The question naturally arises how these groups of nearly allied forms
may originally have been produced. Granting a common origin for all of
them, the changes may have been [51] simultaneous or successive.
According to the geographic distribution, the place of common origin
must probably be sought in the southern part of central Europe, perhaps
even in the vicinity of Lyons. Here we may assume that the old _Draba
verna_ has produced a host or a swarm of new types. Thence they must
have spread over Europe, but whether in doing so they have remained
constant, or whether some or many of them have repeatedly undergone
specific mutations, is of course unknown.
The main fact is, that such a small species as _Draba verna_ is not at
all a uniform type, but comprises over two hundred well distinguished
and constant forms.
It is readily granted that violets and whitlowgrasses are extreme
instances of systematic variability. Such great numbers of elementary
species are not often included in single species of the system. But the
numbers are of secondary importance, and the fact that systematic
species consist, as a rule, of more than one independent and constant
subspecies, retains its almost universal validity.
In some cases the systematic species are manifest groups, sharply
differentiated from one another.
Pages:
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71