THE FROG 35 



we shall find better examples of serial homology among 

 the Invertebrates. 



The limbs of the frog have undergone considerable modi- 

 fication in connection with its swimming and leaping habits ; 

 the limbs of man are more typical, but the best examples of 

 typical limbs are to be found among the long-tailed amphibia 

 and the reptiles. 



When we say that a limb or any other organ is typical, we 

 mean that it conforms to a pattern which we recognise as 

 being the fundamental standard to which all the limbs belong- 

 ing to members of the group may be referred. This funda- 

 mental pattern does not always exist in Nature, though it does 

 in the case of limbs. It may be, in the case of other organs, 

 an abstraction, a design, of which we form the idea after the 

 comparison of a great number of limbs, all resembling one 

 another in some particulars, but differing from one another 

 in this character and in that. In the case of limbs, the 

 differences are chiefly in the suppression or fusion of parts : 

 it is rare that there is an addition of parts as in the case of the 

 supplemental toe of the frog's foot. 



The examination of a large number of cases leads to the 

 recognition of the following plan of structure for the limbs of 

 amphibia, reptiles, birds, and beasts ; those vertebrata which 

 we consequently class together as Pentadactyla. 



Suppose the animal to be placed on the table, belly down- 

 ward, and its head pointing away from the observer; a 

 straight line drawn from the mouth to the tip of the tail 

 divides it into equal and symmetrical right and left halves. 

 This line is called the principal axis of the body, and as the 

 halves into which it divides the animal are alike, the animal 

 is called bilaterally symmetrical. The skull and vertebral 

 column coincide with the principal axis, hence we have 

 referred to them as the axial skeleton. Let two lines be 

 drawn at right angles to the principal axis from the points 

 where the limbs are joined on to the body ; these will be the 

 two secondary axes. Let us suppose the limbs to be 

 straightened out along the secondary axes, as in the diagram, 

 palms downward. Then we find that the humerus in the 

 arm, and the femur in the leg, correspond with the secondary 

 axes. In the fore-arm the radius lies in front of the secondary 

 axis, and is called pre-axial ; the ulna lies behind it, and is called 



