INTRODUCTORY 



vertebrates ; although recent discoveries have shown evidence of a 

 more or less marked affinity between the most generalised mammals 

 and a peculiar group of extinct reptiles known as the Anomodontia 

 (or Theromora), which are themselves nearly related to the equally 

 extinct Labyrinthodont amphibians of the Palaeozoic and Mesozoic 

 epochs. 



In the gradual order of evolution of living beings, mammals, 

 taken altogether, are certainly the highest in organisation, as, with 

 the possible exception of birds, they were the last to appear on 

 the earth's surface. But, as in speaking of all other large and 

 greatly differentiated groups, this expression must not be understood 

 in too limited a sense. The tendency to gradual perfection for 

 their particular station in life, which all groups manifest, leads 

 to various lines of specialisation, or divergence from the common 

 or general type, which may or may not take the direction of 

 elevation. A too complex and sensitive condition of organisation 

 may in some circumstances of life be disadvantageous, and modifi- 

 cation may then take place in a retrograde direction. Thus in 

 mammals, as in other classes, there are low as well as high forms, 

 but by any tests that can be applied especially those based on 

 the state of development of the central nervous system it will 

 be seen that the average exceeds that of any other class ; that 

 the class contains many species far excelling those of any other 

 in perfection of structure, and especially one form which is un- 

 questionably the culminating point yet arrived at amongst organised 

 beings. - 



With regard to the time of the first appearance of mammals 

 upon the earth, the geological record is provokingly imperfect. At 

 the commencement of the Tertiary period they were abundant, and 

 already modified into most of the leading types at present existing. 

 It was at one time thought that they first came into being at this 

 date, but the discovery of more or less fragmentary remains of 

 numerous and generally small species has revealed the existence of 

 some forms of the class at various periods throughout almost the 

 whole of the age of the deposition of the Secondary or Mesozoic 

 rocks. This subject will be reverted to later on. 



It hardly need be said that mammals are vertebrated animals, 

 and possess all the characteristics common to the members of that 

 division of the animal kingdom. They are separated from the 

 Ichthyopsida (fishes and amphibians), and agree with the Sauropsida 

 (reptiles and birds) in the possession during their development of 

 an amnion and allantois, and in never having external branchiae or 

 gills. They differ from reptiles and resemble birds in being warm- 

 blooded, and having a heart with four cavities and a complete 

 double circulation. They differ from both birds and reptiles in the 

 red corpuscles of the blood being non-nucleated and, with very few 



