22 NATURAL HISTOET. 



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- 

 iarities, 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 sui-hingdoms 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 temperatm-e 

 of the surrounding air is 130 degrees belowthis. 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 onr hands. 



- 23. There arc two classes of the warm-blooded Verte- 

 brates : 1. Mammals, or Mammalia (from the Greek word 

 fia^^a, 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 paHo, to bear. Birds 

 are called oviparous {ovum, egg, and pario), because 

 their young are produced from eggs. 



