Among all the facts brought together in the first section I only know of one group of phenomena which at least permit of an attempt to refer them to a phyletic vital force. This is the occurrence of dark ground-colours in adult larvæ which are of light colours in their young condition. I have already attempted to show that in the _Chærocampa_-larvæ this change of colour depends on a double adaptation, the young caterpillars being adapted to the green colour of the plant and the adults to the soil and dead leaves. This interpretation appears the more correct when we find the same process, viz. the gradual replacement of the original green by brown colours, among species of widely different genera, which, with the dark colouring, possess the necessarily correlated habit of hiding themselves by day when in the adult condition. This is the case with _Sphinx Convolvuli_, _Deilephila Vespertilio_, and _Acherontia Atropos_. Thus far all has been easily explicable by natural selection; but when we also see a “tendency” to acquire a dark colour in the course of development, in those species which neither conceal themselves nor are adaptively coloured, but are very conspicuously marked--and if, further, it can be shown that these species, such for instance as _Deilephila Galii_, actually possess immunity from the attacks of foes,--how can this tendency to the formation of a dark colour be otherwise explained than by the admission of a phyletic vital force urging the variations in this direction? Nevertheless I believe that also on this point an appeal to unknown forces can be dispensed with. In the first place, dark ground-colours can be of use to a species otherwise than as means of adaptation. In _D. Galii_, as well as in _D. Euphorbiæ_, the light ring-spots appear rather at their brightest on the pitchy-black ground; and if this caterpillar must (_sit venia verbo!_) become conspicuous, this purpose would be best attained by acquiring a dark ground-colour, such as that of _D. Euphorbiæ_. The tendency, apparently common to all these _Sphingidæ_, to acquire a dark colour with increasing age, depends therefore on two quite distinct adaptations--first, in species sought by enemies, on an adaptation to the colour of the soil; and secondly, in species rejected by foes, on the endeavour to produce the greatest possible contrast of colour. Moreover, the supposition from which this last plea for a vital force set out is not universally correct, since there are species, such for instance as _D. Nicæa_, which never acquire a dark colour; and in _D. Galii_ also, although all the individuals abandon the protective green of the young stages, they by no means all acquire a dark hue in exchange for this colour; many individuals in their light ochreous-yellow colouring rather strikingly resemble the snake-like caterpillar of _D. Nicæa_. VII. PHYLETIC DEVELOPMENT OF THE MARKINGS OF THE SPHINGIDÆ: SUMMARY AND CONCLUSION.