C-values vary enormously among species. In animals they range more than 3,300-fold, and in land plants they differ by a factor of about 1,000. Protist genomes have been reported to vary more than 300,000-fold in size, but the high end of this range ("Amoeba") has been called into question. Variation in C-values bears no relationship to the complexity of the organism or the number of genes contained in its genome; for example, some single-celled protists have genomes much larger than that of humans. This observation was deemed counterintuitive before the discovery of non-coding DNA. It became known as the C-value paradox as a result. However, although there is no longer any paradoxical aspect to the discrepancy between C-value and gene number, this term remains in common usage. For reasons of conceptual clarification, the various puzzles that remain with regard to genome size variation instead have been suggested to more accurately comprise a complex but clearly defined puzzle known as the C-value enigma. C-values correlate with a range of features at the cell and organism levels, including cell size, cell division rate, and, depending on the taxon, body size, metabolic rate, developmental rate, organ complexity, geographical distribution, or extinction risk (for recent reviews, see Bennett and Leitch 2005; Gregory 2005).