1876.] 
Nature’s Scavengers. 17 1 
contribute at all to the purification of the water, being car- 
nivorous ; but their presence was a proof of the existence of 
other forms of insedt-life, on which they could prey. The 
canal, it must be added, was very sparingly used, and its 
successive pools might be regarded as a series of subsidence 
tanks. Hence it appears that subsidence and exposure to 
air alone will bring excessively polluted water to a state 
which permits of the existence of aquatic vegetation and of 
insedt-life. This point once reached, a further and rapid 
improvement is effected by these natural agencies. In a 
flowing stream, or in a canal at one uniform level and stirred 
up by constant traffic, these conditions would not occur. 
Under ordinary circumstances, therefore, it must be admit- 
ted that natural scavenging arrangements fail in case of 
excessively polluted waters. One error must be here carefully 
guarded against. Trees and shrubs growing in or near the 
water have no direct part in its purification. The oxygen 
given off from their leaves mixes with the atmosphere, and 
only adds upon the surface of the water ; but plants whose 
leaves are on or underneath the surface give off oxygen, 
which in its nascent state comes in contadt with the sus- 
pended or dissolved impurities, and effects their combustion. 
We have lastly to turn to the pollutions of the atmosphere, 
and to inquire whether there are any aerial scavengers — * 
any plants or animals engaged in the removal of volatile 
nuisances. That such nuisances exist cannot be doubted. 
All land-animals throw off from their skins* and from their 
respiratory apparatus a large amount of refuse, in the state 
of gases and vapours, probably also of solid matter in minute 
particles. The decomposing remains alike of plants and 
animals, together with the liquid and solid excrements of 
the latter, and, in short, all putrescent matters whatever, 
pollute the atmosphere, as is proved by the very fadt of the 
evil odours they emit. Many of these impurities are dealt 
with by inorganic agencies with which we have here no 
concern. The carbonic acid breathed out by animals— 
which is, stridtly speaking, a gaseous excrement — is removed 
by plants, which may thus claim a very prominent place 
among Nature’s scavengers. But we have to examine, 
further, if there is any animal especially qualified to attack 
those solid impurities which modern research has detected 
in the air? To this question an answer has lately been re- 
turned in the affirmative, and the animal selected for this 
duty has been the common house-fly ! We have no need to 
* An exception will probably exist in the case of such species as are covered 
with hard, corneous, or chitinous layers. 
