there are 'primary constituents' at certain positions in the body,
but how does it happen that these are able to build up the lost parts
in the proper situation and detail? A theoretical formula might well
be thought out, according to which the determinants of successive
parts would become active successively, and would thus liberate one
another in an appropriate order of sequence, but there would not be
much gained by this, especially as what we already know in regard to
the regrowth of the legs and toes in Triton does not harmonize with
such an assumption. It appears to me more important--though even here
we must still be very vague as to details--to recognize that, in all
vital units, there are forces at work which we do not yet know clearly,
which bind the parts of each unit to one another in a particular order
and relation. We were obliged to assume such forces even in regard
to the lowest units, the biophors, since otherwise they could not be
capable of multiplication by division, on which all organic growth
depends, unless we are to assume, as Nägeli did, a continual _generatio
æquivoca_ of the specific kinds of biophors (his 'micellæ'). But we
shall see later, when we come to speak of spontaneous generation, that
we cannot acquiesce in such an assumption. If, then, we cannot conceive
of a power of division arising from within and depending solely on
growth by means of assimilation, without such attractive and repellent
forces or 'vital affinities' the internal parts would necessarily fall
into disorder at every division. It seems to me therefore that such
'affinities' must be operative at _all stages_ in the life of the vital
units, not only in biophors, but also in the cell, and in the 'person'
as well as in determinant and id. It is true that 'persons' no longer
generally possess the power of multiplying by division, but in plants
and lower animals many do possess it; and the power of giving rise
anew to certain parts is obviously a part of that power of doubling
the whole by division. The ultimate roots of regeneration, then, must
lie in these 'affinities' between the parts, which preside over their
arrangement and are able to maintain it and to give rise to it anew.
In this respect the organism appears to us like a crystal whose broken
points always complete themselves again from the mother-lye after
the same system of crystallization, obviously in this case too as a
result of certain internal directive forces, polarities, which here
again we are unable precisely to define. But the difference between
the organism and the crystal does not--as people have been hitherto
inclined to believe--lie only in the fact that the crystal requires the
mother-lye to complete itself, while the vital unit itself procures the
material for its further growth; it lies also in the fact that such
regeneration is not possible in every organism and at every place, but
that _special_ 'primary constituents' are necessary, without which the
relevant part cannot arise. The indispensableness of these primary
constituents, the determinants, seems to me to depend on the fact
that the new structure cannot be built up simply by procuring organic
material, but _that specially hewn stones, different in every case,
are necessary_, which can only be supplied in virtue of an historical
transmission, or, to abandon the metaphor, because the vital units of
which the organ is to be reconstructed possess a specific character
and have a long history behind them; thus they can only arise from
such vital units as have been handed on through generations, that is,
from the determinants. But these primary constituents are given to the
different forms of life in very varying degrees and in very unequal
distribution, and as far as we can see according to their suitability
to an end.
LECTURE XXII
SHARE OF THE PARENTS IN THE BUILDING UP OF THE OFFSPRING
The ids are 'ancestral plasms'--The reducing division brings
about a diversity of germ-plasm in the germ-cells--Bolles
Lee's 'Neotaxis' even in the primordial germ-cells--Häcker's
observations on the persistent distinctness of the maternal and
paternal chromosomes--Identical twins--The individuality is
determined at fertilization--Unequal share of the ids in the
determination of the offspring--Preponderance of one parent in the
composition of the offspring--Certain ids of the ancestors remain
unchanged in the germ-plasm of the descendants--Struggle of the
Biophors--Alternation of the hereditary sequences in the parts
of the child--Reversion--Datura-hybrids--Zebra-striping in the
horse--Three-toed horses--New experiments in hybridization among
plants by Correns and De Vries--Xenia.
As far as the phenomena of regeneration and budding are concerned, we
have not been able to do much more than bring them under a formula,
which harmonizes with the germ-plasm theory. But the case is different
with the actual phenomena of inheritance in the restricted sense, for
instance, with regard to the transmission of individual peculiarities
_from parent to child_. Here the theory really increases our insight
and lets us penetrate deeper into the causes of the phenomena; it is
here no longer a mere 'portmanteau-theory.'
