en gaining
heat.

In the preservation of the average proportion of land and sea, the
igneous agents exert a conservative power, restoring the unevenness of
the surface which the levelling power of water in motion would tend to
destroy. If the diameter of the planet remains always the same, the
downward movements of the crust must be somewhat in excess, to
counterbalance the effects of volcanoes and mineral springs, which are
always ejecting material so as to raise the level of the surface of the
earth. Subterranean movements, therefore, however destructive they may
be during great earthquakes, are essential to the well-being of the
habitable surface, and even to the very existence of terrestrial and
aquatic species.

_III.--Changes of the Organic World now in Progress_

In 1809 Lamarck introduced the idea of transmutation of species,
suggesting that by changes in habitat, climate, and manner of living one
species may, in the course of generations, be transformed into a new
and distinct species.

In England, however, the idea remained dormant till in 1844 a work
entitled the "Vestiges of Creation" reinforced it with many new facts.
In this work the unity of plan exhibited by the whole organic creation,
fossil and recent, and the mutual affinities of all the different
classes of the animal and vegetable kingdoms, were declared to be in
harmony with the idea of new forms having proceeded from older ones by
the gradually modifying influence of environment. In 1858 the theory was
put on a new and sound basis by Wallace and Darwin, who added the
conception of natural selection, suggesting that variations in species
are naturally produced, and that the variety fittest to survive in the
severe struggle for existence must survive, and transmit the
advantageous variation, implying the gradual evolution of new species.
Further, Darwin showed that other varieties may be perpetuated by sexual
selection.

On investigating the geographical distribution of animals and plants we
find that the extent to which the species of mammalia, birds, insects,
landshells, and plants (whether flowering or cryptogamous) agree with
continental species; or the degree in which those of different islands
of the same group agree with each other has an unmistakable relation to
the known facilities enjoyed by each class of crossing the ocean. Such a
relationship accords well with the theory of variation and natural
selection, but with no other hypo