Regarding only the _adult_ larvæ of the species of _Deilephila_,
these represent in their five groups, five stages in the phyletic
development of the genus; but if we also take into consideration the
developmental history, two more stages must be added, viz., that in
which the caterpillar possesses no particular marking, as was found to
be the case in the first stage of the development of _D. Euphorbiæ_
and _D. Dahlii_; and a second stage with a subdorsal line, but without
any ring-spot formations. Seven stages of phyletic development must
therefore be distinguished.
_Stage 1._--No species with entire absence of marking in the adult form
now occurs.
_Stage 2._--A subdorsal, accompanied by a spiracular line, extends from
the caudal horn to the first segment. This also no longer forms the
final stage of the ontogeny, but is, however, undoubtedly retained in
the second stage of several species (_D. Vespertilio_, _Livornica_,
_Lineata_, and perhaps also _Galii_).
_Stage 3._--The subdorsal line bears a ring-spot on the penultimate
segment; the other markings as in the last stage. _D. Hippophaës_ only
belongs to this stage, a small number of specimens, however, showing
a transition to the following stage by the transference of ring-spots
from the posterior to the anterior segments.
_Stage 4._--Open ring-spots appear on the subdorsal line on all the
segments from the eleventh to the first. _D. Zygophylli_ and the North
American _D. Lineata_ belong here.
_Stage 5._--Closed ring-spots are situated on the subdorsal line. Of
the known species, only _D. Livornica_ concludes its development at
this phyletic stage.
_Stage 6._--A single row of ring-spots replaces the subdorsal line. _D.
Galii_, _Vespertilio_, and _Mauritanica_ represent this stage at the
conclusion of their ontogeny.[95]
_Stage 7._--A double row of ring-spots. Only _D. Dahlii_, _Euphorbiæ_,
and _Nicæa_ attain to this highest stage of _Deilephila_ marking, the
two first species in the fourth stage, and _Nicæa_ in the third stage
of its ontogeny.
Although our knowledge of the history of the development of the
individual species is still so fragmentary, we may conclude with
certainty that the development of the markings has been uniform
throughout--that it has proceeded in the same manner in all species.
All the species appear to be making for the same goal, and the
question thus arises whether there may not be an innate force
urging their phyletic development. The rigorous examination of this
conception must be reserved for a later section. Here, as we are
only occupied essentially in establishing facts, it must be remarked
that retrogression has never been observed. The young larval forms
of a species never show the markings of a later phyletic stage than
the older larval forms; the development takes the same course in all
species, only making a greater advance in the same direction in some
than in others.
Thus, _Nicæa_ and _Euphorbiæ_ have advanced to the seventh phyletic
stage, _Zygophylli_ and _Hippophaës_ only to the third, and some
specimens of _Zygophylli_ to the fourth. But at whatever phyletic
stage the ontogeny of a species may terminate, the young larval
stages always display the older phyletic stages. Thus, _Galii_ in its
last ontogenetic stage reaches the _sixth_ phyletic stage; in its
penultimate stage it reaches the _fifth_ phyletic stage; and in its
third stage; the _fourth_ phyletic stage is represented, so that little
imagination is required to anticipate that in the second stage the
_third_ or _second_ phyletic stage would be pictured.
If we tabulate the development of the various species, indicating the
ontogenetic stages by Arabic numerals, and the stages of the phylogeny
which are reached in each stage of the ontogeny by Roman numerals, we
obtain a useful synopsis of the series of developments, and, at the
same time, it shows how many gaps still remain to be filled up in order
to complete our knowledge even of this small group of species.