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 
, 
PY 
