IN PLANTS HAVING IRREGULAR COROLLAS. 417 
the facts of Variation that the real force and legitimacy of this 
deduction can appear. Since, however, the facts now presented 
lead naturally to this question, it may be right to give a slight 
forecast of the line of thought which they suggest. 
The whole significance, then, of facts of comparative structure, 
and especially of the facts of development, as applied to the study 
of Evolution, lies in the belief that the genetic relations of species 
and genera can be determined from this comparative study of their 
forms and modes of development. In other words, it is supposed 
that the changes by which species have been evolved from each 
other are of such a nature that, speaking generally, it is from 
time to time possible to perceive their relationships by study of 
their forms ; and especially it is anticipated that these changes 
are of such a kind as to leave more or less recognizable traces 
of their occurrence. In proportion as it shall be found that 
variations occur without leaving such traces of the previous 
form of the species, to that degree will comparative study of form 
and development be powerless to solve the problem of Descent. 
Now, though it is a question which needs more investigation, 
it is at all events apparently true that the changes which leave 
traces are continuous changes, while discontinuous changes leave 
little or no trace; so that the principle may probably be stated 
thus, that in proportion as Variation is not a continuous process 
will comparative morphology cease to be an effectual guide to the 
history of Descent. 
The facts of the changes seen in our flowers show, as has been 
said, that when a change of symmetry is concerned, Variation is 
constantly discontinuous, leaving no visible trace. It is there- 
fore in the case of forms of differing symmetries that we must 
expect the evidence of comparative morphology to be absent or 
inadmissible. Yet it is precisely at this very point of alteration 
in symmetry that we want help as to the history of Descent. It 
is easy to conceive the steps between forms differing in the degree 
of expression of some character, such as size or intensity of 
colour, but in trying to pass from a form with one kind of sym- 
metry to a form with another we often cannot even conceive the 
transitional steps. There are some cases in which such steps 
can be conceived and are assumed to have occurred. In Veronica, 
for example, when compared with other Scrophulariacee, it is 
supposed that the flower of the former has been derived from the 
latter by “fusion” of the two posterior petals, by loss of the 
