caterpillars of the Sphingidæ are absolutely without markings when
they emerge from the egg. Thus, for instance, the caterpillars of
_Chærocampa_ (Fig. 116, _A_), of _Macroglossa_ (Fig. 115), and of
_Deilephila_ (Fig. 118, _A_), as well as those of the _Smerinthus_
species, are at first without stripe or mark of any kind; they are
of a pale green colour, almost transparent, and very difficult to
recognize when they sit upon a leaf. How very greatly the different
stages _can be_ independently adapted to the different conditions of
their life, when that is necessary for the preservation of the species,
is shown in the most striking manner by many species. Thus the little
green caterpillar of _Aglia tau_, when it leaves the egg, bears five
remarkable reddish rod-like thorns, which in form and colour resemble
the bud-scales of the young beech-buds among which they live, and which
disappear later on; the full-grown caterpillar shows nothing of these,
but is leaf-green, marked with oblique stripes. Even if the use of
these reddish thorns be other than I have indicated, we have in any
case to deal with a special adaptation of _one_, and that the first
caterpillar-stage, and what can happen at this stage is possible also
at every other. Nor is it only animals which undergo metamorphosis that
can exhibit independent phyletic variation at every stage, but those
also with direct development, and indeed, in the case of these, we may
assume adaptation of this kind at almost every stage in the history
of the organs, as we have already seen, because the great abridgement
of the phylogeny into the ontogeny necessitates a very precise
mutual adaptation of the organ-rudiments and of the diverse rates of
development.
We have thus been led by the facts discussed--and numerous others
from other groups in the animal kingdom might be ranked along with
them--to two main propositions, which express the relation of phylogeny
to ontogeny. The first and fundamental proposition is the one already
formulated. The ontogeny arises from the phylogeny by a condensation of
its stages, which may be varied, shortened, thrown out, or compressed
by the interpolation of new stages. The second proposition refers to
individual parts, and may run as follows: As each stage can undergo
new adaptations by itself, so can every part, every organ; such new
adaptations very often show a tendency to be transferred to the
immediately antecedent stage in ontogeny.
It is not my intention to formulate the laws of ontogeny just now,
otherwise many others might be added to these, such as that of the
regular transference of characters acquired at one end of a segmented
animal to the other segments: I must confine myself here to bringing
the two main propositions into harmony with the principles of our
theory of heredity.
How phylogeny is condensed in ontogeny can be understood readily
enough in a general way, although we cannot profess to have any
insight into the detailed processes. The continuity of the germ-plasm
brings about inheritance, in that it is continually handing over to
the germ-plasm of the next generation the determinant-complex of the
preceding one. Every new adaptation at any stage whatever depends
on the variation of particular determinants within the germ-plasm,
and this in its turn depends on germinal selection, that is, on the
struggle of the different determinant-variants among themselves, and
on the variation in a definite direction which arises from this, as
we have already shown. A new kind of determinant can never arise of
itself, but always only from already existing determinants, and through
variation of these. But as spontaneous variation never causes all the
homologous determinants of a germ-plasm to vary in quite the same way,
but only a majority of them, there always remains a minority of the
old determinants, which may, under certain circumstances, predominate
again, as is proved by the aberrations in _Vanessa_ species due to
cold, and by many other kinds of reversion.
But it is not this variation which leads to the prolongation of
ontogeny, and the repetition of the phyletic stages within it. In this
case it is rather that a new character takes the place of an old one,
not that it is added to it. A black spot may arise instead of a red
one, but not first a black spot and then a red one. Of course we still
know far too little in regard to the intimate succession of events in
the stages of ontogeny to be able to say definitely that, in such
apparently simple transformations, the older stage does not, in every
ontogeny, precede the more recent one as a preparation for it, though
it may be only for a brief and transient period.
It is certain, however, that variations such as the addition of a new
stage in ontogeny are undergone, and that this implies the occurrence
of something really quite new. Therefore such a new stage can arise
only from the germ-plasm, by the duplication, and in part variation, of
the determinants of the preceding stage. If, for instance, the body of
a Crustacean be lengthened by a segment, this must be due to a process
of this kind, and in such a case it is intelligible enough that the new
segment can be formed in the ontogeny only after the development of the
older preceding one, for its determinants come from that, and are from
the beginning so arranged that they are only liberated to activity by
the formation of the preceding segment.
Now, if in the course of the phylogeny numerous new segments were
added to the body of the Crustacean, the ontogeny would be materially
prolonged, and condensation would become necessary in the interests of
species-preservation. To bring this condensation about, whole series of
segments which were added successively in the phylogeny succeeded each
other with gradually increasing rapidity in the ontogeny, until finally
they appeared _simultaneously_: the determinants of the segments _n_,
_n_ + 1, _n_ + 2, ... _n_ + _x_ varied in regard to their liberating
stimuli, and were roused to activity no longer successively, but
simultaneously, in the cell complexes controlled by them. We have thus
recapitulation, but with abridgement and compression, of the phyletic
stages in the ontogeny. Thus in the nauplius of _Leptodora_ we see the
rudiments of five of the pairs of legs of the subsequent thorax (Fig.
