IF we could (and it is by no means impossible; only rather tedious and difficult) get a moving-picture record of our Wealdcn oakwood, making our exposures at hourly intervals throughout the year, we could contract a year to less than ten minutes. This would condense the happenings of a long summer’s day to about one second and day and night would flash on and oil our screen with rather unpleasant rapidity. We should, therefore, choose a more ‘open ‘or large scale so that we had a day of fifteen to twenty seconds. Comparison with large- and small-scale maps, which enable us to see a town, a county or a continent entire, will elucidate the point further.

Pictures of this kind could not give us complete understanding of the annual succession, but they would be a great enlargement of our understanding of the shifting balance. We could also use the trick to see the stages by which such a succession is established on entirely unoccupied territory. In this matter, however, we can get enlightenment by less elaborate and costly methods. By providing territory which has been artificially cleared of all living things we can study the stages by which it is populated. This would seem to be a very easy thing to do but in practice a great many difficulties arise. To start with, an area devoid of all life is a matter requiring no little thought and care to obtain. The sea is a favourable place to conduct such experiments, but that will tell us little or nothing about the process on land. It may assist us to establish some general principles, but it cannot answer all our questions. So much depends on the question which we wish to answer by means of experiments on living organisms

Our question must be asked with great care and we must try to be certain that we do not ask one thing and mean another. If all we are able to do by way of experiment is to remove in April all large plants from an area of meadow-land one yard square and observe the subsequent events, we must not assume that the answers we shall get will be equally true for an area twenty yards square cleared in May.

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