Seasonal dimorphism has already been designated by Wallace as
alternation of generation,[37] a term which cannot be disputed so long
as it is confined to a regular alternation of dissimilar generations.
But little is gained by this definition, however, unless it can be
proved that both phenomena are due to similar causes, and that they
are consequently brought about by analogous processes. The causes of
alternation of generation have, until the present time, been scarcely
investigated, owing to the want of material. Haeckel alone has quite
recently subjected these complicated phenomena generally to a searching
investigation, and has arrived at the conclusion that the various
forms of metagenesis can be arranged in two series. He distinguishes
a progressive and a retrogressive series, comprising under the former
those species “which, to a certain extent, are still in a transition
stage from monogenesis to amphigenesis (asexual to sexual propagation),
and the early progenitors of which, therefore, never exclusively
propagated themselves sexually” (_Trematoda_, _Hydromedusæ_). Under the
other, or retrogressive form of metagenesis, Haeckel includes a “return
from amphigenesis to monogenesis,” this being the case with all those
species which now manifest a regular alternation from amphigenesis to
parthenogenesis (_Aphides_, _Rotatoria_, _Daphniidæ_, _Phyllopoda_,
&c.). Essentially I can but agree entirely with Haeckel. Simply
regarding the phenomena of alternation of generation as at present
known, it appears to me to be readily admissible that these multiform
modes of propagation must have originated in at least two different
ways, which can be aptly formulated in the manner suggested by Haeckel.
I will, however, venture to adopt a somewhat different mode of
conception, and regard the manner of propagation (whether sexual or
asexual) not as the determining, but only as the secondary cause. I
will further hazard the separation of the phenomena of alternating
generations (in their widest sense) into two main groups according to
their origin, designating the cases of one group as true metagenesis
and those of the other as heterogenesis.[38] Metagenesis takes
its origin from a phyletic series of dissimilar forms, whilst
heterogenesis originates from a phyletic series of similar forms--this
series, so far as we can at present judge, always consisting of
similar sexual generations. The former would thus nearly coincide
with Haeckel’s progressive, and the latter with his retrogressive
metagenesis. Metagenesis may further originate in various ways. In the
first place, from metamorphosis, as for example, in the propagation
of the celebrated _Cecidomyia_ with nursing larvæ. The power which
these larvæ possess of propagating themselves asexually has evidently
been acquired as a secondary character, as appears from the fact that
there are many species of the same genus the larvæ of which do not
nurse, these larvæ being themselves undoubted secondary forms produced
by the adaptation of this stage of phyletic development to a mode of
life widely different from that of the later stages. In the form now
possessed by these larvæ they could never have represented the final
stage of their ontogeny, neither could they have formerly possessed
the power of sexual propagation. The conclusion seems inevitable that
metagenesis has here proceeded from metamorphosis; that is to say, one
stage of the ontogeny, by acquiring asexual propagation, has changed
the originally existing metamorphosis into metagenesis.
Lubbock[39] is undoubtedly correct when, for cases like that just
mentioned, he attempts to derive alternation of generations from
metamorphosis. But if we exclude heterogenesis there still remain a
large number of cases of true metagenesis which cannot be explained
from this point of view.
It must be admitted, with Haeckel, that the alternation of generations
in the Hydromedusæ and Trematoda does not depend, as in the case of
_Cecidomyia_, upon the larvæ having acquired the power of nursing,
but that the inferior stages of these species always possessed this
power which they now only preserve. The nursing Trematode larvæ now
existing may possibly have been formerly able to propagate themselves
also sexually, this mode of propagation having at the present time
been transferred to a later phyletic stage. In this case, therefore,
metagenesis was not properly produced by metamorphosis, but arose
therefrom in the course of the phyletic development, the earlier
phyletic stages abandoning the power of sexual reproduction, and
preserving the asexual mode of propagation. A third way in which
metagenesis might originate is through polymorphosis. When the
latter is combined with asexual reproduction, as is especially the
case with the Hydrozoa, metagenesis may be derived therefrom. The
successive stages of transformation of one and the same physiological
individual do not in these cases serve as the point of departure
for alternation of generation, but the different contemporary forms
living gregariously into which the species has become divided through
functional differentiation of the various individuals of the same
stock. Individuals are here produced which alone acquire the power
of sexual reproduction, and metagenesis is thus brought about,
these individuals detaching themselves from the stock on which they
originated, while the rest of the individuals remain in combination,
and retain the asexual mode of propagation. No sharp distinction can be
otherwise drawn between this and the cases previously considered.[40]
The difference consists only in the whole cycle of reproduction being
performed by one stock; both classes have the common character that
the different phyletic stages never appear in the same individual
(metamorphosis), but in the course of further phyletic development
metagenesis at the same time arises, i.e. the division of these stages
among a succession of individuals. We are therefore able to distinguish
this _primary metagenesis_ from the _secondary metagenesis_ arising
from metamorphosis.
