LIMBS OF VERTEBRATES 



235 



Monotremata, however, are more usually met with amongst the 

 extinct animals of past geological periods. They indicate the 

 paths along which the more highly organized groups have pro- 

 gressed during their evolution from more lowly organized 

 ancestral groups, which latter may or may not still be represented 

 by surviving forms at the present day. 



It would be difficult to explain the occurrence of such graduated 

 series of organs and organisms by the theory of special creation, 

 and it would be no less difficult to explain in this way those 

 remarkable facts of comparative anatomy which are grouped 



FIG. 90. Shoulder Girdle and part of Sternum of a Babbit, seen from below. 



;la (= Vestige of C< 

 pula; St. Sternum. 



Cl. Clavicle; Cor. Coracoid process of Scapula (= Vestige of Coracoid); Gl. Glenoid 

 cavity; B. Eibs; Sc. 



together under the terms homology and analogy (or homoplasy) 

 to which we shall next refer. 



We have already had occasion to point out that all living 

 organisms are more or less perfectly adapted to the conditions under 

 which they exist, and are accordingly provided with organs suitable 

 for their various requirements. Thus all typical vertebrates have 

 organs of locomotion formed from two pairs of limbs, but these 

 differ very greatly in form and structure in accordance with their 

 adaptation to very diverse conditions of life. In terrestrial 

 vertebrates amphibians, reptiles and mammals both pairs of 

 limbs usually take the form of ambulatory legs, adapted for 

 moving the body on dry land. In birds the hind limbs are 

 generally used for the same purpose and similarly constructed, but 



