HEREDITY 



173 



corresponding details of the other. But in comparing the arm 

 of man with the "hmb" 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 ca-se. 



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 outcome 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 w^e know, homology is never 

 produced in any other way, therefore the actual ])resence of 

 homologies among animals or plants im])lies, 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 Ufe, we c^u §eQ that this trunk represents 



Fig. 105. — Diagram of wings, showing homol- 

 ogy and analogy: a, wing of fly; 6, wing of 

 bird; c, wing of bat. 



