genes influencing the pituitary from exerting any major harmful effect.
But when these " buffering" genes are diluted or reduced in number
by crossing, the potential abnormality of the pituitary can become
This fact of recombination is the source of a whole category of
variation unsuspected by Darwin; much that is new in evolution is
due, not to wholly new genes produced by mutation, but only to new
combinations of old genes.
To sum up, most of the raw material of evolution is produced in
the first instance by mutation of genes into new forms. Owing to
the fact that they arc not blended in crosses, this new variation does
not have to be accepted or rejected immediately, but can be stored
in reserve, so to speak. If not acceptable in itself, it can even be
rendered acceptable by combination with other genes. And, in the
second place, recombination of old genes is capable of producing a
large further supply of new variation.
Still another fraction of the raw material of evolution depends on
the fact that the genes arc arranged in a row along a series of visible
(but of course microscopic) threadlike bodies called the chromosomes.
Owing to accidents in cell reproduction, whole sets of chromosomes
may be added or subtracted. Doubling of the normal complement
of chromosomes is a frequent subsidiary method of evolution in plants.
The polyploids, as the types with increased chromosome-number are
called, arc often more resistant to extreme conditions: for instance,
polyploids constitute an unusually large proportion of the varieties
found in the arctic and mountain regions that have become re-
colonized since the retreat of the ice after the Ice Age.
Chromosome-doubling may also occur after a cross between two
true species. In this case, a new species is formed at one jump—a
process which would have shocked most of Darwin's nineteenth-
century followers, who believed that all evolution was gradual.
Sometimes such new types arc weakly, and die out: in other cases
the new combination of genes gives them exceptional vigour, and
they may even oust both their parents. The classical example of
this comes from the rice-grasses, Spartina, which live on mud-flats.
During the last half-century a new type of rice-grass appeared in
Western Europe, and has been so successful that the Dutch have used
it to reclaim land from the sea. Investigation has proved that this is
a new polyploid species produced by the crossing of an original Euro-
pean species with one accidentally imported from America. In some
areas the European species has been virtually, exterminated by the