John Janovy (jjanovy at unLinfo.unL.edu) repeats my comment that:
>Dave - I'm not sure there are "millions" of worms in a
>__Leucochloridium__ brood sac, more like hundreds, and they are all
>progeny of a single original larva produced asexually. Actually in
>terms of numbers offspring, __Leucochloridium__ is not unusual among
>trematodes, but in all of them, the increased reproductive potential
>in the snail is a result of asexual polyembryony. Maybe some of the
And then writes:
>snail people will respond, but in my view the important question is
>not so much how the famous __Leucochloridium__ brood sac evolved, but
>why the same type of phenomenon has evidently not evolved
>independently in a number of other trematode groups. Might have
>something to do with the fact that __Leuchloridium__ passes its
>larval phases in land snails.
At the risk of becoming a repeated nay-sayer, I would counter that
evolutionary questions framed in the negative (that is "why not")
are among the simplest to answer -- it just didn't happen. Admittedly
trivializing to an extent, one could formulate the negative question,
"Birds fly, some mammals fly, some arthropods fly... why have no
anellids adapted to flying?" Any infinite number of answers could
be formulated, none of which are testable. Such questions are more
metaphysical than scientific. Moreover, such questions appear to
presuppose that such a thing _should_ occur which draws us dangerously
close to the fallacies of progressivism or teleology.
More compelling and scientific evolutionary questions of adaptation
surround those relating to why something might have ocurred MORE than
once independently (wherein the independent derivations are corroborated
phylogenetically as opposed to having been assumed), not around those
relating to why something occured less-than-more-than-once.
Mark E. Siddall "I don't mind a parasite...
mes at vims.edu I object to a cut-rate one"
Virginia Inst. Marine Sci. - Rick
Gloucester Point, VA, 23062