The best proof is afforded by the varieties which grow wild in
localities where they are quite isolated from the species, and where for
this reason, no possibility of crossing disturbs the significance of the
proof. As one instance the rayless form of the wild camomile, or the
_Matricaria Chamomilla discoidea_ may be mentioned. Many systematists
have been so strongly [157] impressed with its absolute constancy and
its behavior as an ordinary species, that they have elevated it, as it
is called, to the rank of a species. As such it is described under the
name of _Matricaria discoidea_ DC. It is remarkable for its rapid and
widespread distribution, as of late years it has become naturalized in
different parts of America and of Europe, where it is to be seen
especially in France and in Norway. Experimentally I raised in
succeeding years between 1000 and 2000 seedlings, but observed no trace
of reversion, either in the strongest or in the numerous very small and
weak individuals which appeared in the cultures.
The tansy-ragwort or _Senecio Jacobaea_ may be chosen as a second
instance. It is a perennial herb with short rootstocks and stout stems
bearing numerous short-peduncled heads in large compact corymb; it
multiplies itself abundantly by seeds and is very common on the sand
dunes of Holland.
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