It is not here my intention to enter into the ultimate causes of
metagenesis; in this subject we should only be able to advance by
making vague hypotheses. The phenomenon of seasonal dimorphism,
with which this work has mainly to deal, is evidently far removed
from metagenesis, and it was to make this clear that the foregoing
observations were brought forward. The characters common in the origin
of metagenesis are to be found, according to the views previously
set forth, in the facts that here the faculty of asexual and of
sexual reproduction is always distributed among _several_ phyletic
stages of development which succeed each other in an ascending series
(progressive metagenesis of Haeckel), whereas I find differences only
in the fact that the power of asexual propagation may (in metagenesis)
be either newly acquired (larva of _Cecidomyia_) or preserved from
previous ages (_Hydroida_). It seems that in this process sexual
reproduction is without exception lost by the earlier, and remains
confined solely to the most recent stages.
From the investigations on seasonal dimorphism it appears that a
cycle of generations can arise in an entirely different way. In this
case a series of generations originally alike are made dissimilar by
external influences. This appears to me of the greatest importance,
since seasonal dimorphism is without doubt closely related to that
mode of reproduction which has hitherto been exclusively designated
as heterogenesis, and a knowledge of its mode of origination must
therefore throw light on the nature and origin of heterogenesis in
general.
In seasonal dimorphism, as I have attempted to show, it is the
direct action of climate, and indeed chiefly that of temperature,
which brings about the change in some of the generations. Since
these generations have been exposed to the alternating influence
of the summer and winter temperature a periodical dimorphism has
been developed--a regular cycle of dissimilar generations. It has
already been asserted that the consecutive generations of a species
comport themselves with respect to heredity in a manner precisely
similar to that of the ontogenetic stages, and at the same time such
succeeding generations point out the parallelism between metamorphosis
and heterogenesis. If influences capable of directly or indirectly
producing changes operate on any particular stage of development,
these changes are always transmitted to the same stage. Upon this
metamorphosis depends. In a precisely similar manner changes which
operated periodically on certain generations (1, 3, 5, for instance)
are transmitted to these generations only, and not to the intermediate
ones. Upon this depends heterogenesis. We have just been led to the
comprehension of heterogenesis by cyclical heredity, by the fact
that a cycle is produced whenever a series of generations exists
under regularly alternating influences. In this cycle newly acquired
changes, however minute in character at first, are only transmitted to
a later, and not to the succeeding generation, appearing only in the
one corresponding, i.e. in that generation which exists under similar
transforming influences. Nothing can more clearly show the extreme
importance which the conditions of life must have upon the formation
and further development of species than this fact. At the same time
nothing shows better that the action of these conditions is not
suddenly and violently exerted, but that it rather takes place by small
and slow operations. In these cases the long-continued accumulation
of imperceptibly small variations proves to be the magic means by
which the forms of the organic world are so powerfully moulded. By the
application of even the greatest warmth nobody would be able to change
the winter form of _A. Levana_ into the summer form; nevertheless, the
summer warmth, acting regularly on the second and third generations
of the year, has, in the course of a lengthened period, stamped these
two generations with a new form without the first generation being
thereby changed. In the same region two different climatic varieties
have been produced (just as in the majority of cases climatic varieties
occur only in separate regions) which alternate with each other, and
thus give rise to a cycle of which each generation propagates itself
sexually.
