CHAP. XVI.] SUMMARY AND CONCLUSION. 101 



It has, however, received successive infusions of higher types 

 from the north, which now mingle in various degrees with its 

 lower forms. At an early period it must have received a low 

 form of Primates, which has been developed into the two peculiar 

 families of American monkeys ; while its llamas", tapirs, deer, 

 and peccaries, came in at a later date, and its opossums and 

 extinct horses probably among the latest. The Australian region 

 alone, after having been united with the great northern 

 continent at a very early date (probably during the Secondary 

 period) has ever since remained more or less completely isolated ; 

 and thus exhibits the development of a primeval type of 

 mammal, almost wholly uninfluenced by any incursions of a 

 later and higher type. In this respect it is unique among all 

 the great regions of the earth. 



We see, then, that each of our six regions has had a history 

 of its own, the main outlines of which we have been able 

 to trace with tolerable certainty. Each of them is now 

 characterised — as it seems to have been in aU. past time of 

 which we have any tolerably full record — by well-marked 

 zoological features ; while all are connected and related in the 

 complex modes we have endeavoured to unravel. To combine 

 any two or more of these regions, on account of existing 

 similarities which are, for the most part, of recent origin, would 

 obscure some of the most important and interesting features 

 of their past history and present condition. And it seems no 

 less impracticable to combine the whole into groups of higher 

 rank ; since it has been shown that there are two opposing modes 

 of doing this, and that each of them represents but one aspect 

 of a problem, which can only be solved by giving equal attention 

 to all its aspects. 



For reasons which have been already stated, and which are 

 sufficiently obvious, we have relied almost exclusively on the 

 distribution of living and extinct mammalia, in arriving at these 

 conclusions. But we believe they will apply equally to elucidate 

 the phenomena presented by the distribution of all terrestrial 

 organisms, when combined with a careful consideration of the 



