SYSTEM OF INSECTS. 397 
forms, we cannot point out any certain diagnostic by which 
in all cases a species may be distinguished from a va- 
riety ;—for those characters that in some are constant, in 
others vary. In general, where there is no difference in 
form, appendages and organs, proportions, sculpture and 
larve,—colour alone, especially in insects inhabiting the 
same district, only indicates a casual variety. Thus Apho- 
dius luridus ¥. has sometimes pale elytra with the strize 
black (Scarabeus nigro-sulcatus E.B.): 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 E.B.); and lastly, in a fourth they are in- 
tirely black (Sc. gagates E.B.) ;—yet all these in every 
other respect precisely correspond. But the converse 
of this will scarcely hold good; for doubtless minor dif- 
ferences of structure are sometimes produced by a dif- 
ferent food and climate: which may probably account for 
some variations observable in the individuals apparently 
of the same species obtained from different countries. 
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 five 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 prevalent, no one of them can 
be deemed universal. ‘The binary number we see exem- 
plified when two branches, so to speak, diverge from a 
common stem,—as in the Vegetableand Animal kingdoms ; 
the terrestrial and aquatic Predaceous beetles; in the 
thalerophagous and saprophagous Lamellicorn ones; in 
