206 THE ENTOMOLOGIST. 
market-gardens, must have a tendency to favour the multiplica- 
tion of certain species, such as Abraxas grossulariata and Biston 
hirtaria. 
(4.) Species which do not belong to the original fauna, but 
have been introduced, and have now become common. These 
are chiefly household pests, as certain ants and cockroaches ; 
and insects which occur on cultivated plants, especially in hot- 
houses. 
(5.) Species which do not belong to the fauna, but have been 
introduced, accidentally or otherwise, and have not succeeded in 
establishing themselves. These have mostly occurred as single 
individuals, as, for instance, at the docks, where exotic species 
are frequently to be found. The means whereby an insect may 
be conveyed into Middlesex are now so numerous and varied, 
that the occurrence of almost anything is possible. The propriety 
of recording such accidental importations in a faunal list may be 
questioned ; but it has been thought best not to omit them in the 
present case, partly because it is not always easy to be certain 
whether an insect was imported, and partly because we can only 
get a proper knowledge of the value of this factor in distribution 
by observing and recording the instances. 
(6.) Species which do not belong to the fauna, but occa- 
sionally wander into Middlesex from their native districts. Such 
are the chalk species, common in Kent and Surrey; as, for 
example, Lycena corydon. Looking through the list, several 
species may be noted which come under this head. 
There is also.a possible seventh group, which, if it has any 
real existence, is of great importance. This would consist of 
endemic species or varieties, and those having their origin and 
metropolis within the limits of the county, if us actually con- 
fined to it. 
Nobody supposes that there are any species of insects 
peculiar to Middlesex; but if we permit ourselves to include the 
immediate neighbourhood of London on the south side of the 
Thames, and perhaps a small portion of Essex, there seems to 
be good evidence for the existence of a few characteristic forms 
of melanism, which have originated independently in the London 
district, and chiefly in Middlesex. That most or all of them have 
also been found far from London does not necessarily prove that 
the melanic varieties in Middlesex came from other counties, as, 
though they may not have spread from the London district any 
great distance, there is no reason why similar forms should not 
have arisen elsewhere, perhaps in several distant but suitable 
localities, quite independently. Supposing this to be the case, it 
would probably be found that when a species had two melanic 
races, say one in London and the other at Manchester, the facies 
of the two, when long series were compared, would be somewhat 
different. 
