THE ONE BIG DIGIT OF THE HORSE 257 



thumb with only two phalanges can be readily made out. True, 

 but in this discussion I am taking into consideration not only a 

 broad evolution reaching to the most ancient forms in which we 

 can clearly identify what we call a hand and a foot, but also tera- 

 tological, or monstrous specimens. Now and again these peep out 

 of oblivion to reveal something of the invisible past, and therefore 

 cannot be ignored in framing evolutionary theories. 



Although the thumb and big toe are readily identifiable in 

 series of Mammals, from their having only two phalanges, it 

 would appear that the thumb before it got shortened and partially 

 atrophied, and separated from the other digits, also had three 

 phalanges, like the other digits ; for in Fleming's translation of 

 Chauveau's work ^ an abnormal hand of a Pig is given (Fig. 89 of 

 these pages), in which the five digits are present, and the thumb is 

 shown with three phalanges : ^ moreover, its metacarpal bone is 

 amalgamated with the trapezium of the carpus. 



Then in the Ichthyosaurs there does not seem to be any dis- 

 tinction between the thumb and the other digits ; and when we 

 consider the large number of phalanges there is in the hand and 

 foot of these extinct animals, we begin to see that homology, 

 beyond a certain stage, cannot always be traced simply from the 

 number of bones in a limb. The carpal bones in their descent 

 have undergone so much alteration and displacement that no 

 accurate conclusion can be drawn from this source in all cases. 

 Even in Monkeys, the majority of which have five digits, there is 

 a section with eight, and another with nine carpal bones. 



Why this so-called archetypal form of limb gave rise to such 

 a vast series of five-digited animals we cannot tell. We may, 

 however, assert with some confidence that it was not the five digits 



^ Comparative Anatomy of Domestic Atiimals,f. 122. 

 ^ Supposing this not to be an oversight. 



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