But even if seasonal dimorphism is to be ascribed to heterogenesis, it
must by no means be asserted that those cases of cyclical propagation
hitherto designated as heterogenesis are completely identical with
seasonal dimorphism. Their identity extends only to their origin
and manner of development, but not to the mode of operation of the
causes which bring about their transformation. Both phenomena have a
common mode of origination, arising from similar (monomorphic) sexual
generations and course of development, a cycle of generations with
gradually diverging characters coming into existence by the action of
alternating influences. On the other hand, the nature of the changes
by which the secondary differs from the primary generation may be
referred to another mode of action of the exciting causes. In seasonal
dimorphism the differences between the two generations are much less
than in other cases of heterogenesis. These differences are both
quantitatively less, and are likewise qualitative, affecting only
characters of biological insignificance.[41] The variations in question
are mostly restricted to the marking and colouring of the wings and
body, occasionally affecting also the form of the wing, and in a few
cases the size of the body (_Plebeius Amyntas_), whilst the bodily
structure--so far at least as my investigations extend--appears to be
the same in both generations.[42]
The state of affairs is quite different in the remaining cases of
heterogenesis; here the entire structure of the body appears to be
more or less changed, and its size is often very different, nearly all
the internal organs differing in the two generations. According to
Claus,[43] “we can scarcely find any other explanation of the mode of
origination of heterogenesis than the gradual and slow advantageous
adaptation of the organization to important varying conditions of
life”--a judgment in which this author is certainly correct. In all
such cases the change does not affect unimportant characters, as it
does in butterflies, but parts of biological or physiological value;
and we cannot, therefore, consider such changes to have originated
through the _direct_ action of altered conditions of life, but
_indirectly_ through natural selection or adaptation.
Thus, the difference between seasonal dimorphism and the other known
cases of heterogenesis consists in the secondary form in which the
species appears in the former originating through the direct action
of external conditions, whilst in the latter this form most probably
originates through the indirect action of such influences. The first
half of the foregoing proposition is alone capable of provisional
proof, but it is in the highest degree probable that the latter half
is also correct. Naturally we cannot say to what extent the direct
action of external conditions plays also a part in true heterogenesis,
as there have been as yet no experiments made on its origin. That
direct action, working to a certain extent co-operatively, plays only
a secondary part, while the chief cause of the change is to be found
in adaptation, no one can doubt who keeps in view, for instance, the
mode of propagation discovered by Leuckart in _Ascaris nigrovenosa_. In
this worm, the one generation lives free in the water, and the other
generation inhabits the lungs of frogs, the two generations differing
from one another in size of body and structure of internal organs to an
extent only possible with the true Nematoda.
To prevent possible misunderstanding, let it be finally noted--even
if superfluous--that the changes causing the diversity of the two
generations in seasonal dimorphism and heterogenesis are not of such
a nature that the value of different “specific characters” can be
attached to them. Distinctly defined specific characters are well
known not to occur generally, and it would therefore be erroneous to
attach but little value to the differences in seasonal dimorphism
because these chiefly consist in the colouring and marking of the
wings. The question here under consideration is not whether two animal
forms have the value of species or of mere varieties--a question which
can never be decided, since the reply always depends upon individual
opinion of the value of the distinctions in question, and the idea
of both species and varieties is moreover purely conventional. The
question is, rather, whether the distinguishing characters possess
an equal constancy--that is, whether they are transmitted with the
same force and accuracy to all individuals; and whether they occur,
therefore, in such a manner that they can be practically employed
as specific characters. With respect to this, it cannot be doubtful
for a moment that the colouring and marking of a butterfly possess
exactly the same value as the constant characters in any other group
of animals, such as the palate-folds in mice, the structure of the
teeth in mammals, the number and form of the wing and tail feathers in
birds, &c. We have but to remember with what wonderful constancy often
the most minute details of marking are transmitted in butterflies. The
systematist frequently distinguishes between two nearly allied species,
as for instance in the _Lycænidæ_, chiefly by the position of certain
insignificant black spots on the under side of the wing (_P. Alexis_
female, and _P. Agestis_); and this diagnosis proves sufficient, since
_P. Alexis_, which has the spots in a straight row, has a different
caterpillar to _P. Agestis_, in which the central spot is nearer the
base of the hind wing!