111, _IV-VIII_), and in the Zoæa larva the rudiments of six thoracic
legs may be seen behind the already developed swimming-leg (Fig. 114,
_VI-XIII_).
But in the course of the phylogeny a segment may also become
superfluous, and we know that it then degenerates and is ultimately
eliminated altogether. Thus in a parasitic Isopod, which lives
within other Crustaceans, a segment of the thorax is wanting in the
relatively well-developed larva, and in the Caprellidæ among the
Amphipod Crustaceans the whole abdomen of from six to seven segments
has degenerated to a narrow, rudimentary structure. In such cases
the gradual degeneration of the relative determinants has preceded
step for step the degeneration of the part itself, and when this is
complete the ontogeny shows nothing of what was previously present,
and so we may speak of a 'falsification' of the phylogeny. But that
the complete disappearance of the determinants only comes about with
extreme slowness, so that whole geological periods are sometimes not
enough for its accomplishment, we have already learnt from our study
of rudimentary organs, instances of which can be demonstrated in every
higher animal, bearing witness to the presence of the relevant organs
or structures in the ancestors of the species.
We can infer with certainty, from the observational data at our
disposal, that the disappearance of useless parts is regulated by
definite laws; but it is too soon to attempt to formulate these laws,
or even to trace them back to their mechanical causes. As we have
already said, a much more comprehensive collection of facts, and
above all one which has been made on a definite plan, is a necessary
preliminary condition to this. But so much at least we may gather
from the facts before us, that the degeneration of an organ begins
at the final stage, and is transferred gradually backwards into the
embryogenesis. Thus the two fingers of birds which have disappeared
since Cretaceous times are still indicated in every bird-embryo,
though they subsequently degenerate. In various mammals 'pre-lacteal
tooth-germs' have been demonstrated in the jaws of embryos, which show
us that not only did ancestors exist whose dentition was the modern
'milk-teeth,' but that still more remote ancestors possessed another
set of teeth, which was crowded out by the 'milk-teeth'; thus the teeth
of the ancestors of the modern right whale (_Balæna mysticetus_) are
only represented in the embryo of to-day in the form of dental pits.
And, as we saw already, the Os centrale so characteristic of the wrist
of lower vertebrates only appears in Man at a very early embryonic
stage, and disappears again as such in the further course of the
embryogenesis.
We may perhaps give a preliminary statement of this law as follows: It
is impossible that any part or organ should be removed suddenly from
the ontogeny without bringing the whole into disorder, and the least
serious disturbance of the course of development will undoubtedly be
caused if the final stage of the part in question become rudimentary
first. Only after this has happened, and the neighbouring parts have
adapted themselves to the disappearance, can this extend to the stages
immediately preceding it, so that these too degenerate, and allow
the surrounding parts to adapt themselves. The further back into the
ontogeny the disappearance extends the greater will be the number of
other structures affected in some way or other by the degeneration, and
these must not all be brought suddenly into new conditions, else the
whole course of development would suffer. Thus at first only those
determinants may disappear--and can disappear according to the laws
of germinal selection--which control the final form of the useless
organ, then those just preceding them, which controlled, let us say,
its size, and thus more and more of the previously active determinants
disappear, and hand in hand with this disappearance there is variation
of all the parts correlated with the dwindling condition of the organ,
so that their own development and that of the animal as a whole suffers
no injury. If it were otherwise, if when a part became useless its
collective determinants were all to disappear at the same time, the
whole ontogeny would totter, in fact it would be much as if a man who
wished to remove the breadth of a window from a house standing on
pillars were to begin by taking away the foundation pillar.
It is, of course, to be understood that these processes go on so
exceedingly slowly that personal selection takes a share in them, at
least at the beginning. Later on, the further degeneration of a useless
organ or rudiment has no effect on the individual's power of life, and
therefore depends solely upon the struggle of the parts within the
germ-plasm (germinal selection).
