CHAP. XVI.] 
SUMMARY AND CONCLUSION. 
161 
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 all 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 
VOL. II. M 
