speak, of thousands of short-range diversifications producing separate
species. Some of the peculiarities of these separate species are due
to non-selective accidents; but all the rest have been closely guided
and moulded by natural selection.
Darwin introduced time into biology, and forced us to regard
human history as the extension of a general process of change, operat-
ing by an automatic natural mechanism. Darwinism to-day has fully
confirmed these general conclusions, but has, in addition, enabled us
to distinguish between different types of change, and to link up human
with biological history more fruitfully by introducing the idea of pro-
gress and the criterion of desirable or undesirable evolutionary
The modern extension of Darwinism has also enabled us to analyse
the process of selection in a way that was impossible in Darwin's day.
In the first place, the intensity of selection may vary very consider-
ably, and this will be reflected in its results. Where a group is freed
from the full normal pressure of competitors or enemies, it is enabled
to evolve in quite unusual directions. The classical examples of this
are found on remote oceanic islands. In such areas of biological low
pressure, the few types which manage to find their way thither pro-
ceed to radiate out in many new directions. The best instance is
that of the birds called sickle-bills (Drepanididae) on the Hawaiian
archipelago. Derived from some kind of honey-creeper, they have
in their oceanic isolation evolved into no less than 18 separate genera,
adapted to an extraordinary range of habits, from nut- to insect-
eaters, from woodpecker-like types to nectar-sippers, each with a
characteristic form of bill.
In the Great Lakes of Africa, nature has conducted a demonstrative
experiment by permitting powerful predatory fish to reach some lakes
but not others. The little fish known as Cichlids exist in all the lakes.
Where predators are present, as in Lake Albert^ only four different
Cichlid species have evolved since the Ice Age; but where predators
are absent, as in Lake Victoria, there are over fifty Cichlid species,
adapted to many new habitats and ways of life. Predator-pressure
has had a restrictive effect on the diversification of prey.
The same sort of thing has happened in Australia, where the early
or marsupial type of mammal was isolated before the more efficient
placental type had been evolved. Accordingly, as everyone knows,
the marsupials in Australia have produced dozens of types, such as
kangaroos, Tasmanian wolf, and flying phalanger, not found either
living or fossil in any other part of the world. Elsewhere the pressure
of more efficient competitors has prevented this efflorescence, and