THE ENGLISH SPARROW AM) ITS ALLIES 283 



stroys large quantities of grain in the field, as well as 

 many kinds of garden produce, so that, on the whole, the 

 English sparrow must be reckoned destructive to agri- 

 culture. Of late years it has come into our Southern 

 markets as a substitute for the rice-bird. 



Increase of Exotic Species. - The extraordinary spread 

 of the English sparrow after importation to this country 

 is not wholly explained by its large fecundity; for although 

 equally reproductive in Europe, it ^increases less rapidly 

 there than here. Also it is not due to any peculiarity of 

 our country, for the bird is a similar pest in Australia. 

 Similar facts concerning the spread of other animals lead 

 us to conclude that it is the new country which permits 

 the rapid spread and consequent destructiveness. Thus 

 when the cabbage-butterfly (Pieris rapce) was brought to 

 this country it spread with such rapidity that, starting in 

 1860 at Quebec, it has now spread all over the United 

 States as far as the Rocky Mountains. Again, the grape- 

 vine insect pest, Phylloxera, a native of this country, but 

 not particularly destructive here, has been accidentally 

 transported to France, and there it lias wrought great 

 havoc in the vineyards. Another instance, this time of 

 an aquatic animal, shows the same result : the periwinkle, 

 Littorina littoria, now the commonest snail on the seashore 

 north of New York, has migrated down the shore from 

 Halifax since 1868. This old species in the new country 

 lias almost driven out the other shore mollusks, to such an 

 extraordinary degree has it multiplied. Now why should 

 animals in a new country develop with such unusual rapid- 

 ity ? It is because coming into a new country they have 

 left behind them their natural enemies, and there has not 

 yet been time for them to acquire new ones. Eventually 



