18 EVOLUTION AND DISEASE. 
and found the muscular coat increased in thickness. 
This simple experiment has been varied by other 
observers with similar results. 
The large breast muscles (pectorals) of birds associated 
with and varying according to the expanse of wing, 
furnish a good example of the relation of increased size 
with augmented use ; these may be compared with the 
Fic. 9.—A medium longitudinal section through the third 
finger of ahorse, and man. M, metacarpal bone ; I, 2, 3, 
phalanges. The ossicle, N, situated at the junction of the 
second and third phalanges is the navicular. 
powerful leg muscle of the frog (known as the gastro- 
cnemius) so important in the act of swimming, and the 
large muscles of the human buttock, useful in helping 
man to maintain the erect position. These are striking 
instances of the inherited effects of increased use of a 
part. 
‘ 
ee 
