ORGANIC EVOLUTION 223 



birds, and whales with the fishes. That the very many 

 different sorts of things living in the water were for a long 

 time merely fishes, is witnessed by the common names they 

 still bear: shell-fish, crayfish, jelly-fish, cuttle-fish, etc. 

 Such classification was based on the recognition of the most 

 superficial characters only. Generally the more funda- 

 mental characters are the less obvious ones, and are found in 

 internal organs, and in developmental phenomena. 



The earliest anatomical classification of land animals, 

 based on the number of feet bipeds, quadrupeds, hexapods, 

 octopods, decapods, centipedes and millipedes was 

 vastly improved when the bipeds and quadrupeds and 

 fishes got together on the basis of the common possession of 

 a spinal column as the group Vertebrata, and all the others 

 were dissociated therefrom as Invertebrata. But the 

 development of embryological knowledge in a later period 

 showed that there are characters more fundamental than 

 the vertebrae ; that certain of the invertebrates possess in 

 common with all the vertebrates, pharyngeal gill clefts and a 

 notocord; hence Cordata replaces Vertebrata as the 

 more comprehensive group name. 



Homologies and analogies. Our judgment of the like- 

 nesses between organisms, or between the parts of a single 

 organism, is based on that essential identity of parts that we 

 call homology*. Two organs are homologous when com- 

 posed of like parts in similar relations, each to each. Thus, 

 the hand of a man (fig. 261) and the fore foot of a sala- 

 mander (fig. 262) are homologous, since they are com- 

 posed of the same parts put together in essentially the 



*A few exceptional organisms, like certain bacteria, are so simple 

 in structure that differences in their bodily organization are hardly 

 discoverable : and their recognition depends in part at least on 

 their manner of growth in culture media, and in the nature of the 

 by-products of their activity. 



