252 PRINCIPLES OF ANIMAL BIOLOGY 



(Fig. 207). Both are used for absorbing oxygen but are wholly different 

 in structure. 



The foregoing analogous organs are so unlike in structure that no one 

 would be led to classify together the animals that possess them. Not 

 always, however, are the structural differences so obvious externally. A 

 whale swims by means of paddles and a flattened tail which greatly 

 resemble fins, and the early naturalists regarded whales as fishes. Yet 

 the whale is a warm-blooded air-breathing animal with a four-chambered 

 heart and some hair on the skin and has also the other characters of 

 mammals, while the fishes are cold-blooded and aquatic, and have a 

 two-chambered heart and scales in the skin. A close resemblance is also 

 exhibited by certain lizards (Amphisbaenidae) to a group of snakes 

 (Typhlopidae), because the former are blind and legless and have a short 

 tail. These external similarities have apparently arisen in evolution 



A B 



Fig. 207.— Analogous structures; respiratory organs: A, gills of salamander; B, lung of 



frog. {From Wienian, "General Zoology.") 



independently of one another and for that reason are not an indication of 

 kinship. 



Homology. — In judging of kinship by means of structural similarities, 

 therefore, care must be taken to employ only those structures that 

 have had similar origins in evolution. It is sometimes difficult to deter- 

 mine now whether similar structures in two groups of animals arose in 

 evolution in the same way, or have converged for some reason from 

 originally distinct beginnings. In general, if two or more groups of 

 animals have one or a few structures in common while all others are 

 different, it is safer to assume that the common structures arose inde- 

 pendently, or at least that their recent evolutionary developments have 

 been independent, and that the groups are therefoi'e not closely related. 

 The lobate feet of the several groups of birds mentioned above fall in 

 this category. If, however, a gi-eat many features of two groups of 

 animals are closely similar, the probability is that such similarities could 

 only have come from similar or identical origins in evolution. The work 

 of the taxonomist therefore becomes, in large measure, the recognition of 

 those characters in different animals whose similarities are due to com- 

 mon evolutionary origin. 