We are well aware, especially from observation on ourselves, that is,
on Man, that the children of a pair often resemble one another but
are never alike, and that one child frequently resembles one parent,
another the other, while a third may exhibit a mingling of both
parents. How does this come about? Since the germinal substance of both
parents is derived from that of the ovum, from which they themselves
have arisen--and must therefore be the same in all the germ-cells
to which they give rise--new determinants cannot be added, and old
ones cannot be dropped out, and variation of the determinants, the
possibility of which is granted, would still not directly bring about
the familiar mingling of resemblances to the two parents, but would at
most give rise to something new and strange.
Here the theory helps to elucidate matters. We found ourselves obliged
to assume that the germ-plasm is composed of ids, that is, of
equivalent portions of germ-plasm, each of which contains all the kinds
of determinants appertaining to the building up of an individual, but
each of these kinds in a particular individual form. I have already
called these ids 'ancestral plasms,' and the term is appropriate, in
so far that in every fertilization an equal number of ids from the
father and from the mother are united in the ovum, so that the child
is built up of the ids of his two nearest 'ancestors.' But as the ids
of the parents are derived from those of the grandparents, and these
again from those of the great-grandparents, the ids are in truth the
idioplasm of the ancestors.
The expression, however, has been very frequently misunderstood, as
if it were intended to mean that the ids retained _unchanged_ for all
time the character of their respective ancestors, and I have even
been credited with supposing that our own ids still consist of the
determinant-complexes of our fish-like or even Amœba-like ancestors.
But in reality no id exactly or completely corresponds to the type,
that is, to the whole being of any one of the ancestors in whose
germ-plasm it was formerly contained, for each of the ancestors had
many ids in his germ-plasm, and his entire constitution was not
determined by any one of these alone, but by the co-operation of them
all. The individual arising from a germ-cell must necessarily be the
result of all the ids which make up his germ-plasm, but undoubtedly the
share taken by some of them may be much stronger than that taken by
others. It is also clear that, if we leave out of account any possible
variation on the part of the ids, each of them belongs, not to one
ancestor only, but to a whole series of ancestors, and must have taken
part in their development, so that it is not the idioplasm of any
particular ancestor, but only ancestral plasm in the general sense. In
this sense we may quite well retain the designation, 'ancestral plasm,'
for the id.
Thus, according to our view, the germ-plasm consists of ids, each of
which contains all the determinants of the whole ontogeny, but usually
in individually different quality.
Returning for a moment to the processes by which the reduction of the
chromosomes, that is, of the nuclear rods of germ-plasm in the ovum
and sperm-cell is brought about, we recall the fact that this happens
at the last two divisions of the germ-cell, the so-called 'maturing
divisions.' In these the nuclear substance, as we have seen, is
divided between the two daughter-nuclei in a manner quite different
from the usual one, for a longitudinal splitting of the rods, bands,
or spheres in the equatorial plane of the nucleus does not take
place, but half the number of rods move into the right and half into
the left daughter-nucleus without previous division, so that in each
daughter-nucleus the number of rods is reduced to half (Fig. 76).
[Illustration: FIG. 76. Diagram of the maturation divisions of the
ovum. _A_, primitive germ-cell. _B_, mother-egg-cell, which has grown
and has doubled the number of its chromosomes. _C_, first maturation
division. _D_, immediately thereafter; _Rk_ 1, the first directive cell
or polar body. _E_, the second maturation spindle has been formed; the
first polar body has divided into two (2 and 3); the four chromosomes
remaining in the ovum lie in the second directive spindle. _F_,
immediately after the second maturation division; 1, the mature ovum;
2, 3, and 4, the three polar cells, each of these four cells containing
two chromosomes.]
