534 ORIGIN OF MAMMALS xix. i- 



1. Classification (cont.) 



Infraclass 2. Metatheria 



Order Marsupialia 

 Infraclass 3. Eutheria (= Placentalia) 



# Order inc. sed. Triconodonta. Jurassic. Europe and N. America 

 *Amphilestes; *Triconodon 



2. The characteristics of mammals 



The idea that the mammals include the highest of animals can 

 easily be ridiculed as a product of human vanity; we shall find, how- 

 ever, that in many aspects of their structure and activities they do 

 indeed stand apart as animals that are 'higher' than others, in the 

 sense in which we have used the word throughout this study. The 

 mammals and the birds are the vertebrates that have become most 

 fully suited for life on land; among them are many species in which 

 the processes of life are carried on under conditions far remote from 

 those in which life first arose. The mammalian organization includes 

 a great number of special features that together enable life to be 

 supported under conditions that seem to be extravagantly improb- 

 able or 'difficult'. For example, the surface of the body is waterproofed, 

 and elaborate devices for obtaining water are developed. A camel and 

 the man he is carrying through the desert may perhaps contain more 

 water than is to be found in the air and sandy wastes for miles around. 

 This is only an extreme example of the 'improbability' of mammalian 

 life, which is one of its most characteristic features. 



The faculty of maintaining a high and constant temperature has 

 opened to the birds and mammals many habitats that were closed to 

 the reptiles. Besides making life possible under extremely cold con- 

 ditions, such as those of the polar regions, the warm blood vastly 

 extends the opportunities for life in more temperate climates. Mam- 

 malian races, which can feed all through the winter, can of course 

 expand more rapidly than their reptilian cousins, which must hiber- 

 nate for much of the year, during which period they consume rather 

 than produce living matter. 



The success of the mammals in maintaining life in strange environ- 

 ments is largely due to the remarkable powers they possess of keep- 

 ing their own composition constant. All living things tend to do 

 this, but it seems probable that the mammals maintain a greater 

 constancy than any other animals, except perhaps the birds. Claude 

 Bernard's famous dictum 'La fixite du milieu interieur c'est la con- 

 dition de la vie libre' may be doubtful of vertebrates in general, but it 



