This word is no longer in common use outside of specialty journals, and indeed is not in the current Webby database at www.m-w.com. It seems to find use only in botany, where it is sometimes applied to flowers and has the meaning of radial symmetry. The clearest example of this usage comes from Webby's distant cousin, the 1911 Encyclopedia Brittanica, which includes this entry on "Scrophulariaceae":
When a terminal flower is present it becomes regular as in toadflax, where radial symmetry is produced by
development of a spur to each petal—such flowers are termed peloric ...
http://www.1911ency.org/ under Scrophulariaceae
Webby's Darwin reference comes from "Origin of Species" and appears to be this one:
... the central flower thus becoming peloric or regular.
See also Peloria which has a bit more detail.