Although the distribution of the rods in this manner takes place twice
in succession, the normal number is not, as we have already seen,
reduced to a quarter, because, long before the occurrence of the first
maturing division, a duplication of the rods by means of longitudinal
division had taken place, and thus the first division differs from an
ordinary division in that the splitting of the rods does not take place
during the process of dividing but long beforehand. Only the second
maturing division differs from all other nuclear divisions known to
us, since it is not associated with any splitting of the rods at all,
but conveys half of the existing rods into each daughter-nucleus. It
is the time reducing division, through which the number of the rods is
reduced to one half[4].
[4] Recent investigations have shown that the reduction of the
chromosomes does not always take place exactly in accordance with the
scheme here indicated, but that it differs from it in many cases. But
as investigations on this point are as yet by no means complete, I need
not go into the question further; the ultimate result is the same in
any case.
This numerical reduction must, however, have other consequences; it
must make the germ-cells of the same individual qualitatively unlike,
that is, in relation to their value in inheritance. Let us assume only
four chromosomes of the rod-form ('idants') as the nuclear elements
of a species, two of which, _A_ and _B_, come from the mother, and
other two, _C_ and _D_, from the father, the last maturing division
may, as far as we can see, result either in removing the combination
_A_ and _B_ from _C_ and _D_, or _A_ and _C_ from _B_ and _D_, or _A_
and _D_ from _B_ and _C_; there is thus a possibility of one of six
different combinations of rods in any one germ-cell. What is the same
thing, six different kinds of germ-cells differing in their hereditary
primary constituents may _be developed in the same_ individual. As this
new combination, or, as we may call it, neotaxis of the germ-plasm
elements, takes place in female as well as in male individuals, there
is a possibility that, in fertilization, 6 × 6 = 36 individuals with
different primary constituents may arise from the germ-cells of the
same two parents. Of course the number of possible combinations
increases very considerably in proportion to the normal number of
rods, for with eight of these it comes up to 70, and with sixteen to
12,870; the number of individuals differing in their inherited primary
constituents would thus be enormous, for each of the 70 or of the
12,870 different hereditary minglings of the ovum could combine in
amphimixis with 70 or 12,870 different sperm-cells, so that 70 × 70
and 12,870 × 12,870 offspring individually different in their primary
constituents might arise from the same two parents. In Man there are
said to be sixteen nuclear rods; so that in his case the last-mentioned
number of parental hereditary minglings might occur. This may seem a
disproportionately high number as compared with the small number of
children of a human pair, but we must not judge from the case of Man
alone, and in plants and animals, which we have already discussed, the
number of descendants is very much larger, and is often enormous. We
saw what significance this apparent extravagance on the part of nature
has, for without it adaptation to changed conditions of life would
not be possible, since, if only so many were born as could attain to
reproduction, no selection of the fittest could take place. The same
would be the case if all the young of a species were alike, and even if
all the descendants of a single pair were alike, effective selection
would be excluded, since only as many individualities could be selected
as there were pairs of parents. It is easy to understand that selection
works more effectively the larger the number of descendants of a
species and the more they differ from each other. The chance that the
best possible combination of characters will occur is thereby increased.
Although we cannot calculate how many individuals of different
combinations of characters natural selection requires to work upon in
order to direct the evolution of the species[5], we can understand
that only as large a choice as possible can secure that the best
possible adaptations of all parts and organs are brought about and
maintained. Precisely in the fact that in every generation such an
enormous superfluity of individuals is produced lies the possibility
of such intensive processes of selection as must continually take
place, if the adaptation of all parts is to be explained. For if among
the thousands of descendants of a fertile species each hundred were
alike among themselves, these hundreds would have, as far as natural
selection was concerned, only the value of a single variant. But such
an all-round adaptation as actually exists in the structure of species
requires as many variants as possible; it requires that each individual
should be a _peculiar complex of hereditary characters_; that is, that
all the fertilized germ-cells of a pair should possess an individually
well-marked character.
[5] For this reason I have left the number of id-combinations given
above unaltered, though, according to the most recent researches into
the processes of maturation, they are probably too high, since every
conceivable combination does not actually occur. We are here concerned
less with the exact number than with the principle.
