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the passage of long periods of time.
    Let’s look first at heritable variation. This means that any group of creatures will differ in their appearance or constitutions from one another, and that this variation is inherited from their parents. Unless they are identical siblings, the children in a family will inherit different traits from their parents, to different degrees. Some will be taller, some shorter, some darker, some fairer. For example, if you gathered every adult male (or adult female) in your town and measured them, you’d find that they’d vary greatly in height. You’d have to group men and women separately, as height is in part related to gender—on average, the men in any given population are taller than women from the same population. You’d find that most people would be middling in height, somewhere between 1.5 and 1.9 meters tall. People much shorter or taller than this are relatively rare. Any population is varied, but variation tends to cluster around a “mean” or “average” value. Calculating an average value is easy: add all the heights together, and divide what you get by the number of people you’ve measured.
    The more people you measure, the better, because your result will be a better approximation of reality. If you can’t measure everyone in your neighborhood, say, you should still try to measure as large a sample as possible. If you can’t do that, you should try to ensure that the people you measure are picked at random. For example, if you measured the heights of the first three people you met, and they happened to be a coven of very small witches, or from a team of very tall basketball players, you shouldn’t be surprised that your sample is unrepresentative of people in your neighborhood in general.
    When you see reports of preference in the press, such as peoples’ voting intentions, or whether their cats prefer ex-battery chicken of one brand over another, you should look out for the small print saying that the evidence comes from a poll of, say, 1,000 people chosen at random. It’s important to get lots of people, and to pick them by chance. This chance element is vitally important. There’s the probably apocryphal story of a market researcher who found that ninety-nine of a hundred people asked ate porridge for breakfast: it turned out that the people asked all came from the McPherson page of the Inverness telephone directory. This, without meaning any offense to residents of the fine city of Inverness who happen to be called McPherson, is probably not a representative sample of people as a whole.
    From this it is clear that variation acts at different levels. As peoplevary in height even in your neighborhood, so do people from different places. Different populations have different average heights. The average American man is 1.76 meters tall, whereas the average American woman is 1.62 meters tall. 1 Dutch men and women tend to be taller, on average—1.87 and 1.69 meters respectively, 2 whereas urban men and women of the east African nation of Malawi tend to be shorter, 1.67 and 1.55 meters. 3 This means that although men tend to be taller than women in general, the average Dutch woman will be taller than the average Malawian man. Because people tend to marry within their locality or ethnic group, the figures for average height differ from place to place.
    Although people vary in all sorts of ways, and even though traits might be influenced by other things, such as nutrition and the environment, it’s plain that height tends to run in families—that is, variation is inherited. Tall parents tend to have tall children. My own daughters are among the tallest in their year groups—but I am relatively tall for an Englishman (1.83 meters, against the average of 1.75), and my wife is very much taller than the average Englishwoman (1.8 against 1.6 meters). 4 She also comes from a family of tall women, who tended to marry guardsmen—not just tall, but proverbially tall. Hmm.