These types must be very old and constant, fluctuating always within the
same distinct and narrow limits. No slow, gradual changes can have taken
place. In different countries their various habitats are as old as the
historical records, and probably many centuries older. They are quite
independent of one another, the distance being in numerous cases far too
great for the exchange of pollen or of seeds. If slow and gradual
changes were the rule, the types could not have remained so uniform
throughout the whole range of these two species. They would necessarily
have split up into thousands [41] and thousands of minor races, which
would show their peculiar characteristics if tested by cultures in
adjacent beds. This however, is not what happens. As a matter of fact
_V. tricolor_ and _V. arvensis_ are widely distributed but wholly
constant types.
Besides these, there occur distinct types in numerous localities. Some
of them evidently have had time and opportunity to spread more or less
widely and now occupy larger regions or even whole countries. Others are
narrowly limited, being restricted to a single locality. Wittrock
collected seeds or plants from as many localities as possible in
different parts of Sweden and neighboring states and sowed them in his
garden near Stockholm.
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