We now come to the study of those varieties, which
differ in more than one character. Of these there are two types. In the
first the points of dissimilarity are intimately connected with one
another, in the second they are more or less independent.
The mutually related peculiarities may be termed correlative, and we
therefore speak, in such cases, of correlative variability. This
phenomenon is of the highest importance and is of general occurrence.
But before describing some examples, it is as well to note that in the
lecture on fluctuating variability, cases of a totally different nature
will be dealt with, which unfortunately are designated by the same term.
Such merely fluctuating variations are therefore to be left out of the
present discussion.
The purple thorn-apple, which is considered by some writers as a variety
of the white-flowered species or _Datura Stramonium_, and by others as a
separate species, _D. Tatula_, will serve as an illustration. But as its
distinguishing attributes, as far as we are concerned with them here,
are of the nature described above as characteristic of varietal
peculiarities no objection [143] can be made to our using them as a case
of correlative variability.
The essential character of the purple thornapple lies in the color of
the flowers, which are of a very beautiful pale blue.
Pages:
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154