BRITISH ANNELIDS.
109
With this systematic arrangement of the genus before us, it will
now be necessary to inquire how the various species of Allolobophora
may be differentiated. To render this point more easy I have arranged
the genus in natural groups, depending upon a number of clearly
defined characteristics. All the species at present known to science
fall under some four natural heading; but as one of these groups is
represented in Great Britain by only one solitary species at present,
I shall limit the group to three.1 These I will again arrange in a
systematic way in order to show their principal characteristics at a
glance.
Tabular View of the Genus Allolobophora.
It is but right to state that each of these groups runs into the
other, so that there is no hard and fast line to be drawn between
them; just as we cannot absolutely differentiate the genus Lumbricus
from Allolobophora so as to include every species known to science.
Every system breaks down if pressed too hard, and even among
the earthworms we find that Nature makes no freaksome leaps, but
progresses on regular lines.
I purpose now taking the three groups of worms belonging to
1 Since this article was written I have not only added to the list of British Worms, but
have also seen Rosa's excellent arrangement. On the strength of these, it will be necessary
eventually to modify the proposed plan somewhat; but, for the present, I allow my arrangement
to stand unaltered.