Morphology. 55 



short and all run together, while the chief expanse 

 of the wing is composed of the shoulder and fore- 

 arm. In frogs and lizards, again, we find hands 

 more like our own ; but in an extinct species of 

 flying reptile the modification was extreme, the 

 wing having been formed by a prodigious elonga- 

 tion of the fifth finger, and a membrane spread 

 over it and the rest of the hand. (Fig. 5.) Lastly, 

 in serpents the hand and arm have disappeared alto- 

 gether. 



Thus, even if we confine our attention to a single 

 organ, how wonderful are the modifications which 

 it is seen to undergo, although never losing its typical 

 character. Everywhere we find the distinction be- 

 tween homology and analogy which was explained 

 in the last chapter the distinction, that is, between 

 correspondence of structure and correspondence of 

 function. On the one hand, we meet with structures 

 which are perfectly homologous and yet in no way 

 analogous : the structural elements remain, but are 

 profoundly modified so as to perform wholly different 

 functions. On the other hand, we meet with struc- 

 tures which are perfectly analogous, and yet in no 

 way homologous : totally different structures are 

 modified to perform the same functions. How, then, 

 are we to explain these things? By design mani- 

 fested in special creation, or by descent with adaptive 

 modification ? If it is said by design manifested in 

 special creation, we must suppose that the Deity 

 formed an archetypal plan of certain structures, and 

 that he determined to adhere to this plan through 

 all the modifications which those structures exhibit. 

 But. if so, why is it that some structures are selected 



