92 ORGANIC EVOLUTION 



and we give expression to this fact when we establish the 

 two major divisions of the plant kingdom ; namely, Phanero- 

 gamia and Cryptogamia. These phenomena of taxonomy 

 or classification were unintelligible until the theory of evolu- 

 tion gave us the talismanic word relationship. 



Homology. 



There are other phenomena of comparative anatomy 

 fully as important to the student of evolution. The phe- 



a b c d 



FIG. 14. Skeletons of fore limbs of various vertebrates, 

 a. Wing of a bird. b. Fore leg of a dog. c. Arm of man. d. Wing of bat. 



nomena of homology are of great interest. The wing of a 

 butterfly and that of a bird serve the same purpose and 

 are built on the same mechanical principle, but they are 

 fundamentally different in their structure. On the other 

 hand, the wing of a bird and the fore leg of a dog, while 

 used for very different purposes and appearing superficially 

 to be very different, are in reality very much alike in their 

 fundamental structure (Fig. 14). Each has four chief divi- 

 sions, upper arm, fore arm, wrist, and hand, and in each 

 we find the same bones, except that the number of fingers has 



