[_Communicated by_ Mr. W. H. EDWARDS, _November 15th, 1879_.]
The experiments with this species were made in June, 1879, on pupæ
from eggs laid by the summer form _Umbrosa_ of the second brood of the
year, and obtained by confining a female in a bag on a stem of hop.
As the pupæ formed, and at intervals of from six to twenty-four hours
after pupation (by which time all the older ones had fully hardened),
they were placed in the ice-box. In making this experiment Mr. Edwards
had three objects in view. 1st. To see whether it was essential that
the exposure should take place immediately after pupation, in order to
effect any change. 2ndly. To see how short a period would suffice to
bring about any change. 3rdly. Whether exposing the summer pupæ would
bring about a change in the form of the resulting butterfly. Inasmuch
as breeding from the egg of _Umbrosa_, in June, in a former year,[64]
gave both _Umbrosa_ (11) and _Fabricii_ (6), the butterflies from the
eggs obtained, if left to nature, might be expected to be of both
forms. The last or fourth brood of the year having been found up to the
present time to be _Fabricii_, and the 1st brood of the spring, raised
from eggs of _Fabricii_ (laid in confinement), having been found to be
wholly _Umbrosa_, the latter is probably the summer and _Fabricii_ the
winter form. The two intervening broods, _i.e._ the 2nd and 3rd, have
yielded both forms. This species hibernates in the imago state.
After the pupæ had been in the ice-box fourteen days they were all
removed but 5, which were left in six days longer. Several were dead at
the end of fourteen days. The temperature most of the time was 1°-2°
R.; but for a few hours each day rose as the ice melted, and was found
to be 3°-6° R.
From the fourteen-day lot 7 butterflies were obtained, 3 males and
4 females. From the twenty-day lot 4 males and 1 female; every one
_Umbrosa_. All had changed in one striking particular. In the normal
_Umbrosa_ of both sexes,[65] the fore wings have on the upper side on
the costal margin next inside the hind marginal border, and separated
from it by a considerable fulvous space, a dark patch which ends a
little below the discoidal nervule; inside the same border at the inner
angle is another dark patch lying on the submedian interspace. Between
these two patches, across all the median interspaces, the ground-colour
is fulvous, very slightly clouded with dark.
In all the 4 females exposed to cold for fourteen days a broad black
band appeared crossing the whole wing, continuous, of uniform shade,
covering the two patches, and almost confluent from end to end with the
marginal border, only a streak of obscure fulvous anywhere separating
the two. In the case of the females from pupæ exposed for twenty days,
the band was present, but while broad, and covering the space between
the patches, it was not so dark as in the other females, and included
against the border a series of obscure fulvous lunules. This is just
like many normal females, and this butterfly was essentially unchanged.
In all the males the patches were diffuse, that at the apex almost
coalescing with the border. In the 3 from chrysalides exposed fourteen
days these patches were connected by a narrow dark band (very different
from the broad band of the females), occupying the same position as
the clouding of the normal male, but blackened and somewhat diffused.
In the 4 examples from the twenty-day pupæ, this connecting band was
scarcely as deeply coloured and continuous as in the other 3. Beyond
this change on the submarginal area, whereby a band is created where
naturally would be only the two patches, and a slight clouding of the
intervening fulvous surfaces, there was no difference of the upper
surface apparent between these examples of both sexes, and a long
series of natural ones placed beside them.
On the under side all the males were of one type, the colours being
very intense. There was considerably more red, both dark and pale, over
the whole surface, than in a series of natural examples in which shades
of brown and a bluish hue predominate. No change was observed in the
females on the under side.
It appears that fourteen days were as effective in producing changes
as a longer period. In fact, the most decided changes were found in
the females exposed the shorter period. It also appears that with
this species cold will produce change if applied after the chrysalis
has hardened. The same experiments were attempted in 1878 with pupæ
of _Grapta Comma_. They were put on ice at from ten minutes to six
hours after forming, and subjected to a temperature of about 0°-1° R.
for eighteen to twenty days, but every pupa was killed. Chrysalides
of _Papilio Ajax_ in the same box, and partly exposed very soon after
pupation, were not injured. It was for this reason that none of the
_Interrogationis_ pupæ were placed in the box till six hours had passed.
It appears further that cold may change the markings on one part of the
wing only, and in cases where it does change dark or dusky markings
melanises them; or it may deepen the colours of the under surface (as
in the females of the present experiment). The females in the above
experiment were apparently most susceptible to the cold, the most
decided changes having been effected in them.
The resulting butterflies were all of one form, although both might
have been expected to appear under natural circumstances.
_Dr. Weismann’s remarks on the foregoing experiments._--The author
of the present work has, at my request, been good enough to furnish
the following remarks upon Mr. Edward’s experiments with _G.
