INTRODUCTION. 8l 



gence : they are the most fruitful in resources, least 

 subject to mere instinct, and most susceptible of pro- 

 gressive improvement. "With but a moderate quan- 

 tity of respiration, they are, in general, intended for 

 locomotion, by walking with strength and continuity, 

 and hence, all the articulations of their skeletons 

 have the forms very exact ; thereby determining, 

 with equal precision, the nature of their movements. 

 Though the typical form is intended for walking on 

 the earth, some fly through the air, by means of 

 membranes attached to their limbs, and a supple- 

 mentary structure fitting for that purpose ; while 

 others have the extremities so short, that they move 

 with ease only in water : but both retain, in every 

 other respects, all the general characters of the class. 

 All mammalia are endowed with warm blood, 

 which results from the great development of the re- 

 spiratory apparatus ; the heart is double, and con- 

 tains four cavities, viz. an auricle and a ventricle 

 on the right side, and the same on the left. The 

 veinous blood passes through the cavities in the 

 right side, and is distributed through the lungs, 

 where it combines wdth the oxygen or vivifying 

 principle of the air ; it is then conveyed by the pul- 

 monary veins to the left auricle, from whence it 

 flows into the ventricle, and is propelled through 

 the arterial system. The females, suckling their 

 young with milk, secreted in breasts or mamma?, 

 are viviparous or ovo-viviparous, therefore pla- 

 cental or implacental; the placental including all 

 the highest organised mammals, from man to the 



