THEIR FLORA AND FAUNA. 209 



among the mammalia alone that this gigantic size and 

 peculiarity of form make their appearance ; for birds, rep- 

 tiles, and fishes partake of similar characteristics, and point 

 to the same favourable conditions of growth, and to the 

 same great law of structural relationship and development. 

 We say structural relationship, for, as has been well re- 

 marked by Professor Jukes, " in speaking of these extinct 

 animals as forming links between our existing forms, we 

 must never forget that the living forms are not the types, 

 but the variations from the types. "We are apt to assume 

 that the forms with which we are most familiar are the 

 most simple and natural ; but the scientific naturalist often 

 finds some extinct form as the simple archetype, from which 

 numerous others have departed more and more by variation 

 and combination of parts in subsequent periods." We are 

 right when we speak of these tertiary mammals as holding 

 an intermediate place between some existing forms, but we 

 are wrong if we consider them on that account to be either 

 more complex in structure or varied in function. "A 

 three-toed horse (hippotherium) would now be looked on," 

 says Mr Woodword, " as a lusus naturae; but in truth, the 

 ordinary one-toed horse of the present day is by far more 

 wonderful." 



We have already remarked, that though the distribution 

 of the tertiary seas and lands differed considerably from the 

 present, there must still have been a certain approximation 

 to the present arrangement, inasmuch as the tertiary flora 

 and fauna of every region, and especially of the later ter- 

 tiaries, bear a considerable resemblance to the plants and 

 animals that yet flourish there. There may be local differ- 

 ences among the tertiary basins of Europe and Asia ; but 

 still throughout the whole there is, if we may so phrase it, 

 an aspect of Old World forms. The species and genera 

 may differ, and there may be forms that stand intermediate 



o 



