You may perhaps object that the bringing of the Trachean _Arachnida_ and the _Myriapoda_ into the class _Insecta_ will render the approximation of them to a natural arrangement more difficult, since it will be impossible at the same time to connect the _Myriapods_ with the _Crustacea_, and the _Trachean_ with the genuine _Arachnida_. I admit the validity of your objection, but by no arrangement of insects in a simple series can we attain this object: the difficulty, however, may perhaps be obviated in this way. The distribution of organized matter, to adopt Mr. Wm. MacLeay's metaphor[43], begins in a dichotomy, constituting the animal and vegetable branches of the great tree of nature, and from these two great branches, by means of infinite ramifications, the whole system is formed, and, what is remarkable, these branches unite again so as to represent a series returning into itself, a discovery due to the patient investigation and acumen of our learned friend just mentioned. Now, in considering the _Aptera_ order, we find at first setting out from the Hexapods, a dichotomy, where the _Anoplura_ Leach branch off on the one side, and the _Thysanura_ Latr. on the other--the former, by means of the _Pediculidæ_, taking their food by suction, particularly _Phthirus_ Leach, or the Morpion (in which the segments of the trunk and abdomen become indistinct[44]) approach the _Octopods_ by the hexapod _Acari_ L.--the latter by _Machilis polypoda_ tending towards the _Myriapods_. In the Octopod branch a further dichotomy takes place, from which you proceed on one side to the _Araneidæ_ in the _Arachnida_, by _Phalangium_, &c.; and in the other by _Chelifer_, &c. to _Scorpio_. Again, the _Myriapod_ branch also divides, going by the _Iulidæ_ to one branch of the _Isopod Crustacea_, and by the _Scolopendridæ_ to another.
But there is another view of this subject before alluded to, which may be repeated here, and which seems to prove that the types of form in one natural group or class are reproduced in another; this appears to result from the following parallel series:
_Neuropterous_ _Aptera._ _Arachnida._ _Crustacea._ _Larvæ._
Myrmeleon _Phalangium_ Aranea { Decapoda { brachyura.