THE FROG AS A REPRESENTATIVE VERTEBRATE 13 



Fig. 4. — Other representatives of the Phylum Chordata. 



A, an. attached tunicate, Sub-phylum Urochordala. a, larval free-swimming stage of 

 same. B, Amphioxus, Sub-phjlum Cephalochordata. C, Balanoglossus, Sub-phylum 

 IJemichordala. 



A species may be divided into varieties, if it shows different 

 types that do not warrant specific distinction. The amount of 

 difference between the individuals compos- 

 ing any species can be appreciated only if 

 one examines specimen after specimen and 

 makes exact comparisons. For such a 

 purpose, collections of shells, containing 

 large numbers of individuals, present con- 

 venient illustrative material. In general, 

 the intra-specific differences that consti- 

 tute varieties are slight, as are also the 

 differences between species. Whether a 

 given group shall be a variety of an exist- 

 ing species, a new species, or even a new 

 genus, depends upon the judgment of the 

 individual taxonomist making the classi- 

 fication. Some tend to make many species, 

 others few species, of the same material. 

 Taxonomists exhibiting these tendencies 

 are termed " spHtters " and " lumpers," 

 respectively, by those who pursue the oppo- 

 site course. 



The foregoing catalogue of unfamiHar 

 names is less interesting than many phases 

 of Zoology, but it illustrates the principles 

 of classification that are consistently fol- 



Fig. 5. — Skeleton of a 

 Permian stegocephal- 

 ian, Eryops, an extinct 

 type of the Class Am- 

 phibia. 



(Photo, by courtesy of 

 the American Museum of 

 Natural History.) 



