246 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



After these illustrations, we come back to define 

 again these terms. 



Analogy is founded on function. Homology on com- 

 mon origin by descent. Parts of most diverse origin 

 may be modified to perform the same function, and 

 therefore assume a general resemblance. This is anal- 

 ogy. Parts of the same origin — really the same parts in 

 different species — by modification for different functions 

 may become so different that they are no longer easily 

 recognized as the same part. This is homology. In 

 the one case the parts seem like the same and behave 

 like the same, but are really very different ; in the other 

 they are really the same part, but they seem to the su- 

 perficial observer to be very different both in appear- 

 ance and inbehavior. In the one case there is a superfi- 

 cial resemblance produced by functions easily observed, 

 and therefore determining popular names; in the other 

 there is a deep-seated resemblance shown by essential 

 structure and structural relations, but more or less ob- 

 scured by adaptation to different functions. This deep- 

 seated resemblance can only be found by wide com- 

 parison in the animal series and in the embryonic series, 

 for it is in this way only that we find the steps of grada- 

 tion connecting. 



There are therefore two ideas underlying homology, 

 viz., common descent and adaptive modification. Things 

 having common origin by descent may be so modified 

 to adapt them to different functions as to conceal their 

 common origin. It is the duty of the morphologist to 

 trace out the evidences of common descent in spite of the 

 obscurations of adaptive modification. 



The idea of homology or common origin with obscura- 

 tions by adaptive modification, lies at the very basis of 

 biology and must be universal. We have therefore an ex- 

 cellent example of it in essential cell structure charac- 



