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the border-line between a mere variety and an obviously "good"
species; he worked out for himself some of the results to be expected
of sexual selection (i.e. competition for mates between rival males).
But that, together with the indirect evidence provided by comparative
anatomy and geographical distribution, was about all.
With this meagre body of knowledge at his disposal, his genius was
able to put evolution on the map; but he could not proceed to the
further task of mapping evolution itself. That was reserved for the
slow cumulative work of several later generations of biologists.
It is not easy to sum up the chief results of that later work in brief
and intelligible form; but it must be attempted. First, there is the
formation of new species. These, we now know, originate in many
different ways, and even those with the same type of origin may come
to differ later in size and internal structure. The chief method of
origin is through physical isolation. Once two groups are physically
isolated so that they can no longer interbreed, they inevitably come
to diverge from each other in the new mutations and the new gene-
recombinations which they accumulate under the influence of natural
selection. And after a certain time the differences in their constitution
reach such a pitch that, even if the two stocks are brought together
once more, they are partially or wholly infertile on crossing.
In addition, when an isolated group is small in numbers, it can be
shown on mathematical grounds that it is likely to pick up and in-
corporate some mutations and recombinations that are useless or even
slightly unfavourable. Thus, some of the diversity of life is, bio-
logically speaking, purely accidental.
These effects, both of physical isolation and of small populations,
are well illustrated by the plants and animals of islands. A popula-
tion on an island is more or less completely isolated from other groups:
and, accordingly, -islands have a disproportionate number of dis-
tinctive sub-species and species, different from the species inhabiting
the nearest mainland and from those inhabiting other near-by islands.
The extraordinary number of distinctive species of giant tortoises
and of ground-finches on the Galapagos archipelago was one of the
main facts met with by Darwin in his voyage on H.M.S. Beagle which
convinced him of the reality of evolution. Again, there is only one
form of mouse-deer on the whole of Sumatra and Borneo, while the
Rhio-Linga archipelago close by, with only r-Jhrth of the area, boasts
no less than seven distinct subspecies.
In the Adriatic a large number of islands have been formed by
subsidence of the land since the end of the Ice Age. Many of them
are inhabited by distinctive races of lizards. A recent study has