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 modification 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 unquestionably 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.