282 ZOOLOGY. 



3. Ampliioxus Primitive and Specialized Characters, 

 Thus while in the broadest facts of structure Amphioxus 

 remains at a stage left behind by very distant ancestors of 

 the other types, it has not retained all the characters of 

 those ancestral forms unchanged, but has developed, in the 

 course of its descent from them, special peculiarities of its 

 own that adapt it more and more fully to the conditions of 

 its life. This is expressed by saying that Amphioxus, 

 though belonging to a very primitive group of Vertebrata 

 (all the other members of which are extinct), is specialized 

 on lines of its own. Instances of this specialization are the 

 forward extension of the notochord, the large number of 

 gill-slits, and the great development of the atrium. We are 

 so much in the dark, however, as to the nature of the 

 common ancestor of Amphioxus and the Oraniates, that it 

 is difficult to say of many of the peculiarities of Amphioxus 

 whether they are primitive or specializations. 



4. Specialization of Dogfish and Frog. Of the other 

 types we may speak with greater certainty. The dogfish, 

 though retaining many characters that must have been 

 possessed by the common ancestor of the Craniates, has 

 specialized in the direction of greater care of its young 

 laying, as it does, but two eggs at a time, well-supplied with 

 yolk and protected by a horny case, instead of the enormous 

 number of ill-cared-for eggs that the bony fishes and the 

 frog continue to produce. The hyostylic condition of the 

 skull, the detailed structure of its paired fins, its pouched 

 gill-slits, and the elaborate arrangement of afferent and 

 efferent vessels, are also specializations which cannot be 

 regarded as intermediate between the conditions of the 

 homologous parts in Amphioxus and the frog. 



The frog again shows great specialization in the reduced 

 number of its vertebrae and complete loss of its tail ; in the 

 structure of its pelvic girdle and hind-limbs, and many 

 other characters which could not possibly have existed in 

 ancestors common to it and the rabbit. 



5. Palaeontological Evidence. Fortunately, we are 

 not condemned to mere speculation as to the extinct common 



