[July, 
392 
The Parasites of Civilisation. 
In this manner numerous vile weeds have been introduced 
into Australia and California ; the house-fly, the ‘ cabbage 
white,” and the Hessian fly into North America; the 
Phylloxera into France ; the bed-bug and the cockroach into 
Europe generally ; the chigoe into tropical Africa ; and 
multitudes of other undesirable species have been imported 
into regions where they were formerly unknown, and where 
their room would be much more desirable than then 
company. # . , 
Cases have happened where parasitical animals have been 
introduced by man into new countries knowingly and wil- 
fully, but from ignorance of or mistake as to their probable 
behaviour. Under this head may be mentioned the con- 
veyance cf the thistle to Australia, and of the European 
sparrow to the United States, -both unfortunate experi- 
ments, regretted by the public at large, if not by theii ill- 
judging perpetrators. 
If we cast a glance at the entire series of these parasites 
we find them, at least as far as animals are concerned, dis- 
tinguished by a relatively small size. Hence they are able 
the more readily to conceal themselves from hostile seaich. 
Conjoined to small size, we find prodigious fecundity. Hence 
though the individual is feeble, the species is mighty. We 
may think them extirpated, but, if only a very few have 
escaped our pursuit, a short time sees them as numerous 
and as destructive as ever. 
Another important feature in parasites, or at least in a 
very large proportion of them, is a noCturnal or what 
amounts almost to the same thing an underground life. 
Everyone will here be reminded at once of the mouse and 
the rat, the snail and slug, the bed-bug and cockroach, the 
mole-cricket, the larvse of the cockchafer, the wirewoim, 
the crane-fly, which work havoc beneath the soil,— not to 
speak of the Phylloxera, which attacks principally tne loots 
of the vine. It needs little reflection to show how noCturnal 
and subterranean habits must withdraw any animal fiom 
ordinary human observation, and as a matter of course from 
attack. 
Another feature, almost universal, is tenacity of life, and 
power of supporting great differences of climate, and othei 
of the conditions of life. Without such tenacity, indeed, 
few plants could be weeds, and fewer animals could rank as 
vermin. For the latter, to be omnivorous is an important 
qualification. The rat and the mouse feed either upon 
vegetable or animal matter, whether living or dead. Almost 
all the slugs and snails will consume animal food, egg, 
