The more aberrant
any form is, the greater must be the number of connecting forms which have
been exterminated and utterly lost. And we have evidence of aberrant
groups having suffered severely from extinction, for they are almost always
represented by extremely few species; and such species as do occur are
generally very distinct from each other, which again implies extinction.
The genera Ornithorhynchus and Lepidosiren, for example, would not have
been less aberrant had each been represented by a dozen species, instead of
as at present by a single one, or by two or three. We can, I think,
account for this fact only by looking at aberrant groups as forms which
have been conquered by more successful competitors, with a few members
still preserved under unusually favourable conditions.
Mr. Waterhouse has remarked that when a member belonging to one group of
animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to Marsupials; but in
the points in which it approaches this order, its relations are general,
that is, not to any one Marsupial species more than to another. As these
points of affinity are believed to be real and not merely adaptive, they
must be due in accordance with our view to inheritance from a common
progenitor. Therefore, we must suppose either that all Rodents, including
the bizcacha, branched off from some ancient Marsupial, which will
naturally have been more or less intermediate in character with respect to
all existing Marsupials; or that both Rodents and Marsupials branched off
from a common progenitor, and that both groups have since undergone much
modification in divergent directions.
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