Some girls find older chaps pretty kinky

Common mnemonic for memorising the biological taxonomy groups (see The Five Kingdoms).

Respectively, in order of decreasing similarity, these are:

  1. Kingdom
  2. Phylum
  3. Class
  4. Order
  5. Family
  6. Genus
  7. Species

In case you've ever wondered the common couplet of latin words found in biology textbooks are the species and the genus, and two species with the same genus share a common ancestor, for example

Homo sapiens are related to Homo erectus in genetic terms*

It is of course perfectly permissible for the species and genus to be identical, for example in the case of ratus ratus - a creature we all probably know to some degree.

Mathematicians of course will recognise the taxonomy system as hierachical ordering, indeed a very similar system is in use for the very inorganic computer DNS system. Such a system of classification is useful wherever objects must be grouped in distinct classes, with each subsuquent class giving further information about the object.

*Not strictly true in some religions