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Another instance is the crossing of the two poppies Papaver nudicaule
and P. striatocarpum, the offspring of which arc quite distinct from
either parent, are fully fertile, and breed true.
Single chromosomes or groups of them may also be added or sub-
tracted to give favourable results: a cytological accident of this sort
gave rise, it seems, to the very successful branch of the rose family
which later produced the apples and pears and their relatives.
Finally, bits of chromosomes may be shifted about. Small sections
may be repeated, thus increasing the total number of genes available.
Sections may be inverted, a process which tends to isolate the genes
they contain from those contained in the uninvcrtcd section. Or
chromosomes may exchange sections, which will help in the repro-
ductive isolation of the new strain.
All these kinds of chromosome mutations, too, provide a source of
variation unknown to Darwin, thus helping to account for the almost
incredible profusion of distinct species in life (nearly a million in in-
sects alone!). But the most important raw material of evolution
seems to consist of gene mutations. In the early clays of Mcndelism
the existence of mutation was taken to mean evolution by big jumps,
and to run counter to Darwin's conception of steady and gradual
change. This, however, was merely due to the fact that attention
was, quite naturally, first concentrated on those mutations which
could be readily detected—in other words, those with large effects.
Just because they have large effects, however, they arc apt to throw
the hereditary machinery out of gear, and so not to be of much value
for evolution. Later, it was discovered that the majority of gene
mutations are of small extent, often quite difficult of detection save
by the most refined techniques. And the accumulation of such small
mutations, constantly buffered by new recombinations, will give pre-
cisely the type of change that Darwin had in mind. Evolution does
go by jumps, but in most cases the jumps are so small that they hardly
ever take the new type outside the range of variation already existing
in the species, and the visible result is a gradual one. Discontinuity
of variation is thus translated by selection into continuity of evolution-
ary change: life marches up a ramp, not a staircase.
So much for the mechanism of evolution. But Darwin was almost
equally unprovided with knowledge about the actual course pursued
by evolution in different groups and in different conditions. He was
aware of the fact that fossils from an earlier epoch differed from the
modern inhabitants of the region, though resembling them in general
type; he was aware that isolation might play a role in the production
of new species; he knew of animal or plant groups which were on