656 The Outline of Science 



beetle remains unconquered, and continues to levy a very serious 

 toll. Yet no one can say that man was much to blame. 



4. Occasionally man's knotting of the web of life is ac- 

 cidental, as in the diagrammatic case of the Gipsy Moth (Ocneria 

 dispar), which a naturalist, Trouvelot by name, imported about 

 1869, for some scientific purpose, from Europe to Massachusetts. 

 By an accident some of the caterpillars got free, and although 

 Trouvelot did all he could to avert the consequences he did not 

 succeed in retrieving all the escapes, and the Gipsy Moth "caught 

 on." Along with another introduction, the Brown-tail Moth, it 

 continues unconquered to do terribly destructive work in defoliat- 

 ing trees in the States. 



But let us look at a brighter picture. In his valuable book 

 on Organic Evolution (1917) Professor R. S. Lull writes: 



One instance where Nature's balance has been restored 

 after being upset by human interference is in the case of a 

 scale insect accidentally introduced into California from 

 Australia on some young lemon-trees. This multiplied until 

 it became a most pernicious pest which various mechanical 

 remedies failed to control. Search was made in Australia, 

 and a natural enemy, a lady-bug, was brought over to Cali- 

 fornia, with the result that the scale was not only reduced but 

 almost completely eliminated. It was then found that the 

 lady-bug depended upon the scale for food to such an extent 

 that it died in turn, and now protected colonies of scale 

 insect and lady-bug are kept in readiness to control future 

 outbreaks of the pest. 



This importation of a natural enemy to counteract the de- 

 structiveness of an introduced alien has been tried in a number of 

 cases with great success. 



5. Sometimes man disturbs the Balance of Nature by elimin- 

 ating, not by fostering. 



There is an Australian story which reads as if written 

 for our instruction. On certain Murray River swamps 



