But in granting this, we decidedly refute the statement that constancy and variability are inherent properties of certain forms of marking. This reasoning is based on the simple fact that a given type of marking comprises both species of great constancy and of (relatively) great variability. Thus, the fore-wings of _Sphinx Ligustri_ and _S. Convolvuli_ are extremely constant, whilst the very similarly marked _Anceryx (Hyloicus) Pinastri_ is exceedingly variable. Similarly _Deilephila Euphorbiæ_ is known by its great variability of colouring and marking, whilst _D. Galii_, which resembles this species so closely as to be sometimes confounded with it, possesses a high degree of constancy, and further, the Corsican and Sardinian _D. Dahlii_ is very variable. Among the family _Arctiidæ_, _Callimorpha Hera_ and the Alpine _Arctia Flavia_ are cases of constancy, whilst _A. Caja_, which is so similar to the last species, is so generally variable that two perfectly identical specimens can scarcely be found together. The same can be shown to hold good for the markings of caterpillars. Thus, the larva of _D. Dahlii_ shows very considerable variability, whilst that of _D. Galii_ is very constant in marking (disregarding the ground-colour). So also the larva of _Vanessa Urticæ_ is very variable and that of _V. Antiopa_ very constant, &c. The great differences with respect to constancy or variability which are displayed by the different stages of one and the same species, must therefore find their explanation elsewhere than in the type of the marking itself. The explanation must be found in the circumstance that each stage changes independently of the others, and at different periods can enter a new phase of variability. We are here led in anticipation to the main question:--Are changes produced by internal or external causes? is it the physical nature of the organism which is compelled to become remoulded spontaneously after the lapse of a certain period of time? or does such modification only occur when produced directly or indirectly by the external conditions of life? In the cases before us the facts undoubtedly indicate a complete dependence of the transformations upon external conditions of life. The independent appearance of variability in the separate stages of the metamorphosis might, however, be regarded as only apparent. It might still be attempted to attribute the changes to a purely inherent cause, _i.e._, to a phyletic vital force, by assuming that the latter acts periodically in such a manner that at first one and then the following stage becomes variable, until finally the entire species is transformed. There is but little to be said in reply to this if we once take refuge in entirely unknown forces, the operation of which can be arbitrarily conceived to be either constant or periodic.