1 6 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



brothers belonged to different species. Thus it is often doubly unsatis- 

 factory when a species is established on the strength of a single specimen 

 — (a) because the constancy of the specific character is undetermined ; 

 (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. But the characters of a single 

 specimen are sometimes so distinctive that the zoologist is safe in 

 making it the type of a new species, or even of a new genus. 



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. 



To sum up, a species is but a relative conception, convenient when 

 we wish to include under one title all the members of a group of 

 individuals who resemble one another in certain characters. There is 

 no absolute constancy in these specific characters, and one species often 

 melts into another, with which it is connected by intermediate varieties. 

 At the same time, the characters, on account of which the naturalist 

 gives a specific name to a group of individuals, should be greater than 

 those which distinguish the members of any one family, should show 

 a relative constancy from generation to generation, and should be 

 associated with reproductive peculiarities which tend to restrict the 

 range of mutual fertility to the members of the proposed species. 



Tabular Survey of Chief Classes.- 

 Reference). 



METAZOA CHORDATA. 



-(For Future 



Aves. 



fEutheiia. 

 Mammalia. 1 Metatheria. Marsupials. 



(Prototheria. Monotremes. Oviparous. 

 ( Carinatae. Keeled flying birds. 

 S Ratjtas. Keel-less running birds. 

 (Extinct reptile-like birds. 



(Crocodilia. Crocodiles and alligators. 

 Ophidia. Snakes. 

 Lacertilia. Lizards. 

 Rhynchocephalia. Spkenodon. 

 Chelonia. Tortoises and turtles. 

 Extinct Classes. 

 rAnura. Tail-less frogs and toads. 

 I Urodela. Tailed newts. 

 Amphibia. A Gymnophiona, e.g. Ccecilia-. 



I Labyrinthodonts and other extinct 

 ^ Amphibians. 

 f Dipnoi. Mud-fishes. 

 Pisces. -I Teleostei. Bony fishes, 



j Ganoidei, e.g. Sturgeon. 

 L Elasmobranchii. Cartilaginous fishes. 



Cyclostomata. -! H fli sh Wyxine), and Lamprey 



( KPetromyzon). 

 Cephalochorda. Ampkioxus. 

 Urochorda. Tunicates. 

 Hemichorda. Balanoglossus, CepJiafodiscus. 



\ .3 s^< 



