TRANSFORMATION OF ORGANS OF ANIMALS 43 



the rods nearest to the free extremity of the 

 principal rod were considerably reduced in size. 



Two series of transformations should now be 

 followed that of limbs which have not yet 

 ceased to be aquatic, and that of limbs adapted 

 to terrestrial existence. 



Among aquatic creatures, the Pleuracanthides 

 and the Dipneusti (Xenacanthus and Ceratodus) 

 have best preserved the bipinnated fin. In 

 Ceratodus especially it is exhibited in almost the 

 primitive condition. Gradually, however (as in 

 Orthacanthus), the rods situated along one edge 

 of the principal rod disappeared, and the fin, no 

 longer bipinnated, became unilateral. This pro- 

 gressive transformation entailed the disappearance 

 of nearly half of the fin. 



With creatures which have become adapted to a 

 terrestrial life, the limbs so far as can be judged 

 from what is known at present appear to have 

 undergone the following transformations : The bi- 

 pinnated fin (such as that of Ceratodus) (fig. 18) 

 is always the starting-point. Then the lateral rods 

 of one half almost completely disappear (as in 

 (Protopterus Amphilius). Next, the other half 

 follows (as in P. annecteus) (fig. 19), and finally 

 only the principal rod remains (as in Lepidosiren) 

 (fig. 20). 



At this point the lirnbs are reduced to mere 

 lopped stems ; they have not, however, atrophied ; 

 the degeneration which has accompanied this de- 



