3o THE STRUCTURE AND LIFE OF BIRDS chap. 



well first to explain what is meant by the anatomical 

 terms rudiment, and homology and analogy. 



A rudiment is an organ which survives, though it has 

 become wholly or almost functionless. Often it is much 

 reduced in size. The Python has hind legs, the claws 

 of which are sometimes just perceptible through the 

 skin. Men have a muscle for moving the ear forward, 

 but very few are able to use it. The Sea-lion has claws 

 at the end of his toes, but the skin projects far beyond 

 them, so that the claws are absolutely useless. Crabs 

 which live in caves to which no light penetrates have 

 eye-stalks with no eyes on the top of them. Perhaps 

 the most startling rudiment of all is the Pineal body 

 found in lizards, birds, and mammals, and believed to 

 represent a central eye. 



Two organs are said to be analogous when their 

 functions are the same ; they are homologous when 

 they are the same in nature and origin. The wing 

 of a bird and the wing of an insect are analogous 

 to one another because they do the same work. 

 The origin of an insect's wing is not known for 

 certain, but there is no doubt that it is not a fore- 

 limb. The relationship, therefore, is one of analogy 

 only. The wing of a bat is only analogous to the 

 wing of a bird ; it is not homologous, for, besides 

 the fact that all five fingers are found in it, it is 

 supported by the leg as well as the arm. There is 

 a small fish called Periophthalmus Kolreuteri, which 

 suns itself upon rocks with only its tail in the water. 1 



1 See Professor Haddon's paper, Nature, January 17, 1889. 

 The fish soon died when its caudal fin was coated over with gold- 

 size. See also Professor Hickson's Naturalist in Celebes, p. 30. 



