ANATOMICAL PROBLEMS BEARING UPON EVOLUTION. 17 



Similar variations occur in birds, reptiles and fishes, 

 and in the light of comparative anatomy, and of the con- 

 comitant and correlated variations in other organs {e.g., 

 the nervous system) their morphological value becomes 

 extremely restricted. They are indeed merely individual 

 variations in the attachment of the limb to the trunk. 



Muscular and Vascular Variations. — Similarly with 

 other organs and systems, the capacity for variation is 

 restricted to narrowly prescribed limits. In the muscular 

 system the complexity of structure and arrangement is so 

 great that any variation, as a rule, implies the minutest 

 change in the complexity of the system. All the muscular 

 variations possible taken together give no clue to the 

 evolution of the system or the ancestry of man. A 

 deficiency of the diaphragm may recall the reptile ; but 

 on the other hand, it may only be an individual example 

 of arrested development. 



The variations in the vascular system are of com- 

 paratively small importance morphologically if we except 

 the great vessels. The mode of formation of blood-vessels 

 and their habit of inosculation, allow readily of variation, 

 mainly because of the occurrence of concomitant varia- 

 tions in neighbouring structures. Any slight obstruction 

 will cause an alteration in the origin and course of an 

 artery. 



The Value of Anatomical Variation, 



Variations appear thus to be unreliable guides except 

 within narrow limits. They are exceptions that prove 

 the rule. From abnormalities great or small, we are not 

 justified in drawing any large conclusion. They serve to 

 corroborate or confirm the mode of development of an 

 organ. They serve to indicate morphological or physio- 

 logical disturbance in the district in which they occur. 



