
IN NATURAL HISTORY 15 
and tail fins have no fin rays, and the bones of the side fins are like those 
of the fore leg of a quadruped. In all these points they differ from fishes 
and agree with other mammals. 
The Genus! is next below the family and includes animals that have 
some character or characters in common, though differing from one an- 
other in smaller, or specific characters. Like the groups already dealt 
with a genus may contain one, or many species; for one species may dif- 
fer so much from any other as to require a place, or genus, by itself, while 
a number of distinct species may possess some common character. Thus 
a large proportion of al] squirrels belong in the genus Sciurus, and the 
true cats, great and small, the lion and the common cat (for size has 
nothing to do with relationship) are included in the genus Felis; the 
short-tailed cats are placed in the genus Lynz, and the hunting leopard 
or cheetah, whose claws are only partly retractile, and cannot be drawn 
within their sheaths as in other cats, in the genus Cynelurus. 
Lastly comes the Species,? whose members constantly resemble one 
another in all essential particulars of form, size and color, the exceptions 
being the geographical races or subspecies mentioned elsewhere. The 
species may be called the unit of classification, and subspecies may be 
looked upon as fractions. 
Still using the lion as an illustration, the various groups to which 
this animal belongs are: Phylum, Vertebrata; Class, Mammalia; Order, 
Fere, Suborder, Fissipedia (split-footed or clawed); Family, Felidae; 
Genus, Felis; Species, Leo. This is clearly shown in the accompanying 
table illustrating the position of the Lion in the Animal Kingdom. 
Anyone who examines a few systems of classification may find that 
they do not agree with one another in all points: this, however, is no 
more surprising than that people differ in matters of religion, politics, or 
schools of medicine. Any system is to some extent an expression of in- 
dividual opinion, and two persons will rarely agree on all questions, 
even in natural history. 
It may appear strange that one order should contain only one or 
two species, while another comprises hundreds, even thousands. But the 
importance of a group does not depend on the number of species it in- 
cludes, but on the extent to which these resemble or differ from those of 
other groups, orders, or families, as the case may be; fifty cents make a 
larger pile than does a single dollar, but they do not form a more im- 
1Latin genus, a race or kind. ; 
?Latin species, a particular sort: be sure never to say sperte. 
