ABSTRACT
School experiments in physics and chemistry often have known answers. If you do not record a value of 9.8 m s
s
for “the acceleration with which an object falls to the earth,” then you know it must be because there was something wrong with your equipment or with how you used it. Similarly, the molar mass of calcium carbonate is 100.09, so any other value would be wrong. The idea that there is a single clear-cut answer to a question is frequently not relevant in biology. Questions such as “How heavy are hedgehogs?” or “What is the length of an earthworm?” do not have a single correct answer. In fact, the variation may be of biological interest in itself. For example, the weights of individual hedgehogs in autumn largely determine whether they will survive their over-winter hibernation. It is thus important to be able to give a precise answer to such questions, and the aim of this chapter is to show how to do it correctly. We will simplify life by concentrating on just those earthworms of one species living in one particular field. Because earthworms are both male and female at the same time, we do not need to specify which sex we wish to measure. Even so, individuals can have one of a wide range of lengths. Why is this? Earthworms can live for a long time, and young earthworms are likely to be shorter than old ones. Like all animals, earthworms have genetic variability — some have a tendency to be short and fat and others to be long and thin. Those that live in the moister part of the field at the bottom of the slope might be more active and have a better food supply, so they will grow more quickly and may tend to be longer than those in a less favourable part of the field. Meanwhile, those living near the footpath along one side of the field tend to be shorter because they are
infested with a parasite or because they have recently escaped from a tussle with a bird. How then should we measure and describe the length of worms in this field?
