16 GENERAL SURVEY OF THE ANIMAL KINGDOM. 
characters. On the other hand, che distinction between one species and 
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 ; (4) 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. Zhe 
members of a spectes 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 
Eutheria. Bd 
Mamma.ta. Metatheria. Marsupials. aa 
Prototheria. Monotremes. Oviparous.) & & 
Carinate. Keeled flying birds. ‘ 
Aves Qdontolcz. Extinct toothed birds. 
= Ratite. Keel-less running birds. 
Extinct reptile-like birds. 
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. 
Gymnophiona, e.g. Cecilia. 
Labyrinthodonts and other extinct 
Amphibians. c 
{eles Mud-fishes. 
Sauropsida. 
REPTILia. 
Gnathostomata 
(z.e. jawed). 
Craniota 
(with skulls). 
AMPHIBIA, 
Ichthyopsida. 
Teleostomi. Bony fishes, etc. 
Elasmobranchii. Cartilaginous fishes. 
Hag-fish (A/yxine), and Lamprey 
(Petromyzon). 
CErHALOCHORDA. Amfphioxus. } 
Pisces. 
CycLOsTOMATA. { 
Urocuorpa. Tunicates. 
Hemicuorpa. Balanoglossus, Cephalodiscus 