Interrogationis_:--
The interesting experiments of Mr. Edwards are here principally
introduced because they show how many weighty questions in connexion
with seasonal dimorphism still remain to be solved. The present
experiments do not offer a _direct_ but, at most, only an _indirect_
proof of the truth of my theory, since they show that the explanation
opposed to mine is also in this case inadmissible. Thus we have here,
as with _Papilio Ajax_, two out of the four annual generations mixed,
_i.e._, consisting of summer and winter forms, and the conclusion is
inevitable that these forms were not produced by the _gradual_ action
of heat or cold. When, from pupæ of the same generation which are
developed under precisely the same external conditions, both forms of
the butterfly are produced, the cause of their diversity cannot lie
in these conditions. It must rather depend on causes innate in the
organism itself, _i.e._, on inherited duplicating tendencies which
meet in the same generation, and to a certain extent contend with
each other for precedence. The two forms must have had their origin
in earlier generations, and there is nothing against the view that
they have arisen through the gradual augmentation of the influences of
temperature.
In another sense, however, one might perceive, in the facts discovered
by Edwards, an objection to my theory.
By the action of cold the form _Umbrosa_, which flies in June, was
produced. Now we should be inclined to regard the var. _Umbrosa_ as
the summer form, and the var. _Fabricii_, which emerges in the autumn,
hibernates in the imago state, and lays eggs in the spring, as the
winter form. It would then be incomprehensible why the var. _Umbrosa_
(_i.e._, the summer form) should be produced by cold.
But it is quite as possible that the var. _Umbrosa_ as that the var.
_Fabricii_ is the winter form. We must not forget that, in this
species, _not one of the four annual generations is exposed to the cold
of winter in the pupal state_. When, therefore, we have in such cases
seasonal dimorphism, to which complete certainty can only be given by
continued observations of this butterfly, which does not occur very
commonly in Virginia, this must depend on the fact that the species
formerly hibernated in the pupal stage. This question now arises, which
of the existing generations was formerly the hibernating one--the first
or the last?
Either may have done so _à priori_, according as the summer was
formerly shorter or longer than now for this species. If the former
were the case, the var. _Fabricii_ is the older winter form; were the
latter the case, the var. _Umbrosa_ is the original winter form, as
shall now be more closely established.
Should the experiments which Mr. Edwards has performed in the course of
his interesting investigations be repeated in future with always the
same results, I should be inclined to explain the case as follows:--
It is not the var. _Fabricii_, but _Umbrosa_, which is the winter
generation. By the northward migration of the species and the relative
shortening of the summer, this winter generation would be pushed
forward into the summer, and would thereby lose only a portion of the
winter characters which it had till that time possessed. The last of
the four generations which occurs in Virginia is extremely rare, so
that it must be regarded either as one of the generations now supposed
to be originating, or as one now supposed to be disappearing. The
latter may be admitted. Somewhat further north this generation would
be entirely suppressed, and the third brood would hibernate, either
in the imago state or as pupæ or caterpillars. In Virginia this third
generation consists of both forms. We may expect that further north, at
least, where it hibernates as pupæ, it will consist entirely, or almost
entirely, of the var. _Umbrosa_. Still further north in the Catskill
Mountains, as Edwards states from his own observations, the species
has only two generations, and one might expect that the var. _Umbrosa_
would there occur as the winter generation.
Should the foregoing be correct, then the fact that the second
generation assumes the _Umbrosa_ form by the action of cold, as was
the case in Edward’s experiments, becomes explicable. The new marking
peculiar to this form produced by this means must be regarded as a
complete reversion to the true winter form, the characters of which are
becoming partly lost as this generation is no longer exposed to the
influence of winter, but has become advanced to the beginning of summer.
_The foregoing explanation is, of course, purely hypothetical_;
it cannot at present be asserted that it is the correct one. Many
investigations based on a sufficiently large number of facts are still
necessary to be able to attempt to explain this complicated case with
any certainty. Neither should I have ventured to offer any opinion on
the present case, did I not believe that even such a premature and
entirely uncertain explanation may always be of use in serving the
inventive principle, _i.e._, in pointing out the way in which the truth
must be sought.
