THE WORLD OF LIFE 453 



Numbers of Flowering Plants 2 



Square Miles. Species. 



The County of Surrey 760 840 



A portion containing 60 660 



A portion containing 10 600 



A portion containing 1 400 



The above figures were given by the late Mr. II. C. Watson, one 

 of onr 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 % 60 acre, or less than the M.00000 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 %5oooooo 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 Increase of Plants and Animals 



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 ele- 

 phant, 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 illustra- 

 tive of this part of the subject, the lecturer referred at some length 

 to the cases of the bison and the passenger pigeon in North Amer- 

 ica, 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 Linnaeus estimated that three of these flies could eat 

 up a dead horse as quickly as a lion. 



2 Other tables illustrating similar facts in other parts of the world were 

 prepared, but not exhibited, as being likely to distract attention from the lec- 

 ture itself. 



