THE ANIMAL KINGDOM. 23 



tion, there are nevertheless Microzoa that float hither and 

 thither on the bosom of the atmosphere, notwithstanding it 

 appears to us so transparent and pure. 



The invisible organisms that people the air constitute, 

 according to Humboldt, quite a special fauna. But irre- 

 spective of these meteoric Infusoria, the existence of which, 

 according to this illustrious philosopher, cannot be doubted, 

 the atmosphere carries an immense quantity of ordinary 

 animalcules, both alive and dead, which its currents take up 

 and transport to all parts of the globe. Sometimes they 

 abound to such an extent in the air as to intercept the 

 light and suffocate travellers. 



Ehrenberg, on analyzing a shower of fine dust which, at 

 a distance of 380 miles from the coast of Africa, enveloped 

 vessels in a thick fog, found eighteen species of silicaplated 

 animalcules. 



But microscopic life does not invade air, earth, and water 

 only. It is met with again full of vigor and vitality in the 

 interior of animals and plants. No organ, however vigor- 

 ous or well protected, can elude it. Not only do animal- 

 cules pour into every cavity of the animal which is in com- 

 munication with the outer world, but thev are met with in 

 parts closely sealed up from the air. The complete system 

 of arteries and veins by which the circulation of the blood 

 is effected, though closed from the outer world at every 

 point, nevertheless at times contains Microzoa mixed with 

 the blood globules, apparently living quite at their ease in 

 the midst of the incessant flow of the blood. Supposing 

 that this traverses its circuit every day more than 2800 

 times, and that, not taking account of the capillary ramifi- 



