ORGANIC EVOLUTION 



2 59 



the separate coracoid bone of the vertebrate shoulder girdle 

 is at once evident when its development is studied; for it 

 arises as a separate bone and has at first the usual position 

 and relations of the coracoid, and later becomes anchylosed 

 with the scapula. Such difficulties as exist in determining 

 what are the principal veins of the wing of an insect, and 

 what is the mode of their branching, disappear when one 

 studies in the immature wing of a primitive insect, such as 

 the stonefly shown in figure 158, the arrangement of the 

 tracheae, about which subsequently the veins are formed. 



The correspondence between ontogeny and phylogeny 

 suggests an explanation of hosts of vestigial structures 

 found in embryos. They are inheritances out of the past. 

 Such are the rudimentary incisor teeth of cattle which 

 though present in the embryo never cut the gums, and are 

 wanting in the adult animal. Some of them are useless old 

 heir-looms, of no practical consequence; and many of them 

 are of no value save as they condition the development of 

 other parts. Such, also, are the gill-pouches of bird and 

 mammal embryos; these appear , only to disappear again 

 (save the foremost slit, in connection with which the eusta- 

 chian tube of the ear is developed) , and no gills are formed 

 in connection with them. 



Ontogeny often explains the absence in the adult of 

 parts that might be expected. Thus, the carpus of a bird 

 appears to be represented by but two bones, if we study only 

 the bones of the adult bird; but in the young bird an 

 additional row of separate carpal bones is found (fig. 159) 

 in much the same relations as in other vertebrates. 



The tendency of the individual in its development to 

 repeat and reproduce ancestral characters is but the out- 

 ward sign of that inward force we designate as heredity 

 the force which tends to make like produce like. Perhaps 

 it is only a sort of developmental inertia. Racial history 



