I think there is a "solution" to this question, although I'm not sure the
question is completely valid. It really depends upon what you mean by feet.
For example, starfish have five "feet," or at least limbs, which is an odd
number. If you don't count starfish limbs as "feet" then you really can't
count the feet of insects either, since we're talking about analogous
structures rather than evolutionarily homologous ones. If we are limited to
strictly homologous structures, then the only animals that have "feet" are
the vertebrates (and not all of them have feet either, e.g. fish).
Vertebrate animals are limited to four limbs due to choices that were made
very early in chordate evolutionary history.

My solution: Bilateral symmetry in the animal body plan produces the even
number of limbs, but those animals that develop with radial symmetry (like
the starfish) can have odd numbers of limbs. Why does bilateral symmetry
predominate in the animal kingdom would perhaps be a better question. My
answer to that question is: pure dumb luck.

Thomas M. Greiner, Ph.D.
Assistant Professor
Department of Anatomy
New York Chiropractic College
Seneca Falls, NY 13148-0800 USA

Phone: (315) 568-3183
Fax: (315) 568-3017
Email: tgreiner@nycc.edu

> -----Original Message-----
> From: Dr. Chris Kirtley [SMTP:kirtley@CUA.EDU]
> Sent: Tuesday, May 15, 2001 2:12 PM
> Subject: Summer Science Quiz #2
> Dear all,
> My little quiz seems to have proven very popular. So, since it's Summer
> (at least in the Northern hemisphere), when academics languish like
> chimnies, here's another...
> This one is from none other than Aristotle himself, who in 350 BC asked
> the following question in his "On the Gait of Animals" <
> http://classics.mit.edu/Aristotle/gait_anim.html >
> "Why do all animals have an even number of feet?", or in other words,
> why are there no three-legged animals, for example?
> Unlike the previous question, this one has no solution (apart from
> Aristotle's, which I find personally rather dissatisfying).
> Chris
> --
> Dr. Chris Kirtley MD PhD
> Associate Professor
> HomeCare Technologies for the 21st Century (Whitaker Foundation)
> NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
> Dept. of Biomedical Engineering, Pangborn 105B
> Catholic University of America
> 620 Michigan Ave NE
> Washington, DC 20064
> Tel. 202-319-6247, fax 202-319-4287
> Email: kirtley@cua.edu
> http://engineering.cua.edu/biomedical
> Clinical Gait Analysis: http://guardian.curtin.edu.au/cga
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