56 LAND MAMMALS IN THE WESTERN HEMISPHERE 



third dimension is the successive modification in time of a 

 genetically connected series. The cumulative effect of such 

 modifications is so great that only very elastic definitions 

 will include the earlier and later members of an unbroken series. 

 In attempting to apply the Linnaean system to the successive 

 faunas {i.e. assemblages of animals) which have inhabited the 

 earth, palaeontologists have employed various devices. One 

 such method is to classify each fauna without reference to 

 those which precede and follow it, but this has the great draw- 

 back of obscuring and ignoring the relationships, to express 

 which is the very object of classification. Another and more 

 logical method is to treat species and genera as though they 

 belonged to the present order of things, for these groups, 

 particularly species, were relatively short-Uved, when regarded 

 from the standpoint of geological time, and either became so 

 modified as to require recognition as new species and genera, or 

 died out without leaving descendants. Groups of higher rank, 

 families, orders, etc., are treated as genetic series and include 

 the principal line or stock and such side-branches as did not 

 ramify too widely or depart too far from the main stem. Under 

 the first arrangement, the horses, a long history of which has 

 been deciphered, would be divided into several families ; under 

 the second, they are all included in a single family. 



One of the most interesting results of palaeontological 

 study is the discovery that in many famiUes, such as the horses, 

 rhinoceroses and camels, there are distinct series which in- 

 dependently passed through parallel courses of development, 

 the series of each family keeping a remarkably even pace in 

 the degree of progressive modification. Such a minor genetic 

 series within a family is called a phylum, not a very happy 

 selection, for the same term had been previously employed 

 in a much wider sense, as equivalent to the subkingdom. In 

 both uses of the term the underlying principle, that of genetic 

 series, is the same ; the difference is in the comprehensiveness 

 of meaning. 



