Sir

For most of the past 2,500 years, the dominant view in the West was that life was graded from better to worse, higher to lower, in a Chain of Being. This could be the product of some creative deity or — as in the eighteenth- and nineteenth-century theories proposed by Denis Diderot, Jean Baptiste de Lamarck and Robert Chambers — a consequence of matter somehow organizing itself through a goal-directed, progressive evolutionary process. Humans invariably occupied the top link.

The scientific literature persists in providing a home for the terminology of the Chain. During the past year, for instance, the Nature.com search facility identifies more than 300 references to 'higher' or 'lower' eukaryotes or organisms.

This usage is misleading, because evolution is not a progressive process. Selection may, in the short term, lead to increases in fitness. But there is no reason to assume that species adapted to the environments they occupy today are better adapted than their precursors were to their environments, and if evolution does not ensure this it hardly qualifies as progressive. Of course, because the first life forms were very simple, it is not surprising that evolution has resulted in increases in specialization or complexity in many lineages. But there is no sense in which such changes can make one taxon higher than another.

An argument in favour of preserving the terminology of the Chain is that it is useful. Yet there is considerable disagreement over the referents of 'higher' and 'lower'. The status of plants, for example, as higher or lower organisms is very fluid. Yeast is typically a 'lower' organism when compared with animals but can be a 'higher' one when compared with bacteria. Perhaps more disturbingly, and in a usage reminiscent of natural theology, non-human mammals can be relegated to the group of lower organisms when they are compared with humans.

A descriptive terminology hardly qualifies as useful if users disagree over what it describes. Moreover, it is rarely the case that using a consistent and scientifically robust terminology is more difficult than using the inconsistent and misleading terminology of the Chain. Distinguishing between plants and animals, for example, or between flowering and non-flowering plants, or between vertebrates and invertebrates, or between humans and other primates/mammals/animals without resorting to the descriptors 'higher' and 'lower' is easy — it's just been done.