and metacarpal bones of the two parts in question agree 



