apter. I hope
to be able to show that, although at first sight somewhat fragmentary and
disconnected, this work is really the development of a clear and definite
theory, and its application to the solution of a number of biological
problems. That theory is, briefly, that the distribution of the various
species and groups of living things over the earth's surface, and their
aggregation in definite assemblages in certain areas, is the {532} direct
result and outcome of a complex set of causes, which may be grouped as
"biological" and "physical." The biological causes are mainly of two
kinds--firstly, the constant tendency of all organisms to increase in
numbers and to occupy a wider area, and their various powers of dispersion
and migration through which, when unchecked, they are enabled to spread
widely over the globe; and, secondly, those laws of evolution and
extinction which determine the manner in which groups of organisms arise
and grow, reach their maximum, and then dwindle away, often breaking up
into separate portions which long survive in very remote regions. The
physical causes are also mainly of two kinds. We have, first, the
geographical changes which at one time isolate a whole fauna and flora, at
another time lead to their dispersal and intermixture with adjacent faunas
and floras--and it was here important to ascertain and define the exact
nature and extent of these changes, and to determine the question of the
general stability or instability of continents and oceans; in the second
place, it was necessary to determine the exact nature, extent and frequency
of the changes of climate which have occurred in various parts of the
earth,--because such changes are among the most powerful agents in causing
the dispersal and extinction of plants and animals. Hence the importance
attached to the question of geological climates and their causes, which
have been here investigated at some length with the aid of the most recent
researches of geologists, physicists, and explorers. These various
inquiries led on to an investigation of the mode of formation of stratified
deposits, with a view to fix within some limits their probable age; and
also to an estimate of the probable rate of development of the organic
world; and both these processes are shown to involve, so far as we can
judge, periods of time less vast than have generally been thought
necessary.

The numerous facts and theories established in the First Part of t