For the reasons just given, I maintain that it is neither justifiable
nor useful to designate the di- and polymorphism of butterflies as
di- and polychroism, and thereby to attribute but little importance to
these phenomena.[44] This designation would be only justifiable if the
differences of colour were due to other causes than the differences of
form, using this last word in a narrow sense. But it has been shown
that the same direct action of climate which originates new colours,
produces also in some species differences of form (contour of wing,
size, &c.); whilst, on the other hand, it has long been known that many
protective colours can only be explained by the indirect action of
external conditions.
When I raise a distinction in the nature of the changes between
seasonal dimorphism and the remaining known cases of heterogenesis,
this must be taken as referring only to the biological or physiological
result of the change in the transformed organism itself. In seasonal
dimorphism only insignificant characters become prominently changed,
characters which are without importance for the welfare of the species;
while in true heterogenesis we are compelled to admit that useful
changes, or adaptations, have occurred.
Heterogenesis may thus be defined either in accordance with my proposal
or in the manner hitherto adopted, since it may be regarded as more
morphological than the cyclical succession of differently formed sexual
generations; or, with Claus, as the succession of different sexual
generations, “living under different conditions of existence”--a
definition which applies in all cases to seasonal dimorphism. Varying
conditions of existence, in their widest sense, are the result of the
action of different climates; and a case has been made known recently
in which it is extremely probable that the climatic differences of
the seasons have produced a cycle of generations by influencing the
processes of nutrition. This case is quite analogous to that which
we have observed in the seasonal dimorphism of butterflies, but with
the distinction that the difference between the winter and summer
generations does not, at least entirely, consist in the form of the
reproductive adult, but almost entirely in its ontogeny--in the mode
of its development. A comparison of this case with the analogous
phenomenon in butterflies, may be of interest. In the remarkable
fresh-water Daphnid, _Leptodora hyalina_ Lillejeborg, it was proved
some years ago by P. E. Müller,[45] who studied the ontogeny, that this
last was direct, since the embryo, before leaving the egg, already
possesses the form, members, and internal organs of the adult. This
was, at least, the case with the summer eggs. It was subsequently
shown by Sars[46] that this mode of development only holds good for
the summer brood, the winter eggs producing an embryo in the spring
which possesses only the three first pairs of limbs, and, instead of
compound eyes, only a single frontal eye, thus exhibiting briefly, at
first, the structure of a _Nauplius_, and gradually acquiring that
of _Leptodora_. The mature form derived from the winter eggs is not
distinguishable from the later generations, except by the presence
of the simple larval eye, which appears as a small black spot. The
generations when fully developed are thus distinguished only by this
minute marking, but the summer generation undergoes direct development,
whilst the winter generation, on the contrary, is only developed
by metamorphosis, beginning with the simplest Crustacean type, and
thus fairly representing the phyletic development of the species. We
therefore see, in this case, the combination of a metamorphic and a
direct development taking place to a certain extent under our eyes. It
cannot be proved with certainty what the cause of this phenomenon may
be, but the conjecture is almost unavoidable that it is closely related
to the origin of the seasonal dimorphism of butterflies, since both
depend on the alternating climatic influences of summer and winter:
it is most probable that these influences have directly[47] brought
about a shortening of the period of development in summer. Thus we have
here a case of heterogenesis nearly related to the seasonal dimorphism
of butterflies in a twofold manner--first, because the cycle of
generations is also in this case brought about by the direct action of
the external conditions of life; and secondly, the winter form is here
also the primary, and the summer form the secondary one.
In accordance with the idea first introduced into science by Rudolph
Leuckart, we have hitherto understood heterogenesis to be only the
alternation of dissimilar sexual generations. From this point of
view the reproduction of _Leptodora_ can be as little ascribed to
heterogenesis as can that of _Aphis_ or _Daphnia_, although the
apparent agamic reproduction of the winter and a portion of the summer
generation is undoubtedly parthenogenesis and not propagation by
nursing.[48] As has already been said, however, I would attribute no
fundamental importance to the criterion of agamic reproduction--the
more especially because we are ignorant of the physiological
significance of the two modes of propagation; and further, because this
principle of classification is entirely external, and only valuable
in so far as no better one can be substituted for it. A separation of
the modes of cyclical propagation according to their genesis appears
to me--especially if practicable--not alone to be of greater value,
but the only correct one, and for this the knowledge of the origin of
seasonal dimorphism seems to me to furnish a possible method.
