30 THE STRUCTURE AND LIFE OF BIRDS cuHap. 
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 Kdlreuteri, which 
suns itself upon rocks with only its tail in the water. 
1 See Professor Haddon’s paper, Mature, 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. 
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