NATURAL SELECTION; SEXUAL SELECTION 65 



species occupies the district or the surroundings best fitted 

 for its habitation. This is manifested in the fact of the extraor- 

 dinary fertility and persistence shown by many kinds of animals 

 and plants in taking possession of new lands which have become, 

 through the voluntary or involuntary interference of man, open 

 to their invasion. Facts of this sort are the "enormous in- 

 crease of rabbits and pigs in Australia and New Zealand, of 

 horses and cattle in South America, and of the sparrows of 

 North America, though in none of these cases are the animals 

 natives of the countries in which they thrive so well ' (Wal- 

 lace). The persistent spreading of European weeds to the 

 exclusion of our native plants is a fact too well known to every 

 farmer in America. The constant moving westward of the 

 white weed and the Canada thistle marks the steady deteriora- 

 tion of our grass fields. The cockroaches in American kitch- 

 ens represent invading species from Europe. The American 

 cockroaches live in the woods. Perhaps a majority of the 

 worst insect pests of the United States are of European or 

 Asiatic origin. Especially noteworthy are cases of this type 

 in Australia and New Zealand. In New Zealand the weeds 

 of Europe, toughened by centuries of selection, have won an 

 easy victory over the native plants. 



Dr. Hooker states that, in New Zealand " the cow grass has 

 taken possession of the roadsides; dock and watercress choke 

 the rivers ; the sow thistle is spread all over the country, growing 

 luxuriantly up to 6,000 feet; white clover in the mountain dis- 

 tricts displaces the native grasses." The native Maori saying 

 is: " As the white man's rat has driven away the native rat, as 

 the European fly drives away our own, and the clover kills our 

 fern, so will the Maoris disappear before the white man himself." 



Prof. Sidney Dickinson gives the following notes on the 

 rabbit and other plagues of Australia: 



"The average annual cost to Australasia of the rabbit plague is 

 700,000, or nearly $3,500,000. The work which these enormous figures 

 represent has a marked effect in reducing the number of rabbits in the 

 better districts, although there is little to suppose that their extermina- 

 tion will ever be more than partial. Most of the larger runs show very 

 few at present, and rabbit-proof fencing, which has been set around 

 thousands of square miles, has done much to check further inroads. 

 Until this invention began to be utilized it was not uncommon to find 