If, as was indicated above, we designate as metagenesis (in the narrow
sense) all those cases in which it must be admitted that a series
of differently aged phyletic stages have furnished the points of
departure, and as heterogenesis those cases in which similar phyletic
stages have been compelled to produce a cycle of generations by the
periodic action of external influences, it is clear that the scope of
heterogenesis is by this means considerably extended, and at the same
time sharply and precisely defined.
Under heterogenesis then is comprised, not only as heretofore the
reproduction of _Ascaris nigrovenosa_, of _Leptodora appendiculata_,
and of the cattle-lice, but also that of the _Aphides_, _Coccidæ_,
_Daphniidæ_, _Rotatoria_, and _Phyllopoda_, and, in short, all those
cases in which we can determine the former identity of the two kinds
of generations from their form, anatomical structure, and mode of
reproduction. This conclusion is essentially supported by a comparison
of the most closely allied species. Thus, for instance, when we see
the genus _Aphis_ and its allies related on all sides to insects which
propagate sexually in all generations, and when we further observe
the great similarity of the whole external and internal structure
in the two kinds of generations of _Aphis_, we are forced to the
conjecture that the apparent asexual reproduction of the _Aphidæ_ is in
reality parthenogenesis, i.e., that it has been developed from sexual
reproduction. Neither can it be any longer disputed that in this case,
as well as in that of _Leptodora_ and other _Daphniidæ_, the same female
alternately propagates parthenogenetically, and produces eggs requiring
fertilization. This was established by Von Heyden[49] some years ago,
in the case of _Lachnus Querci_, and has been since confirmed by
Balbiani.[50]
There can be no doubt that in all these cases the cycle of generations
has been developed from phyletically similar generations. But
instances are certainly conceivable which present themselves with less
clearness and simplicity. In the first place, we do not know whether
parthenogenesis may not finally settle down into complete asexual
reproduction. Should this be the case, it might be possible that from
heterogenesis a mode of propagation would ultimately arise, which
was apparently indistinguishable from pure metagenesis. Such a state
of affairs might result, if the generations settling into asexual
reproduction (as, for instance, the plant-lice), at the same time
by adaptation to varying conditions of life, underwent considerable
change of structure, and entered upon a metamorphosis to some extent
retrogressive. We should then be inclined to regard these generations
as an earlier phyletic stage, whilst, in fact, they would be a later
one, and the idea of metagenesis would thus have been formed after the
manner of heterogenesis.
On the other hand, it is equally conceivable that heterogenesis may
have been developed from true metagenesis in the case of larvæ which,
having acquired the faculty of asexual propagation, are similar in
function to sexually mature insects. This possibility is not at first
sight apparent. If the nursing-larvæ of the _Cecidomyiæ_ were as much
like the sexual insects as are the young Orthoptera to the sexually
mature forms, we should not know whether to regard them as degraded
sexual insects, or as true larvæ which had attained the power of
asexual propagation. Their propagation would be considered to be
parthenogenesis; and as it could not be denied that heterogenesis was
here manifest, the mode of development of their particular kind of
propagation might be proved, i.e., it might be demonstrated, that the
generations now parthenogenetic were formerly mere reproductive larval
stages.
I have only offered these last observations in order to show on what
uncertain ground we are still standing with regard to this subject
whenever we deal with the meaning of any particular case, and how
much still remains to be done. It appears certain that the two forms
of cyclical propagation, heterogenesis and metagenesis, originate in
entirely distinct ways, so that it must be admitted that, under these
circumstances, the idea of the existing conditions respecting the true
genesis may possibly be erroneous. To indicate the manner in which the
cyclical mode of propagation has arisen in any single case, would only
be possible by a searching proof and complete knowledge of existing
facts in addition to experiments.