THE DISTRIBUTION OF SPECIES. 201 
man, open to their invasion. Facts of this sort are the 
“enormous increase of rabbits and pigs in Australia 
and New Zealand, of horses and cattle in South America, 
and of the sparrow in North America, though in none 
of these cases are the animals natives of the countries 
in which they thrive so well.” (Wallace.) The persist- 
ent spreading of European weeds to the exclusion of 
our native plants is a fact too well known to every 
farmer in America. The constant movement westward 
of the whiteweed and the Canada thistle marks the 
steady deterioration of our grass fields. Especially 
noteworthy has been this change in Aus- 
tralia and New Zealand. In New Zea- 
land the weeds of Europe, toughened by 
centuries of struggle, have won an easy 
victory over the native plants. Edward Wakefield, in 
his history of New Zealand, says that “many animals 
and birds acquire peculiarities in the new country which 
would indeed astonish those accustomed to them in the 
old. They usually run to a much larger size and breed 
oftener. They also take to strange kinds of food. 
Birds deemed granivorous at home become insectivorous 
here, and vice versa. Some learn the habits of the na- 
tive species. Skylarks imitate the native wagtail, and 
may often be seen perching on fences and telegraph 
wires. ‘They sing in the nighttime, too, a thing un- 
heard of in the old country, and doubtless acquired from 
the nocturnal habits of the New Zealand birds.” 
The European house fly in New Zealand has com- 
pletely extirpated the large bluebottle fly, which was 
formerly a source of great annoyance to the settlers. 
An account is given of a farmer who filled a bottle with 
house flies and carried them eighty miles into the coun- 
try, liberating them, one by one, in the vicinity of his 
sheepfolds, in order to let them take the place of the 
Invasion of the 
Australian 
realm, 
