2 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 Paleozoic 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 branchie 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 
