356 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



and tibia through the medium of the carpus and tar- 

 sus, and it is these elements therefore which function 

 in the use of the limb. We have here an evident 

 illustration of the effect of disuse in effecting the 

 atrophy of an element, and of use in increasing the 

 size and complexity of an adjacent element, of the 

 same organism. No other explanation seems possible, 

 for the elements which are reduced and those which 

 are enlarged, are subjected in every other respect to 

 the same conditions. 



d. Atrophy of Incisor Teeth. 



This is complete in both jaws of existing Edentata; 

 the upper jaw of Dinocerata and many Artiodactyla, 

 and is partial in the upper jaw in various Chiroptera 

 and Lemuridae. We have already seen (p. 326) that 

 the superior incisors of certain Lemuridae are without 

 utility, owing to the conversion of the inferior incisors 

 into a horizontal comb. I have ascribed the reduction 

 of the superior incisors of bats to disuse consequent on 

 the adoption of a frugivorous diet. 1 Further reason, 

 which is common to the living members of the orders 

 mentioned, is to be found in the disuse which has fol- 

 lowed the use of the tongue as an organ for the pre- 

 hension of food. The fruit-eating bats with most re- 

 duced incisors (Glossophaginae) carry the soft parts of 

 fruits into the mouth with the tongue. The Edentata 

 use the tongue for the collection of both insect and 

 vegetable food, projecting it far exterior to the mouth. 

 The Artiodactyla without superior incisors however, 

 combine the prehensile use of the tongue with a use of 

 the lower incisors, which bite off the grass thus seized, 



\Mechan. Origin, etc., Mammalia, 1889, p. 224. See also Dr. H. Allen, 

 Proceeds. Academy, Philadelphia, 1891, p. 451. 



