WHEN man wishes to exploit any part of the earth’s surface he is rarely, if ever, presented with absolutely unoccupied territory. He has usually to contend with an established population of one or more species. We have tried to see how such a population is not continuously constant; that it varies from time to time, though the population of a particular area in May of one year may be almost identical in May of another. In more formal language we may say that the balance is shifting (or dynamic), not fixed (or static). It is deplorably easy to forget this—the outstanding feature of all living systems. Nobody is more prone to the error than the professional scientist: it is something of which he must continually remind himself. We may take the practice of photography for comparison.

Everybody who has ever taken a snapshot of his friends knows how often the likeness is disappointing. Yet why should we expect that an expression captured in one-twenty-fifth of a second will record a faithful and satisfactory impression of somebody of whom our observation must extend to thousands of times that amount? We may note in confirmation that nobody ever says that a cinematograph picture is ‘not a bit like you.’ Our observations of a living population are equivalent to a sort of snapshot or ‘still ‘picture of a

continuous process of which we can, at best, see or hope to see, fragments only. It is a fairly common trick in the bag of the nature-cinematographcr to take a film of a growing plant with an interval of some minutes between each picture. When the film is projected at the normal rate the plant appears to grow and move about at a most astonishing speed. It is one way of seeing a process which is too slow for our perception in time; we manage to contract or condense the time-dimensions of the operation. We extend that dimension by a similar process when we take a slow-motion picture of a fast-moving object. We ‘give ourselves time ‘to sec a series of movements which happen too fast for our senses.

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