Mysterious Arrivals
Not only fish make their unaccountable appearance in pools and brooks which were just now dry land. I built a shallow concrete pond some two yards in diameter and planted a weeping willow beside it. Though the place is singularly dry the willow grew at great speed and finally learnt to send overland roots into the water. This summer a species of water- cress has established itself on the concrete ; and I know of no other plant of the sort within a mile. The most sudden and complete adaptation to March conditions that I have seen was on the Pilkhem Ridge above Ypres. The many shell holes were frequented by innumerable frogs and water- birds and were richly fringed with rushes and other water-loving plants, some three years after the shells ceased to fall. Some of the many examples of quick repopulation by fish, which continue to reach me from India, are certainly explained by aestivation. Some fish, like the frogs of South Australia, can survive in under-earth retreats. for very long periods.
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