THE EPIC OF EVOLUTIOiN 499 



Likewise there is frequently a convergence of structure on the part 

 of diverse organisms, as a result of adaptation to a single kind of 

 environment. When this happens in the organic world, it is called 

 convergent evolution, and it puts the comparative anatomist on his 

 guard, since resemblance between organisms does not always signify 

 relationship. Pelagic animals of the open ocean, belonging to the 

 quite different groups of coelenterates, molluscs, crustaceans, worms, 

 tunicates, and fishes, often become more or less transparent, which 

 makes it difficult for them to be seen by ravenous fishes from below, 

 or by preying birds diving down from above. Snakes, blind caecil- 

 ians, legless lizards, and eels, all have attained a similar body form, 

 but innumerable other anatomical features that they severally pos- 

 sess prove them to be not closely related, in spite of their external 

 resemblance. Most cases of convergent evolution are functional 

 rather than structural. Organisms show their relationship to each 

 other by their structure, that is, by what they are, rather than by 

 their function, that is, by what they do. 



Vestigial structures, such as the well-known degenerating vermi- 

 form appendix in man, that is absent in the cat but excessively 

 developed in the rabbit, are anatomical parts gradually disappear- 

 ing below the horizon of usefulness. Like certain finicky parlor 

 boarders, they often make trouble and there is no accounting for 

 their presence except upon the theory of evolution. Again, as a 

 final example, may be cited the lumbar plexus, which is a union of 

 spinal nerves to supply the hind legs of vertebrates. The fact that 

 some snakes have a lumbar plexus, although they have no legs to be 

 supplied with nerves, indicates that they are still hanging on to the 

 documentary evidence that establishes their relationship to other 

 vertebrates, although they have diverged far from the ancestral 

 stock. 



Evidence from Embryology 



There is a suggestive parallel between the embryonic develop- 

 ment of the individual and the more extensive course of organic 

 evolution. A whale and a mouse, both mammals, are more alike in 

 their development than a whale and a fish, which are outwardly 

 more similar. Again, that curious living fossil, the horseshoe crab 

 Limulus, which has retained its conservative individuality as a species 

 since paleozoic times, looks anatomically like a crustacean. Its 

 embryological development, however, as Kingsley has demonstrated. 



