458 ANIMALS INTRODUCED UNAWARES 



common to both must be examined with suspicion. Speak- 

 ing generally, the further distant two countries are from one 

 another the easier is it to trace their commerce in living 

 things. 



Consider the influence of the introduction of cabbages 

 or similar cruciferous plants upon the distribution of very 

 different kinds of insects. A common British Butterfly 

 only too familiar the Small White (Pieris rapae], a species 

 widely distributed in Europe from the Mediterranean Sea 

 to Scandinavia, was carried to Quebec by accident about 

 1860. Since then it has spread throughout the whole of 

 Canada and the United States by an invasion, the progress 

 of which, according to Professor Riley, can be traced step 

 by step westwards, from the landing-place on the Atlantic 

 coast. By a further lift it has also become established 

 in the distant isles of Hawaii. It has long been the most 

 serious of Butterfly pests in North America, and there its 

 chance introduction has led to another addition to the fauna 

 the deliberate introduction of the tiny Hymenopterous 

 Braconid (Apanteles glomeratus\ whose grubs feed within 

 and upon the caterpillars of the Cabbage Whites, and with- 

 out whose active assistance cabbage beds in Britain would 

 be wholly given over to ravenous caterpillars. 



A very different insect, the Cabbage Fly (Chortophila 

 (Phorbid] brassicae) is an equally notorious pest. The mag- 

 gots of this small light-grey two-winged (Dipterous) fly, 

 related to and somewhat resembling a House-fly, but on a 

 much smaller scale, bore into the roots of cabbages, cauli- 

 flowers and other crucifers, destroying the plants, even to 

 the extent of tens of thousands of acres in a year. This pest 

 of European origin occurs in northern Africa and has been 

 carried with cruciferous plants to North America. Here it 

 has gradually subdued Canada and the United States, having 

 reached the limit of the continent in 1914, when it did serious 

 damage amongst cabbages, cauliflowers, turnips and radishes 

 in Alaska. 



Another example will suffice, for our purpose, to com- 

 plete the tale of the cabbage and its introductions. In 

 summer and autumn, the outer leaves of cabbages may often 

 be seen wrinkled and covered with myriads of Greenflies 

 the Cabbage Aphis (Aphis brassicae\ whose attacks finally 



