144 GRADUAL MODIFICATION OF ANIMAL FORMS xi 



from the group to which they are most nearly relative in the 

 direction of the other. 



These are among the bunodonts, the little South American 

 peccaries (Dicotyles), and among the selenodonts, in a far 

 greater degree, the chevrotains (Tragulus). The latter in 

 many remarkable characters deviate strongly from the 

 ruminants, and approach the pigs, or rather, as will be shown 

 presently, to the generalised type of the entire group. 



Such being the present condition of the order, what does 

 palaeontology reveal of its past history ? 



In the first place, it is most necessary to bear in mind the 

 provisional character of all classifications of extinct animals, 

 because of our imperfect knowledge of their structure ; but 

 endeavouring to make the best use of what little we possess, 

 I have added in the scheme opposite all the best-known extinct 

 forms somewhere near the position, in relation to the existing 

 forms and each other, in which their affinities would place 

 them, and have shown by the different shading their relation 

 as regards time. 



The tertiary period, with which we are now alone 

 concerned, has here been divided for convenience into six 

 epochs. Of course, it were possible to have gone into minute 

 details and made many more divisions, but it would have 

 made the diagram less clear, and it is best, perhaps, not to 

 attempt to refine too much in this somewhat tentative 

 exposition of a biological history, especially as there is still 

 much uncertainty as to the exact relative age of many of our 

 fossiliferous strata. 



The epochs chosen are the recent (including the pleistocene), 

 the pliocene, late and early miocene, and late and early eocene, 

 each represented by a different shading. It is not meant that 

 if a genus or group is here assigned to one of these epochs, 

 that some of its members may not have extended in some 

 degree beyond its limits (as it must be always remembered 

 that the boundaries of these epochs are quite artificial), either 

 before or after, but that the period assigned to it was that in 

 which it most chiefly flourished. When two shadings are 

 represented, one within another, it signifies that the group 

 existed in both, and of course in all intervening periods. 



