59 



starvation, or through unfavourable physical causes. As examples, I beg 

 leave to mention Locustidse, the Colorado beetle, some Melolonthidse (to 

 one species of which is owing the destruction of thousands of Eucalypti 

 in Monarto), several species of moths, &c. The undue increase of 

 their number could, I firmly believe, be proved to be due to the removal 

 of the check upon it, be it in the form of some predaceous beetle, as most 

 likely ; of some birds, of climate, or condition of soil. In any of these 

 man may, nay, often is, the foremost motor. 



Insect species in general are best off in localities with alternations of 

 wooded and open tracts ; there the soil preserves its humidity for the 

 longest time, because, on the one hand, the water descending as rain is 

 greatly retained, and can percolate freely in the ground at the 

 same time that the decay of dead vegetation is promoted ; on 

 the other hand, the ground being shaded remains always toler- 

 ably cool, conditions favourable to predatory insects — Cara- 

 bidee, &c. The combined effect is to promote from year to year a more 

 luxurious growth of vegetation, well able to withstand the attacks of 

 destructive insects, and sheltering their enemies — predatory insects, 

 birds, &c. In localities where some plant-eating insect increased in- 

 ordinately, mostly a great deficiency of predatory insects and birds may 

 be noticed. Where there are many woody tracts at small distances sur- 

 face springs abound, reacting favourably upon the former, thus improving 

 and conserving the favourable aspect of the place. 



Now man enters upon the scene. 



The arable portions are carefully grubbed, cleared, and ploughed, 

 and on the rest he depastures his cattle and sheep. The summer's heat 

 parches the soil ; scores of insects, lizards, birds, &c, perish ; those birds 

 feeding upon the former, their eggs or larvae, which have not been 

 wantonly shot, withdraw themselves to other regions offering more privacy, 

 or die unable to bring up their young to maturity. Useless weeds and 

 destructive insects, gifted with great fecundity and adaptability, augment 

 apace for a while. The winter's rain descends unchecked, and carries the 

 loose unprotected surface-soil annually in millions of tons to the sea. The 

 shrubs and grasses continually browsed off, and unable to seed properly, 

 wither and die. The trees thinned by the wood-cutter, in whose shade for- 

 merly scores of humbler or younger members flourished, and in return in- 

 suring for their big brother plenty of moisture and coolness, sicken, are 

 attacked def encelessly by some insect, and succumb. Surface springs dry up 

 gradually, elsewhere retentive strata get disturbed, the rain water per- 