The justification of this postulate becomes all the clearer if we take
into consideration the male germ-cells as well as the female. Let us
think of the enormous number of sperm-cells which are produced by many
animals, and indeed by the highest of them--an almost incalculably
large number which certainly goes far beyond millions. Let us assume
that in Man there may be 12,870 million spermatozoa, then, with
sixteen ids, and with an equally frequent occurrence of all possible
combinations of germ-plasm--there would be 12,870--there would be a
million of each type containing identical germ-plasm. The danger that
several ova would be fertilized by identical sperm-cells would be by no
means small.
It cannot, therefore, surprise us that other means have been employed
by Nature to secure re-groupings of the ids. The simplest means would
be, if before each division of the primitive germ-cells the nuclear
rods were to divide, and if the split halves were irregularly
intermingled, then at the formation of the next nuclear spindle an
entirely new arrangement of the halves would result. But in animals, at
least, this is certainly not the case; the processes of reduction are
restricted to the maturing divisions.
Years ago Ischikawa observed that, in the conjugation of _Noctiluca_,
the nuclei of the two animals become closely apposed, but that they do
not fuse, although they behave like a single nucleus in the division
which follows. In this case _paternal and maternal nuclear substance
remain separate_ (Fig. 83, vol. i. p. 317). The same phenomenon has
since been repeatedly observed in many-celled animals, first by Häcker,
then by Rückert in the Copepods, and afterwards by Conklin in the eggs
of a Gastropod (_Crepidula_). But all these observations referred only
to the earlier stages of ovum-segmentation up to twenty-nine cells,
and it could not be affirmed that the distinctiveness of the paternal
and maternal chromosomes lasted till farther on into the ontogeny.
Professor Häcker now informs me, however, that he has been able to
trace this separateness in a Copepod (_Canthocamptus_) not only from
the beginning of segmentation on to the primitive genital cell, but
also through the divisions of this up to the mother-egg-cell[6].
Thus we may now assume that the paternal and maternal hereditary
bodies remain distinct, not only for a time, but throughout the whole
development, a fact which confirms our assumption of the independence
of the nuclear rods, notwithstanding their apparent breaking-up in
the nuclear reticulum of the 'resting' nucleus. This new knowledge
throws fresh light in another direction; it proves to us that the
remarkable and complicated processes which go on in the nucleus
during the maturing divisions have really the significance which I
long ago ascribed to them[7], that of effecting the maximum diversity
of intermingling of the paternal and maternal hereditary elements.
For Häcker has shown that during the second maturation-division
the paternal and maternal chromosomes are no longer united each
in a special group, but occur scattered about in the nucleus, and
subsequently come together again to form two differently combined
groups.
[6] Since this was written Häcker has published his results. See
_Anatom. Anzeiger_ xv (1902), p. 440.
[7] See my essay, _Amphimixis_, Jena, 1891.
If this were not so, if the maternal and paternal chromosomes remained
separate, then the reducing division would cause only one of these
groups to reach each of the germ-cells, and thus each mature ovum
or sperm-cell would contain either only paternal or only maternal
hereditary bodies. But this would make a reversion to more than three
generations back impossible, and as such reversions undoubtedly occur,
we must conclude that manifold new combinations of the paternal and
maternal chromosomes take place. This obviously happens during the
maturation-divisions, at least in the Metazoa.
The more numerous the rods or the free individual ids in a species
are, the more numerous are the possible combinations. Whether all the
mathematically possible combinations actually occur is a different
question, which I should not like to answer in the affirmative just
yet; but in any case the _actual_ number of combinations in a species
with many nuclear elements will be greater than in one with few, and
in this respect those species in which the ids occur as independent
granules will have an advantage over those in which they are combined
into rods or bands (idants). These latter, however, afford us a better
possibility of deducing the new combinations of the ids, although the
idants themselves are not outwardly distinguishable from each other.
I must refrain from going into these highly interesting processes in
more detail just now. So much is certain, that Nature makes use of
_various means_ to bring about the re-combination, and at the same time
the reduction of the ids during the two 'reducing divisions.' This is
proved by the fact recently established by Montgomery, that in many
animal groups reduction results from the _first_ maturing division.