If we could see the determinants, and recognize directly their
arrangement in the germ-plasm and their importance in ontogeny, we
should doubtless understand many of the phenomena of ontogeny and their
relation to phylogeny which must otherwise remain a riddle, or demand
accessory hypotheses for their interpretation. Several years ago Emery
rightly pointed out that the phenomena of the variation of homologous
parts might be inferred by reasoning from the germ-plasm theory. If
one hand has six fingers instead of five, it not infrequently happens
that the other also exhibits a superfluity of fingers, and sometimes
the foot does so too. The phyletic modification of the limbs in the
Ungulates has taken place with striking uniformity in the fore and
hind extremities; no animal has ever been one-hoofed in front and
two-hoofed behind. Although I might suggest that this primarily depends
on adaptation to different conditions of the ground, and that the
Artiodactyls were evolved in relation to the soft marshy soil of the
forest, and the Perissodactyls for the steppes, it cannot be denied
that germinal conditions may have co-operated in bringing about this
uniformity of the direction of variation, especially as the whole
structure of the fore- and hind-limbs exhibits such marked similarity.
Emery is inclined to refer this to 'germ-plasmic correlations,' and
we have assumed from the very first that the different determinants
and groups of determinants do indeed stand in definite and close
relations to one another. But it seems to me premature to say anything
more precise and definite than that in the meantime. I should like,
however, to say that determinants or groups of determinants which
had in old ancestral germ-plasms to give rise to a series of quite
similar structures by multiplication during the ontogeny, and therefore
only needed to be present _singly_ in the germ-plasm, would, in
later descendants, have to shift their multiplication back into the
germ-plasm itself, if necessity required that the homologous parts
which they controlled should become _different_ from each other.
Then the previously single group of determinants in the germ-plasm
would have to become multiple. But as new determinants can only arise
from those which already exist, these new ones must have had their
place beside the old, and would therefore probably be exposed to
any intra-germinal causes of variation in common with them--that is
to say, they will tend to vary even later in a similar manner. For
instance, we might think of the segments of primitive Annelids, which
in form and contents are for the most part alike, as arising from one
germ-rudiment, from which, when, in the higher Annelids, the various
regions of the body had to take a different form, several primary
constituents of the germ-plasm separated themselves off; and in a
similar way the much higher and more complex differentiation of the
somatic segments in the Crustaceans must have been brought about. Thus
we understand how the determinant groups of the germ-plasm multiplied
according to the need for increasing differentiation, but remained in
intimate relation, which exposed them in some measure to a common fate,
that is, to common modifying influences, and in many cases determined
them to similar variation.
But we cannot see directly into the germ-plasm, and are therefore
thrown back on the inductions we can make from the facts presented
to us by the phenomena of visible living organisms. As yet the
material for such inductions is scanty, because it has been got
together haphazard, and not collected on a definite plan. I therefore
refrain for the present from attempting any further elaboration of
my germ-plasm theory. It is only when an abundance of observation
material, collected according to a definite plan, lies at our disposal
that anything more in regard to the intimate structure of the
germ-plasm, or the mutual influences and relations of its determinants
and its modification in the course of phylogeny can be deduced with
any certainty. Meanwhile, we must content ourselves with having,
through the hypothesis of determinants, made intelligible at least
the one fundamental fact, how it is possible that in the course of
the phylogeny single parts and single stages can be thrown out or
interpolated, or even only caused to vary, without giving rise to
variation in all the rest of the parts and stages of the animal.
A theory of epigenesis cannot do this, for, if no representative
particles were contained in the germ-plasm, then every variation of
it would affect the whole course of development and every part of the
organism, and variations of individual parts arising from the germ
would be impossible.
LECTURE XXVIII
THE GENERAL SIGNIFICANCE OF AMPHIMIXIS
Twofold import of amphimixis--It conditions the continual changing
of individuality--Analogy from game of cards--The germ-plasm is at
once variable and persistent--The two roots of individual variation:
germinal selection and new combinations of the ids--'Harmonious'
adaptation conditions amphimixis--Difference between adaptation
and mere variation--Is a 'direct' use of amphimixis to be insisted
upon?--Ceaseless intervention of personal selection in the lineage of
the germ-plasm--Far-reaching effects of personal selection--Fixing
of the arrangements for amphimixis in the course of generations
of species--Increase of the constancy of a character with its
duration--Characters in the same species variable in different
degrees--The upper and under surfaces of Kallima--Wild plants brought
under cultivation do not at first vary--Amphimixis very ancient,
therefore very firmly established--Does amphimixis bring about
equalization (Hatschek, Haycraft, Quetelet)?--Galton's frequency
curves--Ammon's free scope for variations--De Vries' asymetrical
curves of frequency.
We have already made ourselves familiar with the process which in
unicellular organisms is called conjugation and in multicellular
organisms fertilization, and we have seen that its most obvious
significance lay in the fact that through it the germ-plasms of two
individuals are united. Since, according to our view, this germ-plasm
or idioplasm is the bearer of the hereditary tendencies of the
organism concerned, the mingling or amphimixis of two germ-plasms
brings together the hereditary tendencies of two individuals, and the
organism whose development is derived from this mingled germ-plasm
must therefore exhibit traits of both parents, and must to a certain
extent be made up of the traits of both. This is one result attained by
amphimixis.
