16 GENERAL SURVEY OF THE. ANIMAL KINGDOM. 



characters. On the other hand, the distinction between one species ana 

 another should always be greater than any difference between the 

 members of a family (using the word family here to mean the progeny 

 of a pair). For no one would divide mankind into species according to 

 the colour of eyes or hair, as this might lead to the absurd conclusion 

 that two brothers belonged to different species. Thus it is often doubly 

 unsatisfactory when a species is established on the strength of a single 

 specimen (a) because the constancy of the specific character is undeter- 

 mined ; (b] because the variations within the limits of the family have 

 not been observed. Indeed, it has happened that one species has been 

 made out of a male, and another out of its mate. 



3. Although cases are known where members of different species 

 have paired and brought forth fertile hybrids, this is not usual. The 

 members of a species are fertile inter se, but not usually with members 

 of other species. In fact, the distinctness of species has largely depended 

 on a restriction of the range of fertility. 



TABULAR SURVEY. (For Future Reference) 



METAZOA CHORDATA 



fEutheria. "| g g 



-( Metatheria. Marsupials. 

 l^Prototheria. Monotremes. Oviparous. J *$ g 



MAMMALIA. 



AVES. 



REPTILIA. 



AMPHIBIA. 



PISCES. 



CEPHALOCHORDA. Amphioxus. 



UROCHORDA. Tunicates. 



HEMICHORDA. Balanoglossus, Cephalodzscus 



(Crocodilia. Crocodiles and alligators. 

 Ophidia. Snakes. 

 Lacertilia. Lizards. 

 Rhynchocephalia. Sphenodon. 

 Chelonia. Tortoises and turtles. 

 Extinct Classes. 



'Anura. Tail-less frogs and toads. 

 Urodela. Tailed newts. 

 Gymnpphiona, e.g. Ccecilia. 

 Labyrinthpdonts and other extinct 

 , Amphibians. 

 Dipnoi. Mud-fishes. 

 Teleostomi. Bony fishes, etc. 

 Elasmobranchii. Cartilaginous fishes. ^ 

 Myxine), and Lamprey 



