As I have shown in Chapter 1, with the quotation from Whittaker (1969), by about the middle of the twentieth century the three major kingdoms of eukaryotes were finally recognised, and a crucial character difference was their respective modes of nutrition:

1. (a) animals engulf

2. (b) plants photosynthesise

3. (c) fungi absorb externally digested nutrients.

As you might expect, many other differences can be added to these – some general differences, some highly specific. Some of these kingdom-specific differences are absolute, but most have to be qualified in some way. For example, you might, with some reason, say that a characteristic of animals is that they move, and contrast that with the characteristic immobility of plants. But coral reefs are made up of animals and yet are pretty immobile; and the next time you stroll through a meadow in late summer and the breeze stirs up an atmosphere filled with flying seeds, look around and remind yourself: ‘the plants are migrating’.

Consequently, although it is possible to assemble panels of biological characteristics that are specifically expressed by each kingdom, you have to recognise that those characteristics may be subject to the context in which they are expressed and that in some circumstances there may be serious exceptions. When you try to establish evolutionary relationships there are more difficulties, the prime one being how to decide whether a character is ancestral or adapted. Intuitively, you might expect the ancestral character to be the simpler, and the adapted character to be the more complex.