But we went further than this, and saw that there is a second result
implied in amphimixis, namely, that the individual character of
the germ-plasm is being continually altered by new combinations of
the ids contained in it. We inferred from what I believe to be the
demonstrated hypothesis that the germ-plasm is composed of ids, that
its reduction to half the original mass must mean a reduction of the
ids to half the number, and as the ids contain primary constituents
which are individually different, this must effect a new arrangement,
a new mingling of these individual peculiarities. The reduction of the
germ-plasm to half, that is, the diminution of the number of its ids
to half, is a phenomenon generally associated with amphimixis, and
has been established in the case of all animals which have hitherto
been investigated, and of all the most carefully studied plants,
and finally, it has been shown to be very probable in unicellular
organisms, for the processes of conjugation in Infusorians and many
other Protozoa include phenomena very similar to those of reducing
division in the higher animals. The prediction made on theoretical
grounds has here been verified by observation, and it is obvious that
the assumption of ids, that is, of units in the germ-plasm which
are handed on from one generation to the succeeding one, involves a
reduction of their number in each amphimixis. Without this the number
of ids would be doubled at each amphimixis, and would therefore
gradually amount to something enormous. We see therefore why this
normally recurrent reduction of ids before each amphimixis was
established in the course of evolution, and we see that it inevitably
involves that a new combination of ids should be associated with each
amphimixis.
If nothing persists unless it be purposeful, that is, necessary, what
is the meaning of the fact that arrangements for amphimixis occur over
almost the whole known domain of life, from the very simple organisms
up to the highest, in unicellular and multicellular organisms, in
plants and animals alike? Why is it that this arrangement has been
departed from only in a few small groups of forms, while it occurs
everywhere else, in almost every generation, so indissolubly associated
with reproduction that it has even been regarded--with a lack of
clearness--as itself a form of reproduction, and is even now generally
called 'sexual reproduction'? And why is it that in many organisms,
especially lowly ones, it is not associated with every reproduction,
though it recurs at regular or irregular intervals? Such a universal
arrangement must undoubtedly be of fundamental importance, and we
have to ask wherein this importance lies. That is the problem to the
solution of which we must now apply ourselves.
So much we may say at once: The significance of amphimixis cannot be
that of making multiplication possible, for multiplication may be
effected without amphimixis in the most diverse ways--by division of
the organism into two or more, by budding, and even by the production
of unicellular germs. Even though these last are usually in various
ways so organized that they must undergo amphimixis before they can
develop into new organisms, yet there are numerous germ-cells which
are not subject to this condition (e.g. spores), and there are--as we
have seen--many germ-cells, adapted for amphimixis, which always, or in
certain generations, or even only occasionally, emancipate themselves
from this condition under certain external influences: I refer to
egg-cells which develop parthenogenetically.
If amphimixis is not a universal preliminary condition of reproduction,
wherein lies the necessity for its general occurrence among living
organisms?
We have already learned that there are two results produced without
exception by amphimixis; one of these is the antecedent reduction
of the original number of ids by one half, and the consequent new
combination of ids which results from this; the other is the union
of two such halved germ-plasms from two different individuals. The
first we may, with Hartog, compare to the removal of half of a pack
of cards previously mixed, the second to the combination of two
such halves from different packs. The first process brings nothing
new into the complex of primary constituents, but rather removes a
part--larger or smaller--of its characters: not necessarily exactly
half of these, since each individual kind of id may be represented by
doubles or multiples. But the reduction simplifies the composition
of the germ-plasm, and might by itself, through the struggle of the
ids in ontogeny, lead to a resultant different from the parent, that
is, to a new individuality. Through the second process, however, new
individual traits are of necessity added, and make the resultants still
more markedly diverse, that is, if the ids of both parents attain to
expression in the struggle of ontogeny, and this, as we have already
seen, is usually the case, though not always and certainly not always
in all parts. Thus amphimixis, together with the preparatory reduction
of the ids, secures the constant recurrence of individual peculiarities
through the ceaseless new combinations of individual characters already
existing in the species.
When sixteen years ago I first inquired into the actual and ultimate
significance of sexual reproduction, I thought I had found it in
this ceaseless production of new individualities. This seemed to me
a sufficient reason for the introduction of amphimixis into nature,
since the difference between individuals is the basis of the process
of selection, and thus the basis of all the transformations of
organisms, which we may refer to natural or sexual selection. Now
these differences of selection-value are--as I believed then, and do
still--not only by far the most frequent organic changes, but also the
most important, since they not only initiate, but control new lines of
evolution. Therefore I still regard amphimixis as the means by which a
continual new combination of variations is effected a process without
which the evolution of this world of organisms so endlessly diverse in
form and so inconceivably complex, could not have taken place.