444 WILLIS : MORPHOLOGY OF THE PODOSTBMACEÆ 
last to provide a good instance of fixed or phylogenetic 
correlation, to support the few yet adduced. 
It is also evident that the familiar statement that the 
characters of an organ or organism are partly hereditary 
and partly adaptive does not cover the whole truth, and 
that we must allow for the presence of what we may perhaps 
term correlative or induced characters, characters due to 
correlation or to the parallel action of similar causes, and 
not necessarily advantageous, though fixed in the heredity. 
The objection is often brought against the theory of 
natural selection that many specific characters — and less 
often that generic characters — are useless to their possessors. 
Hitherto, however, so far as I am aware, few cases have been 
adduced in which it has not been possible to reply that 
further investigation may prove the usefulness of these 
characters, or that they may have been derived from useful 
characters in ancestral forms. In the few cases not to be 
explained in these ways, correlation has been adduced."’ In 
the Podostemaceæ wo have perhaps the best instance of some 
specific and still more generic and tribal characters, 
apparently due entirely or almost entirely to correlation or 
to direct action of external causes, and not to natural 
selection. Natural selection, however, may well have acted 
largely on the vegetative organs, which are most directly 
concerned with the external conditions of life. 
We are perhaps, at least in Botany, too much in the habit 
of regarding evolution from an extremely analytical point of 
view, taking the characters of an organism singly, and more 
or less unconsciously assuming that the evolution has 
affected one at a time, as represented by a diagram of 
dichotomous branching in one plane, such as that given by 
Darwinf to illustrate the possible formation of new species, 
and thus implying that a change in any organ must neces- 
sarily be accompanied by some advantage to that organ or 
* Cf. Darwin, Origin of Species, 6th ed., p. 116 ; Wallace, Darwinism, 
ch. VI, 
t Origin of Species, 6th ed., p. 90, 
