48 Animal Geography . [January* 
bine and harmonise the mass of unconnected faCts ascer- 
tained, and which should not merely propose an arrangement, 
but should demonstrate it by a careful and exhaustive ana- 
lysis. This deficiency has been supplied by Mr. Wallace in 
a manner which must greatly enhance the well-merited 
esteem in which he is held by naturalists. The result is a 
work which in its department has no equal in any language, 
and which must at once be received as the text-book of 
zoological geography. 
It may, at the first glance, appear an easy matter to de- 
termine the geographical distribution of the animal kingdom. 
We have only, it is said, to take a census of species in every 
country, to compare the returns, and to arrange our divisions 
accordingly ; but the moment we make the attempt diffi- 
culties spring up on all sides. We require a trustworthy 
classification of animals, so that we may know what forms 
can be legitimately included under each species, genus, or 
family. We must then decide whether our classification is 
to be positive or negative, founded on the presence or on the 
mere absence of certain groups. Our own view, like that of 
Mr. Wallace, is that mere negative characteristics can have 
but very limited value. The extirpation of certain striking 
forms of life in a given island, whether effected by human 
agency or by natural causes, cannot give such island a 
higher rank as a zoological province than it had before. To 
distinguish two regions, a and b, we must be able to show 
that each contains something which is wanting in the other. 
Why, for instance, are the claims of Australia to rank as a 
distinct primary region so universally allowed ? Not from 
the mere absence of monodelphic mammals, whether Car- 
nivora, Rodentia, Ungulata, and the like, but because, in 
the stead of all these, there are didelphic groups which to 
some extent replace, or at least simulate, the monodelphic 
orders and families. This brings us to another fundamental 
principle, — the higher the rank of the group present in one 
country and absent in another, the more fundamental is the 
distinction between them. Thus two adjacent islands might 
contain not a single Lepidopterous species in common ; yet 
if all the species belonged to genera common to both islands 
we should rank both in the same region, sub-region, province, 
and district. But suppose that they had no genera or no 
families in common, we should consider it necessary to refer 
them at any rate to distinct sub-regions. If, again, the very 
orders are distinct, as is the case if we compare the mam- 
mals of Australia with those of the rest of the world,* we 
* With the exception of the opossums of North and South America. 
