

16 GENERAL SURVEY OI< THE ANIMAL KINGDOM. 



of eyes or hair, as this might lead to the absurd conclusion that t\\" 

 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 ; 

 (/') because the variations within the limits of the family have not been 

 observed. Indeed, it has happened that one species lias 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. 7 '/it.' 

 members of a species arc fertile inter se, but not usually u'itk 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. 



TAUULAR SURVEY. (For Future Reference.} 



METAZOA CHORDATA. 



f Eutheria. \ 



Metatheria. Marsupials. 

 LPrototheria. Monotremes. Oviparous. I 

 / Carinatae. Keeled flying birds. 

 J Odontolcae. Extinct toothed birds. 

 I Ratitae. Keel-less running birds. 

 V. Extinct reptile-like birds. 



/Crocodilia. Crocodiles and alligators. 

 I Ophitlia. Snakes. 

 | Lacertilia. Lizards. 

 I Rhynchocephalia. Sphf notion. 

 I Chelonia. Tortoises and turtles. 

 I Extinct Classes. 



/"Anura. Tail-less frogs and toads. 

 I Urodela. Tailed newts. 

 J Gymnophiona, e.g. C&cilia. 

 I Labyrinthpdonts and other extinct 

 \_ Amphibians. 

 ( Dipnoi. Mud-fishes. 

 j Teleostomi. llony fishes, etc. 

 I, Elasmobranchii. Cartilaginous fishes. 



s}l <^>f "<'>, and lamprey 

 etromyzon). 



CEPHAI.OCIIORDA. A inpJrio.\i<s. 

 UKOCHORDA. Tunicates. 

 HEMICHORDA. fialanoglossus, 



MAMMALIA. 



AVF.S. 



REPTII.IA. 



AMPHIBIA. 



PISCES. 



CYCLOSTOMATA. 







a 

 o 



rt 



a 

 o 



\ 



rt 

 ri. 



O 



r- O c/i 



.53 



