262 STUDIES IN THE EVOLUTION OF ANIMALS 



instance — ^the Alligator {Caiman latirostris) has only four digits in 

 the hind-limb, thus furnished : — 



1st digit, two phalanges, 



2nd digit, . three phalanges, 



3rd and 4th digits, four phalanges each. 



Then in the Brazilian Tortoise {Testudo tabulata) the hind-limb 

 is as follows; — 



1st digit, . . two phalanges, 



2nd, 3rd and 4th digits, two phalanges each, 



5th digit, may be one, or may be two ; 



and all the bones of the first row of the tarsus are lumped into one 

 bone, so that the modifications in the foot of this Tortoise have 

 been very great. 



If we turn to the birds, we find the modifications in their hands 

 and feet still more astonishing. 



Among mammals, again, we find a curious modification in the 

 hand of the Little Ant-eater {Cycloturus didactylus). The third 

 digit has only two phalanges, a stout and broad metacarpal bone, 

 and the distal row of the carpus is often lumped into one bone. 



These are a few examples of animals in which the bones of the 

 hand have become so modified as to make it very doubtful 

 whether all their homologies can any longer be traced. The study 

 of the young animals where the ossification may be still incomplete 

 will no doubt help in discovering their homologies. But it requires 

 great faith in the acumen of a professor to believe implicitly all he 

 may write or say regarding homologies ; for in this little Ant- 

 eater's hand ^ the bone marked (v.), as the rudiment of a fifth digit, 

 may be one of the carpal bones, and that marked (tm), as the 

 trapezium, may be the metacarpal bone of the first digit ! 



■' Fig. 108 — Flower's Osteology of Mammals. 



