308 Heredity. 



symmetry and general homology, the origin of polymor- 

 phism, the definition of an individual or person, etc.; 

 but the end of a book is not the place to enter upon a 

 a new field, and I am forced to reserve this subject for 

 future discussion, although I will now indicate very 

 briefly the nature of this explanation. 



The basis of modern morphology is the doctrine that 

 homology indicates genetic relationship. 



Homology is fundamental similarity of plan, as distin- 

 guished from difference or similarity in physiological 

 function. For example a man's arm and hand are fitted 

 for grasping, and a bird's wing for flight, and their dif- 

 ferent uses render them unlike each other in a superfi- 

 cial view, although there is below and behind this ob- 

 vious difference a more deep-seated resemblance. The 

 feathers which cover the bird's wing have the same his- 

 tological structure and the same origin as the hairs upon 

 the human arm; the skin which covers the limb has the 

 same character in both cases; the wing, like the arm, 

 has a supporting skeleton, which consists of a shoulder 

 and upper arm, a forearm, a wrist, and a hand; the 

 muscles have the same structure and the same general 

 arrangement, and the way in which they are supplied 

 with nerves and blood-vessels is the same. 



This fundamental identity of structure which is obscur- 

 ed, but not destroyed by the difference of use, is homology. 

 In a superficial view the wing of a bird resembles the wing 

 of a dragon-fly more closely than it resembles a man's arm, 

 but careful examination shows that the insect's wing is 

 not a limb at all, but a peculiar outgrowth from the body. 

 The resemblance between a bird's wing and an insect's 

 wing is not an homology, and it has no morphological 

 significance : it does not indicate that there is any close 

 relationship between a bird and an insect. 