As far as I know, no attempt has yet been made to prove from a general
point of view the interpolation of new generations, or the omission
of single generations from the annual cycle, with respect to causes
and effects. An investigation of this kind would be of the greatest
importance, not only for seasonal dimorphism, but also for the
elucidation of questions of a much more general nature, and would be a
most satisfactory problem for the scientific entomologist. I may here
be permitted to develope in a purely theoretical manner the principles
in accordance with which such an investigation should be made:--
_On the change in the number of generations of the annual cycle._--A
change in the number of generations which a species produces annually
must be sought chiefly in changes of climate, and therefore in a
lengthening or shortening of the period of warmth, or in an increase or
diminution of warmth within this period; or, finally, in both changes
conjointly. The last case would be of the most frequent occurrence,
since a lengthening of the period of warmth is, as a rule, correlated
with an elevation of the mean temperature of this period, and _vice
versâ_. Of other complications I can here perceive the following:--
Climatic changes may be _active_ or _passive_, _i.e._, they occur by a
change of climate or by a migration and extension of the species over
new districts having another climate.
By a lengthening of the summer, as I shall designate the shorter
portion of the whole annual period of warmth, the last generation of
the year would be advanced further in its development than before;
if, for instance, it formerly hibernated in the pupal state, it would
now pass the winter in the imago stage. Should a further lengthening
of the summer occur, the butterflies might emerge soon enough to lay
eggs in the autumn, and by a still greater lengthening the eggs also
might hatch, the larvæ grow up and hibernate as pupæ. In this manner
we should have a new generation interpolated, owing to the generation
which formerly hibernated being made to recede step by step towards the
autumn and the summer. _By a lengthening of the summer there occurs
therefore a retrogressive interruption of generations._
The exact opposite occurs if the summer should become shortened. In
this case the last generation would no longer be so far developed
as formerly; for instance, it might not reach the pupal stage, as
formerly, at the beginning of winter, and would thus hibernate in
a younger stage, either as egg or larvæ. Finally, by a continual
shortening of the summer it would no longer appear at the end of
this period but in the following spring; in other words, it would be
eliminated. _By a shortening of the summer accordingly the interruption
of generations occurs by advancement._
The following considerations, which submit themselves with reference to
seasonal dimorphism, are readily conceivable, at least, in so far as
they can be arrived at by purely theoretical methods. Were the summer
to become shorter the generation which formerly hibernated in the pupal
stage would be advanced further into the spring. It would not thereby
necessarily immediately lose the winter characters which it formerly
possessed. Whether this would happen, and to what extent, would rather
depend upon the intensity of the action of the summer climate on the
generation in question, and on the number of generations which have
been submitted to this action. Hitherto no attempts have been made to
expose a monomorphic species to an elevated temperature throughout
several generations, so as to obtain an approximate measure of the
rapidity with which such climatic influences can bring about changes.
For this reason we must for the present refrain from all hypothesis
relating to this subject.
The disturbance of generations by the shortening of summer might also
occur to a species in such a manner that the generation which formerly
hibernated advances into the spring, the last of the summer generations
at the same time reaching the beginning of winter. The latter would
then hibernate in the pupal state, and would sooner or later also
assume the winter form through the action of the cold of winter. We
should, in this case, have two generations possessing more or less
completely the winter form, the ancient winter generation now gradually
losing the winter characters, and the new winter generation gradually
acquiring these characters.
In the reverse case, _i.e._, by a lengthening of the summer, we
should have the same possibilities only with the difference that the
disturbance of generations would occur in a reverse direction. In this
case it might happen that the former winter generation would become the
autumnal brood, and more or less preserve its characters for a long
period. Here also a new winter generation would be produced as soon as
the former spring brood had so far retrograded that its pupæ hibernated.
I am only too conscious how entirely theoretical are these conjectures.
It is very possible that observation of nature will render numerous
corrections necessary. For instance, I have assumed that every species
is able, when necessary, to adapt any one of its developmental stages
to hibernation. Whether this is actually the case must be learnt from
further researches; at present we only know that many species hibernate
in the egg stage, others in the larval state, others as pupæ, and yet
others in the perfect state. We know also that many species hibernate
in several stages at the same time, but we do not know whether each
stage of every species has an equal power of accommodation to cold.
Should this not be the case the above conjectures would have to be
considerably modified. To take up this subject, so as to completely
master all the facts connected therewith, naturalists would have to
devote their whole time and energy to the order Lepidoptera, which I
have been unable to do.
From the considerations offered, it thus appears that the phenomena
of seasonal dimorphism may depend on extremely complex processes, so
that one need not be surprised if only a few cases now admit of certain
analysis. We must also admit, however, that it is more advantageous to
science to be able in the first place to analyze the simplest cases by
means of breeding experiments, than to concern oneself in guessing at
cases which are so complicated as to make it impossible at present to
procure all the materials necessary for their solution.
[Illustration:
Plate I.
Aug. Weismann pinx. Lith. J.A. Hofmann, Würzburg.
]
[Illustration:
Plate II.
Aug. Weismann pinx. Lith. J.A. Hofmann, Würzburg.
]