Let us in the first place confine our attention to the three chief forms which every Lepidopteron presents, viz. larva, pupa, and imago. With respect to the constancy or variability of these three forms, we actually find in nature all the combinations which are theoretically conceivable. (1.) There are species which possess a high degree of constancy in all three stages, such, for example, as _Limenitis Camilla_, _Pieris Brassicæ_,[171] _Sphinx Ligustri_, and _Euchelia Jacobææ_. (2.) There are species showing a high degree of variability in all three stages. This case must be of rare occurrence, as I am only able to adduce _Araschnia Prorsa-Levana_, a fact which arises from the circumstance that the pupal stage is, as a rule, but seldom variable. (3.) There are species which are variable in two stages and constant in the third. To this class, for example, belongs _Smerinthus Tiliæ_, of which the larva and imago are very variable, whilst the pupa is quite constant. The same is the case with _Lasiocampa Pini_, the well-known fir moth. Many butterflies show this same phenomenon in other combinations, such, for instance, as _Vanessa Urticæ_ and _Polychloros_, in which the larva and pupa are very variable, and the imago very constant. In a less degree the same is also the case with _Vanessa Atalanta_, whilst in _Pieris Napi_ the pupa and imago are variable, and the caterpillar remarkably constant, this likewise being the case with the local form _Bryoniæ_, which, according to my theory, is to be regarded as the parent form of _Napi_ (See Part I. of the present volume). (4.) There are species which are constant in two stages, and variable only in the third. Thus, a few species can be found in which the larva and pupa are constant and the imago variable. This is the case with _Saturnia Yamamai_, the imago of which is well known to present numberless shades of colour, varying from light yellow to greyish black, whilst the green caterpillar shows only slight individual differences of marking, and scarcely any differences of colour. The pupa of this species is quite constant. _Arctia Caja_ and _Hebe_, and _Chelonia Plantaginis_ belong to this same category. There are a very large number of species which possess very constant imagines and pupæ, but extremely variable larvæ. The following are the cases known to me:--_Macroglossa Stellatarum_, _Fuciformis_ and _Bombyliformis_; _Chærocampa Elpenor_, _Celerio_, and _Nerii_; _Deilephila Galii_, _Livornica_, Hübn., _Hippophaës_, _Vespertilio_, and _Zygophylli_; _Sphinx Convolvuli_; _Acherontia Atropos_; _Smerinthus Ocellatus_ and _Tiliæ_; _Callimorpha Hera_; _Cucullia Verbasci_ and _Scrophulariæ_.