Page:Popular Science Monthly Volume 75.djvu/457

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The above figures were given by the late Mr. H. C. Watson, one of our most eminent British botanists, and as he lived most of his life in the country, they are probably the results of his personal observation, and are therefore quite trustworthy.

Continuing the above enquiry to still smaller areas, one perch equalling acre, or less than the  of a square mile, has been found to have about forty distinct species, while on a patch 4 feet by 3 feet in Kent (or about  of a square mile) Mr. Darwin found twenty species.

The same law of increase of numbers in proportion to areas applies to the animal world, if we count all the species that visit a garden or field during the year, though those that can continuously live there are not perhaps so numerous in very small areas.

The powers of increase of plants and animals were next discussed, and were shown to be enormously great. An oak tree may produce some millions of acorns in a good year, but only one of these becomes a tree in several hundred years, to replace the parent. Kerner states that a common weed, Sisymbrium Sophia, produces about three quarters of a million of seeds; and if all these grew and multiplied for three years, the plants produced would cover the whole land surface of the globe.

Equally striking is the possible increase in the animal world. Darwin calculated that the slowest breeding of all animals, the elephant, would in 750 years from a single pair produce nineteen millions. Rabbits, which have several litters a year would produce a million from a single pair in four or five years, as they have probably done in Australia, where they have become a national calamity. As illustrative of this part of the subject, the lecturer referred at some length to the cases of the bison and the passenger pigeon in North America, and the lemmings of Scandinavia. In the insect tribes still more rapid powers of increase exist. The common flesh-fly goes through its complete transformations from egg to perfect insect in two weeks; and Linnæus estimated that three of these flies could eat up a dead horse as quickly as a lion.