406 .SYSTEM OF INSECTS. 
a variety ; — for those characters that in some are con- 
stant, in others vary. In general, where there is no dif- 
ference Inform, appendages and organs, sculpture, propor- 
tions and larvce, — colour alone, especially in insects in- 
habiting the same district, only indicates a casual variety. 
Thus Aphodius luridus has sometimes pale elytra with the 
striae b\ackl(Scarabaus nigro-sulcatus Marsh.): at others it 
has black spots between the striae, as in the type : in a 
third variety the elytra are black at the base and pale at 
the apex (Sc. varius Marsh.) ; and lastly, in a fourth they 
are intirely black (Sc. gagates Marsh.); — yet all these in 
every other respect precisely correspond. But the con- 
verse of this will scarcely hold good ; for doubtless minor 
differences of structure are sometimes produced by a 
different food and climate : which may probably account 
for some variations observable in the individuals appa- 
rently of the same species obtained from different coun- 
tries. 
Having considered the kind and value of the groups 
into which Annulose animals, and more especially insects, 
may be divided, I shall next call your attention to their 
composition. There are Jive numbers and their multi- 
ples which seem more particularly to prevail in nature : 
namely, Two — Three — Four — Five and Seven. But 
though these numbers are jn-evalent, no one of them can 
be deemed universal. The binary number, which af- 
fords the most simple, and for that reason perhaps not 
the least valuable, mode of arrangement, we see exem- 
plified when two branches, so to speak, diverge from a 
common stem, — as in the Vegetable and Animal king- 
doms ; the terrestrial and aquatic Predaceous beetles; 
