76 THE SCIENCE OF LIFE. 



From the earliest times of history down to Cuvier, 

 naturalists were in the habit of regarding similarity of 

 external form and evident purpose as indicating anal- 

 ogies, and so far as functional design is concerned, the 

 principle may be considered right. But purpose and 

 plan for a purpose are different, and modern science 

 seeks for its types in the characters of internal structure 

 and development. 



6. Parts, or organs, having similar origin and develop- 

 ment, and therefore the same essential structure, are 

 called homologous ; while those which are anatomically 

 different, though corresponding in use, are called analo- 

 gous. Thus in the vegetable kingdom the tendril of the 

 Vine, which is a transformation of the flower-stalk ; that 

 of the Pea, which is a prolongation of the leaf-stalk ; that 

 of Gloriosa, which is the point of the leaf itself; and that 

 of StrophantkuS) which is the point of the petal ; are all 

 analogous, but not homologous. The arms of Man, the 

 fore-legs of a Horse, the paddles of a Whale, the wings 

 of a Bird, the front flippers of a Turtle, and the pectoral 

 fins of a Fish, are homologous but not analogous. The 

 wings of the Bird, Flying Squirrel, and Bat are not ho- 

 mologous, since that of the first is developed from the 

 fore-limb only, that of the Squirrel is an extension of the 

 skin between the fore and hind limbs, and that of the 

 Bat is a membrane between the fingers and down the 

 side to the tail. The air-bladder of a Fish is homologous 

 with a lung, but analogous to the air-chamber of the 

 Nautilus. In the functional analogies, perhaps more 

 evidently than in the structural homologies, we trace 

 evidence of purpose, or design. " Blind metaphysical 



