136 THE MAMMALIA. 



to the Hoofed animals was able to maintain, there- 

 fore, that, 'in spite of our as yet knowing with 

 certainty only a very small portion of the fossil 

 animals, still it has come to seem at least probable 

 that not only the variety and very marked character 

 of the forms, but that the number of the species 

 also has continuously increased.' However, this 

 opinion requires modification since the discoveries 

 made in America during the last fifteen years, 

 which discoveries may, to a certain extent, be 

 classed with Filhol's discoveries in France. The 

 Hoofed animals teach us above all things, as has 

 already been said, that we live in a world zoologi- 

 cally very impoverished. 



The fossil species which have not left any 

 living representative of their former existence will 

 receive only our second consideration ; but as this 

 very class contains most of the genera that must 

 be the primary forms of the present species, we 

 shall best attain our object by showing the links 

 which connect the mammals we have around us 

 with those of the primordial world, by starting in 

 a general way from the known Hoofed animals as 

 a few of the highest and still blossoming off- shoots 

 of a mighty tree and thence take a survey back 

 towards the roots. 



