ORGANIC EVOLUTION 223 



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

 different sorts of things Hving 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 ffig. 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 m part at least on 

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

 by-products of their activity. 



