368 



BIOLOGY 



tary canal, similar circulatory, nervous, and excretory systems, 

 and all other parts of their anatomy are essentially alike. Such 



a likeness in struc- 

 ture is sometimes 

 found in quite unex- 

 pected places. One 

 would hardly expect, 

 for example, that the 

 arm of man, the fore 

 leg of a horse, the 

 wing of a bird, the 

 fore leg of a frog 

 and the fin of a fish 

 would be identical 

 structures, since they 

 vary so much in shape 

 and function; but 

 they are all found to 

 be homologous. 



Analogy. A second type of likeness is similarity in function, 

 irrespective of structure. It not infrequently happens that dif- 

 ferent animals develop organs of similar functions but of 

 totally different structure. In this case they are said to be 

 analogous (Gr. ana = according to -+- logos = ratio) but not 

 homologous. For example, the butterfly and the bird have 

 both developed wings for flying, and their wings are hence analo- 

 gous. They are of similar shape and are used much in the same 

 way; but the wing of the bird is made of bones, muscles, nerves, 

 and feathers, while the wing of the butterfly has none of these 

 parts, being simply an outgrowth of the skin containing air 

 tubes. It is not homologous with the bird's wing, in spite of 

 similarity in shape and function. The wing of the bird is, how- 

 ever, both analogous and homologous with the wing of the bat, 

 since both are used for similar purposes and both are made of 

 similar bones and muscles, nerves and blood vessels. As another 



FlG. 149. A SEGMENTED WORM RELATED TO 



THE EARTHWORM, BUT HAVING TENTACLES 

 AND GILLS 



