OF CBYLOIS^ AND INDIA. 
447 
remaining nearly akin to Y in every other respect. The 
increasing dorsiventrality will at least re-act on the floral 
organs — let the mechanism be what it may — and the latter 
will show increased dorsiventrality. Now the phenomena 
of this increasing dorsiventrality seem, as we have seen, to 
follow fairly deflnite laws, and hence it is by .no means 
unlikely that the same phenomena will show in X and Z, 
while they do not show in Y, which has not increased its 
vegetative dorsiventrality. We shall thus get a polyphyletic 
genus formed, with two species X and Z, which may be each 
as nearly akin to Y as to one another, though Y is not 
included in their genus. The genus will be polyphyletic in 
that there is no common ancestor which possesses the generic 
characters. 
There is no necessity, perhaps, for the case to be so simple 
as that just sketched. It is quite possible that species not 
at all closely allied may take the same generic step, so to 
speak, which brings them close together in our schemes of 
classiflcation, though their immediate ancestors might have 
been far apart by the criteria used. 
The same reasoning will of course apply to the case of a 
number of forms taking a similar tribal or sub-ordinal step 
when the conditions occur, and in fact the larger the group 
and the fewer the characters on which it is based, the greater 
the likelihood of its being polyphyletic." It is by no means 
unlikely that some of the many cases of genera where the 
different species seem to have each of them some of the 
generic characters of surrounding genera, while all have 
their own generic character, may be due to some such process 
of evolution as that just sketched. Thus in Hydrobryum it 
is quite possible that the smooth-fruited H. sessile may belong 
to a different phylum from the other species with ribbed 
fruits, or in Farmeria, that the two species may be derived 
from different phyla, though of course the facts may be 
explained on other hypotheses. Just as an idea or a discovery 
An old rule in a new and more easily-fitting dress. 
