438 



EXTINCTION OF SPECIES. 



[Ch. XLII. 



mono 



But 



sucli as dead carcasses, which might taint the air. 

 sooner has the destroying commission been executed than 

 the gigantic power becomes dormant— each of the mighty 

 host soon reaches the term of its transient existence \nd 

 the season arrives when the whole species passes naturally 

 into the egg, and thence into the larva and pupa state. In 

 this defenceless condition it may be destroyed either by the 



mentation 



may 



■mation 



the season proves unfavourable to the hatchino- of the eg-p's 

 or tlie development of the pupse. 



Thus the swarming myriads depart which may have covered 

 the vegetation like the aphides, or darkened the air like 



locusts. In almost everv sf^a.snn thprA nva 



some 



Mil 



ton's spirits, which thronged the spacious halL ' reduce to 



forms 



So thick the aery crowd 



vegetable matter j and, 



Swarm'd and were straitened ; till, the signal given, 

 Eehold a wonder! they but now who seemed 

 In bigness to surpass earth's giant sons, 

 Now less than smallest dwarfs. 



A few examples will illustrate the mode in which this 

 force operates. It is well known that, among the countless 

 species of the insect creation, some feed on animal, others on 



upon considering- a catalogue of 

 8,000 British Insects and Arachnida3, Mr. Kirby found 

 that these two divisions were nearly a counterpoise to each 

 other, the carnivorous being somewhat preponderant. There 

 are also distinct species, some appointed to consume livin_ 

 other dead or putrid animal and vegetable substances. One 

 female, of Musca carnaria, will give birth to 20,000 young ; 

 and the larvse of many flesh-flies devour so much food 

 m twenty-four hours, and grow so quickly, as to increase their 

 weight two hundred-fold ! In five days after being hatched 

 they arrive at their full growth and size, so that there was 

 ground, says Kirby, for the assertion of Linnaeus, that three 



y 



