GENERAL LAWS OF ANIMAL STRUCTURE. 2 6l 



tial identity and derivative origin of parts may be ob- 

 scured or even obliterated. If, on the other hand, it be 

 too small, then the identity may have come in some 

 other way than by common origin — may have been made 

 out of hand at once. Now, this equal balance is found 

 in the skeleton, and especially in the limbs. In the mus- 

 cular system the adaptive modification is too extreme ; it 

 obscures the derivative origin. In the case of the vis- 

 ceral system, on the contrary, there is scarcely enough 

 modification to make any evidence. Next to the skele- 

 tal, the best evidences are found in the nervous system. 

 Here the essential identity of parts in all vertebrates, 

 and yet their modification in each class, is very striking, 

 especially in the brain. We have already explained this 

 (pages 76-78). In the case of the muscular system the 

 modification in passing from the fish to the land verte- 

 brates is so extreme that all hope of homology seems 

 vain. But in land vertebrates, from the amphibia (frogs, 

 etc.) to man, it may probably be traced by careful study, 

 but this has not been attempted, except in a fragmentary 

 way. Undoubtedly a rich field is open here. 



III. SERIAL HOMOLOGY. 



There is an evident correspondence in the several 

 parts of the fore and hind limbs. The hip girdle corre- 

 sponds with the shoulder girdle, and bone for bone, 

 although the parts are more consolidated in the former; 

 the femur corresponds to the humerus ; the two bones 

 of the leg to the two bones of the forearm, each to each ; 

 the seven bones of the ankle to the eight bones of the 

 wrist, two of the former having been consolidated into 

 one ; the five bones of the instep (metatarsal) to the five 

 bones of the palm (metacarpal) and the fourteen bones 

 of the toes to the fourteen bones of the fingers. This 

 introduces us to the idea of a serial repetition of similar 



