HEREDITY 



173 



a 



corresponding details of the other. But in comparing the arm 

 of man with the "limb" of a tree, the arm of a starfish, or the 

 foreleg of a grasshopper, we find no correspondence in details. 

 In a natural classification, or one founded on fact, organisms 

 showing the closest homologies are placed together. An arti- 

 ficial classification is one based on analogies. Such a classifica- 

 tion might place together a cricket, a frog, and a kangaroo, 

 because they all jump, or a bird, a bat, and a butterfly, because 

 they all fly, even though 

 the wings are very dif- 

 ferently made (Fig. 105) 

 in each case. 



The very existence 

 of such terms as animals 

 and plants, insects and 

 mollusks imply relation- 

 ships, and relationships 

 in different degrees. 

 Classification is the 

 process of reducing our 

 knowledge of these 

 grades of likeness and 

 unlikeness to a system. 

 By bringing together 

 those which are funda- 

 mentally alike, and 

 separating those which 

 are unlike, we find that 



these traits are the tmtcome of long-continued influences. 

 Classification is defined as "the rational lawful disposition of 

 observed facts." It rests on the results of the operations of 

 natural laws, or forces which bring about inevitable results. 



For it is a matter of common observation that the closest 

 homologies are shown by those animals which have sprung from 

 a common stock. The fact of blood relationship shows itself 

 always in homology. So far as we know, homology is never 

 produced in any other way, therefore the actual presence of 

 homologies among animals or plants implies, as we shall see in a 

 later chapter, their common descent from stock possessing these 

 same characters. In our primitive use of the trunk of the tree 

 to imply unity in life, we can see that this trunk represents 



FIG. 105. Diagram of wings, showing homol- 

 ogy and analogy: a, wing of fly; b, wing of 

 bird; c, wing of bat. 



