istribution, a
student of the theory of evolution might naturally anticipate that all
groups of allied organisms would be found in the same region, and that,
as he travelled farther and farther from any given centre, the forms of
life would differ more and more from those which prevailed at the
starting-point, till, in the remotest regions to which he could
penetrate, he would find an entirely new assemblage of animals and
plants, altogether unlike those with which he was familiar. He would
also anticipate that diversities of climate would always be associated
with a corresponding diversity in the forms of life.

Now these anticipations are to a considerable extent justified.
Remoteness on the earth's surface is usually an indication of diversity
in the fauna and flora, while strongly contrasted climates are always
accompanied by a considerable contrast in the forms of life. But this
correspondence is by no means exact or proportionate, and the converse
propositions are often quite untrue. Countries which are near to each
other often differ radically in their animal and vegetable productions;
while similarity of climate, together with moderate geographical
proximity, are often accompanied by marked diversities in the prevailing
forms of life. Again, while many groups of animals--genera, families,
and sometimes even orders--are confined to limited regions, most of the
families, many genera, and even some species are found in every part of
the earth. An enumeration of a few of these anomalies will better
illustrate the nature of the problem we have to solve.

As examples of extreme diversity, notwithstanding geographical
proximity, we may adduce Madagascar and Africa, whose animal and
vegetable productions are far less alike than are those of Great Britain
and Japan at the remotest extremities of the great northern continent;
while an equal, or perhaps even a still greater, diversity exists
between Australia and New Zealand. On the other hand, Northern Africa
and South Europe, though separated by the Mediterranean Sea, have faunas
and floras which do not differ from each other more than do the various
countries of Europe. As a proof that similarity of climate and general
adaptability have had but a small part in determining the forms of life
in each country, we have the fact of the enormous increase of rabbits
and pigs in Australia and New Zealand, of horses and cattle in South
America, and of the common sparrow in North