22 NATURAL HISTORY. 
varieties or breeds of dogs and horses, owing to acci- 
dental causes. 
21. The distinction, then, between different species is 
a definite and fixed one. There can be no dispute about 
it in any case where the facts bearing on the question 
are all known; but it is not so with other distinctions, 
for they are not based upon specific and definite pecul- 
larities, and may be varied by different classifiers. A 
genus includes many species that are alike in some things. 
Thus, the genus canis includes dogs, wolves, foxes, jack- 
als, etc., which, though specifically different, are very 
much alike in their teeth, claws, and feet. Then a fam- 
ily includes genera (plural of genus) ; an order includes 
families ; a class, orders; and, finally, orders are included 
in swb-kingdoms or departments. The terms division, 
tribe, and group are variously used by way of conven- 
ience. The term sub-class (under class) is sometimes 
used. It means a grand division of a class, as sub-king- 
dom means a grand division of a kingdom. 
22. The Vertebrates have two grand divisions, the 
warm-blooded and the cold-blooded. The warm-blood- 
ed maintain a high temperature of the blood under vary- 
ing states of the atmosphere. Thus, the blood of man 
is maintained at 98 degrees, even when the temperature 
of the surrounding air is 130 degrees below this. In the 
cold-blooded, on the other hand, the temperature of the 
blood is varied by that of the surrounding air or water. 
The fish when taken out of the water is of the tempera- 
ture of the water, and therefore feels cold to our hands. 
23. There are two classes of the warm-blooded Verte- 
brates: 1. Mammals, or Mammalia (from the Greek word 
fappua, mamma, a breast), animals that suckle their 
young; 2. Birds. The young of Mammals are born alive, 
and therefore Mammals are said to be viviparous, from 
the Latin words vivus, alive, and pario, to bear. Birds 
are called oviparous (ovwm, egg, and pario), because 
their young are produced from eggs. 
