In article <
jhnplb...@mid.individual.net>,
occam <nob...@nowhere.nix> wrote:
>"as rare as..." was the context of the expression of "hens' teeth" that
>I recently came across. It made me smile.
>
>"horsefeathers" is the only other expression I know of that tries to
>convey the same idea. Any others?
Stephen Jay Gould's third collection of essays (published 1983) was
titled _Hen's Teeth and Horse's Toes_, after the title of one of the
essays therein. It is of course about how the odd-toed ungulates
(Perissodactyla), which today have either three toes or one toe on
each foot, evolved from five-toed ancestors, and connects that to
embryology and development (or what was known about it in the early
1980s, anyway). Horses have vestigial toe bones from their second and
fourth toes, and occasionally, horses are born with developmental
abnormalities that allow these long-suppressed toes to develop into
substantial, if non-functional, digits.
The thing about hen's teeth, of course, is that birds have not had
teeth for 60 million years, but if tissue from the right part of a
chicken embryo is grafted into the right part of a mouse, it can
develop into a tooth, demonstrating that chickens still maintain the
developmental machinery of their toothed dinosaur ancestors. Since
this requires human intervention, whereas polydactyly occurs naturally
in horses, hen's teeth have to be rarer than horse's (supernumerary)
toes.
-GAWollman
--
Garrett A. Wollman | "Act to avoid constraining the future; if you can,
wol...@bimajority.org| act to remove constraint from the future. This is
Opinions not shared by| a thing you can do, are able to do, to do together."
my employers. | - Graydon Saunders, _A Succession of Bad Days_ (2015)