The problem with such examples of 'partially evolved structures' is that
they are completely evolved structures in the organisms we see using
them. These creatures have gone into gliding; it works for them.
They've taken their own route; there's no justification for claiming that
they're on some other creature's evolutionary path, only they're just a
bit behind. Lungfish don't have 'partial limbs;' they have fully developed
lobe-fins, suited to the lives they've been living for a hundred million
years or more. Pinnipeds have lost much of their limb structure; do
you think that if they were gradually forced to abandon the sea they
would get that limb structure back?
The evolutionary trend of reduction and specialization of segments
(particularly, serially homologous segments) is ignored in these
speculations. The speculators accept evolutionary reduction in a
desultory way, as an understandable and observable secondary
phenomenon, but despite the absence of evidence for gradual
elaboration in skeletal structure (in terms of number of parts), their
ideas typically involve just such structural elaboration.
--Cliff Lundberg ~ San Francisco ~ cliff@noe.com