1877.] The Geographical Distribution of Animals. 163 
its lower forms. At an early period it must have received a low 
form of Primates, which has been developed into the two pecul- 
iar 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 conti- 
nent 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 characterized — 
as it seems to have been in all past time of which we have any 
tolerably full record — by well-marked zodlogical features, while 
all are connected and related in the complex modes we have en- 
deavored 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 im- 
portant 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 dis- 
tribution 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 
various means of dispersal of the different groups and the com- 
parative longevity of their species and genera. Even insects, 
which are perhaps of all animals the farthest removed from 
Mammalia in this respect, agree in the great outlines of their 
distribution, with the vertebrate orders. The regions are ad- 
mittedly the same, or nearly the same for both, and the dis- 
‘repancies that occur are of a nature which can be explained by 
two undoubted facts, the greater antiquity, and the greater 
ties for dispersal of insects 
