THE ORIGIN OF SPECIES 353 



the fundamental resemblances. This similarity in dis- 

 similarity is brought out by the science of comparative 

 anatomy. A few concrete examples, some of which we are 

 already familiar with, will serve to bring the main facts clearly 

 before us. 



The fore-legs of Frogs and Lizards, the wings of Birds, the 

 fore-legs of the Horse, and the arms of Man are built on the 

 same basic plan. (Figs. 80, 81, 185, 186.) The same is true of 

 the hind-limbs. Clearly all are homologous structures, such 

 variations as exist being brought about chiefly by the modi- 

 fication or absence of one part or another. In short, all the 

 chief parts of both the fore-limbs and the hind-limbs are 

 homologous throughout the series. All are composed of 

 the same fundamental materials disposed in practically 

 the same way nearly all the bones, muscles, blood vessels, 

 and nerves are homologous. Or compare the digestive sys- 

 tems of the same forms, or the excretory and reproductive 

 systems. One has but to recall that, on an earlier page, it 

 was possible to describe in general terms these systems as 

 they exist throughout the Vertebrate series in forms as 

 obviously different as Fish and Man. They are all funda- 

 mentally the same. (Figs. 82-87, 97.) 



Turning to the Invertebrates, we may remind the reader 

 that all the appendages of the Crayfish are built on the same 

 simple biramous plan as exhibited in the swimming legs 

 (swimmerets) of the abdomen. The highly specialized 

 walking legs, great claws, jaws, and feelers (antennae and 

 antennules) are all reducible to modifications of the simple 

 swimmeret type. (Fig. 72.) In short, all are homologous 

 structures, though differing widely in function. This is a 

 most striking example of SERIAL HOMOLOGY, though we have 

 seen the same principle exhibited in the Vertebrates where 

 the fore-limbs and the hind -limbs of each animal are homolo- 