Whether it operates at this stage with rings, bands, double rods,
X-shaped structures, groups of four (tetrads), and so on, all this
serves the same end, the more or less thoroughgoing re-arrangement of
the hereditary vital units. I am convinced that new investigations
into these processes, if they were undertaken from this point of view,
would lead to very important results[8]. It would be important to find
out how great the variations are which thus arise, for it is very
probable that they differ in degree in the different animal-groups.
Even the combination of the ids into rods (idants) indicates that some
species may be more conservative than others in maintaining their
id-combinations, and that there will be among them a greater tenacity
in the hereditary combinations of characters (i.e. of the 'type' of
the parents). If we should succeed in penetrating more deeply into
these processes we should probably also understand why in certain human
families the hereditary characters are transmitted more purely and more
tenaciously than those of other families with which they have mingled,
and so on. It may well be that the persistence of character is due
to the fact that ids which have once combined into rods hold firmly
together, for it seems to me in no way impossible that individual
differences should occur even in these most delicate processes.
[8] Since this was written for the first edition observations of this
process have been considerably increased, and discussions as to the
exact interpretation of these are in full tide; we are surrounded by
a wealth of new observations, facts, and explanations, without having
attained to a consistent and unified theory. Several naturalists, such
as Boveri, Häcker, Wilson, and others, have attempted interpretations,
but these are in many points contradictory to one another. It is
therefore impossible to enter into the question in detail here;
further light from new observations must be awaited. So much we may
say, however, that it is not chance alone which presides over the
re-arrangement of the chromosomes during the reducing divisions;
_affinities_ play a part also; there are stronger or weaker attractions
between the chromosomes, which aid in determining their relative
position to one another.
But let us leave these more intimate questions out of account
altogether, and turn our attention to the more obvious and less
delicate phenomena, and we find that the re-arrangement of ids
(Neotaxis) which we have just discussed affords a simple explanation
of the generally observed phenomenon of the differences between
individuals! Each individual is different from every other, not in
the case of Man alone, but in all species in which we can judge of
differences, and this is true not only of descendants of different
parents, but even of those of the same parents.
Of course the differences between two brothers or two sisters do not
depend entirely on the hereditary basis, but in part also on external
conditions which have affected them from embryonic development onwards.
Let us suppose that of two brothers who have sprung from identical
germ-cells one becomes a sailor, the other a tailor; it would not
surprise us to find them very different in their fiftieth year, one
weather-beaten and tanned, the other pale; one muscular, straight,
and vigorous, the other weakly and of bent carriage. The same primary
constituents develop differently according to the conditions to which
they are exposed. But the two brothers will still resemble each other
in the features of the face, colour of hair, form of eyes, stature and
proportion of limbs, perhaps even in a birthmark, more than any other
human beings of their own or any other family, and this resemblance
will depend upon the identity of the hereditary primary constituents,
on the similar id-combination of the germ-plasm.
Man himself affords a particularly good example in favour of this
interpretation in the case of so-called 'identical twins.' It is well
known that there are two kinds of twins, those that are not strikingly
alike, and often very different, and those that are alike to the extent
of being mistakable for one another. Among the latter the resemblance
may go so far that the parents find it necessary to mark the children
by some outward sign, so that they may not be continually confused.
We have now every reason to believe that twins of the former kind
are derived from two different ova, and that those of the latter kind
arise from a single ovum, which, after fertilization, has divided into
two ova. This not infrequently occurs in fishes and other animals,
and we can bring it about artificially in a number of species by
experimentally separating the two first blastomeres.
We have here, then, a case of absolute identity of the germ-plasm in
two individuals, for the id-combination of the two ova derived from the
same process of fertilization must be exactly the same. That in such a
case, notwithstanding the inevitable differences of external influences
to which the twins are exposed from intra-uterine life onwards, such a
high degree of resemblance should arise is a fact of great theoretical
importance. From the basis of the germ-plasm theory we can very well
understand it, for, according to the theory, only precisely similar
combinations of ids can give rise to identical individuals.