WHAT IS EVOLVED IN EVOLUTION? 2JI 



Example of an Intrinsic Character To take an illustration : 

 the character which distinguishes the Spiriferidae from the 

 Terebratulidae and the Rhynchonellidae, called the brachid- 

 ium, is fundamentally an intrinsic character, because in the 

 fixation and rigidity of parts there is implied an adjustment of 



FIG. 62. Brachial apparatus of (i) Rhynchonella, in which only the crura are developed ; (2) Ma- 

 gellania, showing the crura with the looped bands of the brachidium ; and (3) Athyris, with 

 no loops but the brachidial bands extended in spiral coils. 



the other parts of the organism to these conditions; and in the 

 apprehension, the distribution, the deposition, and the sup- 

 ply of materials for constructing the apparatus, there is implied 

 an adjustment of the whole organism to the work of con- 

 structing this new part. Even though the soft parts were 

 essentially the same in the Orthis and the Spirifer, the modi- 

 fication in the Spirifer is a radical one, involving the whole or- 

 ganism, and not merely the particular part concerned. 



Example of an Extrinsic Character, On the other hand, the 

 character distinguishing the spires of the Atrypidae from 

 those of the Spiriferidae is the permanent turning of the point 

 of the cone toward the centre of the valve in the Atrypa, and 

 toward the upper outer angle of the shells in the Spirifer. 

 This is a matter of adjustment which may involve a slight 

 rearrangement of the relations of parts, but may involve no 

 more; the difference in the shape of the shell itself may 

 occasion such adjustment, as a tight shoe might distort the 

 shape of the foot (Figs. 63, 64). 



