42 THE THEORY OF EVOLUTION 



drawbacks; it is the harmonious result of all of them, 

 pointing in the same direction, which gives great 

 weight to their combined testimony. 

 ^fomparative anatomy, as the name implies, is the 

 comparison of structure and tissues in the whole 

 range of animal life. Such comparison leads to the 

 conclusion that in the animal kingdom there are 

 several distinct types of structure, characterizing the 

 grand divisions of the kingdom. Within any one of 

 these grand divisions may be found almost endless 

 diversities of structure, all of them obviously different 

 modifications of the same fundamental plan. How- 

 ever great the modification, the identity of the plan is 

 clearly to be perceived throughout, just as is the 

 theme of a musical composition, which may be 

 traced through all the variations into which the 

 composer has woven it. A few concrete instances will 

 make clear the application of this principle. Super- 

 ficially examined, the following structures would 

 seem to have little or nothing in common, but rather 

 to be constructed on totally distinct plans for al- 

 together different purposes: the fore leg of a lizard, 

 the wing of a bird and of a bat, the burrowing shovel 

 of a mole, the flipper of a whale, the fore leg of a 

 horse, the human arm and hand. When, however, 

 all of these structures are carefully dissected and the 

 bones, muscles and nerves are compared, it becomes 

 immediately apparent that the plan of structured 

 the same throughout, modified to serve different 

 uses, of running, flying, burrowing, swimming and 



