250 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 

 II. SPECIAL HOMOLOGY. 



But not only do we find a common general plan sug- 

 gesting common origin, but, when examined thought- 

 fully, we find all vertebrate skeletons exactly corre- 

 sponding, part for part, bone for bone, only modified by 

 necessary adaptation for various functions. The modi- 

 fications may be so great as to obscure the essential 

 identity, but extensive study of comparative anatomy 

 and embryology reveal the missing links in the continu- 

 ous chain of change. The evidence here is demon- 

 strative. 



Limbs. — To show this we take the case of the 

 limbs, partly because their structure is better known to 

 common observation, but mainly because we have in 

 them the two ideas of common origin and adaptive 

 modification, both equally illustrated. The modifica- 

 tions here have been great, but not so great as to 

 wholly conceal the common origin. 



(a) Fore Limbs. — For fore limbs we take man's as 

 the type or term of comparison because it is best known, 

 and also because it is really far less modified than many 

 others; although in this regard alone that of reptiles 

 would be the best. 



See, then, the fore limbs of various classes of verte- 

 brates (Figs. 160, 161, 162). The same parts are similarly 

 lettered in all, so that the legend sufficiently explains 

 the corresponding parts. Moreover, dotted lines are 

 drawn through the most important corresponding parts 

 to make the comparison more easy. But it is necessary 

 to say something more in the way of explanation 

 concerning several of these parts. 



1. Shoulder Girdle. — This consists of three important 

 bones — viz., the blade (scapula), the clavicle, and the cora- 

 coid. The type is seen in the reptile (Fig. 162, A) which 



