THE CLASS MAMMALIA u 



water. They possess a branchial respiration in which their 

 gills, acting as lungs, are enabled to extract air from the 

 water instead of from the atmosphere. 



For the present we are only concerned with the first of 

 the foregoing classes. In the Mammalia are all the animals 

 whose organisation is most developed, whose senses are the 

 most delicate, whose intelligence is the most perfect, and 

 which are most closely allied to mankind. They claim more 

 of our attention, not only because they include man himself, 

 but also the whole of the animal creation that is more 

 essential to his immediate welfare. Let us for a moment 

 try to imagine what our life would be if all the members of 

 the class Mammalia were rigidly excluded. Wool, hair, and 

 fur would form no part of our clothing ; our animal food 

 would be restricted to the flesh of birds and fishes ; we 

 should need a substitute for leather or give up wearing 

 boots and shoes ; we should dispense with all articles that 

 are made of bone ; and we should banish our friend the 

 dog and cease to make use of any beast of burden. 



Having decided which is the particular realm of the 

 animal kingdom we desire to explore, we again consult our 

 charts to ascertain what are the furthermost limits within 

 which our exploration will be confined. 



In dealing with the class Mammalia (and it is the same in 

 the other classes), we find that it contains groups of animals 

 in possession of some similarities in form and structure, and 

 conversely exhibiting common dissimilarities from other 

 groups. The mind almost involuntarily draws together to 

 form a section all those animals which have a mutual 

 resemblance to each other in some certain prominent 

 characteristics. 



Each order consists of an associated group, the units in 

 which have certain essential points in common. Neverthe- 

 less, in each order there occur numerous distinct, perhaps 

 almost trifling, points of difference which enable us to 

 separate the members of the order into sub-orders, with a 

 further division into sections and groups, and finally into 

 families, each possessing their own distinctive character- 

 istics. 



