MAN AND THE WEB OF LIFE 87 



and the extension of potato fields afforded an opportunity for 

 prolific multiplication of beetles, and this the natural enemies 

 were no longer able to check. Year after year the beetles 

 extended their march westward till they reached the Atlantic 

 seaboard. In spite of many counteractive measures, they 

 continue to levy a very heavy toll. 



It seemed laudable indeed to introduce European trout into 

 Tasmanian streams, where they have prospered and been 

 profitable. But there is always a tax to pay. The trout 

 have been levying toll on the dragon-flies, and already, as Mr. 

 Tillyard tells us, some characteristic Tasmanian species are 

 becoming rare. A trout's stomach may be found crammed 

 with over a score of dragon-flies. But dragon-flies live on 

 insects which they ,catch on the wing, and many of these 

 insects are injurious to crops and trees. So the introduction of 

 trout into Tasmania means a tax on farmer and fruit-grower. 

 4. Now and again the disturbance may be traced to an 

 unfortunate accident, as in the diagrammatic case of the 

 gypsy moth (Ocneria dispar). About 1869 a French naturalist 

 in Massachusetts, Trouvelot by name, had imported some 

 specimens of this moth from Europe for some scientific purpose. 

 Inadvertently, some of the caterpillars were allowed to escape, 

 and although Trouvelot did all he could to avert the conse- 

 quences of the accident, and did not delay in reporting it 

 to the authorities, the gypsy moth caught on in the States, 

 and, along with another introduction, the brown -tail moth 

 (Euproctis chrysorrhoea), continues to do terribly destructive 

 work in defoliating trees. Both are still unconquered pests. 

 Very suggestive of the " wheels within wheels " is the case 

 of a scale-insect introduced into California along with some 

 young lemon trees from Australia. It became a pernicious 

 pest defying control, but was eventually mastered by import- 

 ing from Australia one of its natural enemies, a lady-bug. 

 The almost complete elimination of the scale left the lady- 

 bugs to die in turn. Nowadays, " protected colonies of scale 

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

 of the pest." * 



1 Prof. R. S. Lull's Organic Evolution (1917), p. 111. 



