448 WILLIS ; MORPHOLOGY OP THE PODOSTEMACEÆ 
is very often made approximately at the same time by more 
than one mind acting on somewhat similar materials in 
similar ways, so we may imagine the same generic or ordinal 
step taken by allied or similar species at about the same 
time or in relation to similar causes. This of course is only 
a supposition, but the question as to whether many genera 
and other groups are or are not polyphyletic is raised on 
sufficient evidence to require that it shall be investigated 
and set at rest, as well as the further question, whether, if 
any genus or other group is found to be polyphyletic, it is 
to be retained as an artificial group or divided into others, 
even though there may be the very closest similarity among 
its members. 
It is evident that if the phenomena here suggested are at 
all common, the process of evolution may in these cases have 
been much swifter than we are usually inclined to believe, 
many forms perhaps having undergone parallel develop- 
ments, so that the evolution may be likened to the climbing 
of steps by several individuals at once. 
To follow up the same line of thought, it is clear that it 
may considerably alter some of the prevailing views upon 
geographical distribution, for if the generic or ordinal step 
may be taken separately by similar or allied species, these 
species may be already separated by large distances, and 
the genus or order may thus conceivably arise in two or 
more phyla at different places. It is no longer absolutely 
necessary to suppose, to take the particular case in hand, 
that the extremely zygomorphic-flowered Eupodostemeæ 
must have spread from a common centre occupied by one 
original ancestral species with the sub-ordinal characters ; 
there may have been, in India and in the other regions 
where this sub-order is found, several species which under 
the influence of similar causes may have taken the steps, so 
to speak, which transformed them to representatives of this 
sub-order. All that is necessary is the final stage in the 
zygomorphism of the andrœceum, reducing it to one single 
or double stamen on the lower side of the flower. 
