It may appear almost unnecessary to bring forward additional proofs in support of this interpretation of the facts, but in a field where the data are so scanty, no argument which can be drawn from them should be considered as superfluous. The variations which occasionally occur in the larvæ, however, to a certain extent furnish a proof of the correctness of the theoretical interpretation offered.
When, in the ontogeny of these species, we actually see before us a series of stages of phyletic development, we must admit that ordinary reversion may occur, causing an adult caterpillar to show the characters of the young. Forms reverting to an earlier phyletic stage must, on the whole, occur but seldom, as this stage is removed further back in the ontogeny. Thus, indications of the subdorsal line must occur but rarely in the _adult_ larvæ of _Euphorbiæ_, and still less frequently in _Nicæa_, whilst they must be expected to be of more common occurrence in _Vespertilio_, and also, as has already been seen, in _Dahlii_. In this last species, as also in _Vespertilio_, the completely-developed subdorsal line is still present in the third stage, whilst it is possessed by _Euphorbiæ_ only in the second stage, and then in a rudimentary condition.
The state of affairs may in fact be thus described: Among several hundred adult larvæ of _Dahlii_ found in Sardinia by Dr. Staudinger, there were some which did not actually possess a distinct subdorsal line, but in place thereof, and as its last indication, a feeble light stripe. One of Dr. Staudinger’s caterpillars showed also a distinct line between the closed eye-spots. In the last stage of _Vespertilio_ this line appears still more frequently, whilst in _Euphorbiæ_ it is extremely rare, and when present it only appears as a faint indication. This is the case with one of the specimens figured in Hübner’s work as an “aberration,” and also with one in Dr. Staudinger’s collection. Of _Nicæa_ I have at most seen only eight specimens, none of which showed any trace of the long-vanished subdorsal line.
It must be expected that any ontogenetic stage would most readily revert to the preceding phyletic stage, so that characters present in the preceding stage are consequently those which would most commonly arise by reversion. This postulate of the theory also finds confirmation in the facts. Caterpillars which, when full grown, belong to the _seventh_ phyletic stage, _e.g._ _D. Euphorbiæ_, not unfrequently show variations corresponding to the _sixth_ stage, _i.e._ only one instead of two rows of ring-spots--the upper and first-appearing series. On the other hand, forms reverting to the _fifth_ phyletic stage (ring-spots with connecting subdorsal line) occur but very rarely. I have never met with such cases in adult living caterpillars of _D. Euphorbiæ_, although in one instance such a larva was found in the fourth ontogenetic stage; but the strikingly dark, brownish subdorsal line which connected the otherwise perfectly developed ring-spots, completely disappeared in the fifth stage of the ontogeny. Those larvæ which, in the adult state, belong to the _sixth_ phyletic stage, not unfrequently show the characters of the _fifth_ stage more or less developed, as, for example, _D. Vespertilio_.[96]