If it be asked why the longitudinal lines were first formed, and then the oblique stripes, it may be replied that the physical constitution of these caterpillars would be more easily able to give rise to simple longitudinal lines than to complicated oblique stripes crossing their segments.[164] It may perhaps also be suggested that the oldest _Sphingidæ_ lived entirely on low plants among grasses, and in the course of time gradually took to shrubs and trees. At the present time the majority of the Sphinx-larvæ still live on low plants, and but few on trees, such caterpillars generally belonging to certain special genera.
The character of oblique stripes becomes perfected by the addition of coloured edges, the latter, as is self-evident, having been added subsesequently.
The third chief constituent of the Sphinx-markings, _i.e._ the spots--whether perfect ocelli or only ring-spots--in two of the special genera here considered, arise on the subdorsal, where they are either deposited (_Deilephila_), or built up from a fragment of this line (_Chærocampa_). That these markings can, however, also originate independently of the subdorsal, is shown by the ocellus of _Pterogon Œnotheræ_, situated on the segment bearing the caudal horn. In this case, however, the ontogeny teaches us that the spot also succeeds the subdorsal, so that we can state generally that all these spot-markings are of later origin than the longitudinal striping.
The question as to the relative ages of the oblique stripes and the spot-marking does not admit of a general answer. In some cases (_C. Elpenor_ and _Porcellus_) the oblique stripes disappear when the ocelli reach complete development, and we may therefore venture to conclude that in these cases the former appeared earlier in the phylogeny. But it is very probable that oblique stripes arose independently at different periods, just as longitudinal lines occur irregularly in quite distinct families. It would be a great error if we were to ascribe the possession of oblique stripes solely to descent from a common ancestor. The oblique markings found on certain species of _Macroglossa_ (_M. Corythus_ from India) have not been inherited from a remote period, but have been independently acquired by this or by some recent ancestral species. They have nothing to do _genetically_ with the oblique stripes which occur in some species of _Chærocampa_ (_e.g._ in _C. Nessus_, from India), or with those of the species of _Smerinthus_ and _Sphinx_. They depend simply on analogous adaptation (Seidlitz[165]), _i.e._ on adaptation to an analogous environment.