356 THE CHANGING GENERATIONS 



between these two feet is wholly superficial and relates to their use in 

 burrowing. They show adaptive convergence in form. 



Endless examples of equally obvious analogies could be cited. To name 

 a few, there are the jumping legs of grasshoppers and kangaroos, the 

 wings of butterflies and birds, the jaws of vertebrates and insects, and 

 the " poison fangs" of snakes and spiders. Organs may be analogous 

 without being very similar in form or they may be quite alike and yet 

 unmistakably analogous, because they occur in organisms otherwise too 

 unlike to be considered close kin. 



Sometimes, however, closer study is needed to detect the analogous 

 nature of a given resemblance. It is not surprising that the older natural- 



Fig. 24.1. Analogous resemblance between a lizard and a snake. Left, the legless burrowing 

 lizard called the "glass snake" (Ophisaurus ventralis). Right, a true snake (Conophis 

 lineatus) of similar appearance and habits. The two are both reptiles, but not at all closely 

 related. {Left, courtesy Zoological Society of Philadelphia; right, photo by Prof. Archie F. 

 Carr, Jr.) 



ists grouped the whales and porpoises (mammals) with the fishes on 

 account of their fishlike tails and streamlined bodies, or that they placed 

 the bivalve mollusks with the bivalved brachiopods because of similarities 

 in the shells. Some caterpillars could easily be mistaken for slugs (naked 

 snails), which they closely resemble in form and in the possession of a 

 creeping sole. The cephalopod mollusks (squid, octopus) have an eye 

 which is very like that of a fish or other vertebrate. Certain legless 

 burrowing lizards closely resemble snakes (Fig. 24.1); the burrowing 

 blind lizards (Amphisbaenidae) and "worm snakes" (Typhlopidae) are 

 blind and short-tailed and almost identical in appearance and habits. 

 But all these cases of resemblance prove to be instances of analogy, when 

 put to the test of slructural plan and mode of development. All of them 

 are the result of adaptation of very distantly related animals to similar 

 modes of life. 



Homology. Structures that are built on the same basic plan and that 

 arise embryologically in the same way are said to be homologous, or to 



