?rule of three?

[P. F. Stevens]

Sermons also tend to have three main points.....  The rank of genus is made
by humans, not nature, although the species contained, whether 3 or 300,
may (I would argue, should) represent a monophyletic group.  As to the
prevalence of genera with three species, yes, this may be a mnemonic device
- but we can also bundle the  species into larger and larger groups,
whether or not we call them genera.   Five (more or less) seems to be a
convenient number for our memory bundling activities; I believe a review of
this issue will shortly appear in  Behaviour, Brain and Neuroscience or
some such place.

Some of the issues of the relationship between taxonomic structure and
mnemonics are discussed in BioScience 47: 243-250.  1997.

Peter S.