They prove above all that the whole future individual is determined at
fertilization, or, to express it theoretically, that the id-composition
of the germ-plasm is decisive for the whole ontogeny. It might have
been supposed that the combination of ids could change again during
development, and that a greater multiplication of some than of others
might take place at certain stages of development, or through certain
chance external influences. It might have been thought that there was a
struggle among the ids in the sense that some of them were suppressed
and set aside. All such suppositions break down in face of the fact of
identical twins, which teaches us that identical germ-plasm evokes an
ontogeny which runs its course as regularly as two chronometers, which
are constructed and regulated alike.
But when I say that a struggle of the ids, in the sense of a material
setting aside of some of them, cannot take place, I by no means intend
to maintain that the influence which each individual id exerts on the
course of development may not be disproportionate to that exerted by
others, and, under some circumstances, very disproportionate indeed.
I must refrain from entering into this subject in detail now, but I
should like to give at least an indication of what I mean.
If the germ-plasm consists of ids, these ids collectively must
determine the structure, the whole individuality--let us say, briefly,
the 'type' of the offspring; it is the resultant of all the different
impelling forces which are contained in the different ids. If these
were all equally strong, and all operating in the same direction,
they would necessarily all have the same share in the resultant of
development, the 'type' of the child. But this is not the case.
Numerous experiments on the hybridization of two species of plant have
taught us that the descendants of such hybridization usually maintain
a medium between the ancestral species; but it is not always the case,
for in many hybrids the character of one species, whether paternal or
maternal, preponderates in the young plant.
We recognize the same thing still more clearly in Man, whose children
by no means always maintain a medium between the characters of the two
parents, but frequently resemble one--the father or the mother--much
more strongly than the other.
How can this fact be theoretically explained? Must we ascribe to the
ids of the father or of the mother a greater determining power? Without
excluding such an assumption as on _a priori_ grounds inadmissible,
I am inclined to believe that we do not require it to explain _this_
phenomenon. For, if we take our stand simply on the fact of the
preponderance of one parent, it follows directly from this that not
all the ids control the type of the child, let the cause of the
non-co-operation of some of them be what it may. But if in this case
only a portion of the ids contained in the germ-plasm controls the
type, this combination of ids suffices to make the child resemble
one parent, the father, for instance, and consequently half the
number of ids is sufficient in some circumstances to determine the
child--taking for granted that the one-sidedness of the inheritance is
complete, which never actually happens. But the half number of ids can
only suffice if it includes the same combinations of ids which have
determined the type in the case of the father; as soon as one or more
ids of this particular combination are replaced by others the paternal
germ-plasm alone is not enough to call forth complete resemblance in
the child.
But, at the reduction, a change of arrangement of ids takes place,
and a new combination arises, and thus each germ-cell receives its
particular group of ids. It may thus happen that, in one particular
sperm-cell, exactly the same group of ids is contained as that which
determined the type of the father, and that the same is true of a
particular egg-cell in regard to the type of the mother. Let us now
assume that a sperm-cell and an egg-cell meet, which contain both those
groups of ids which had determined the type of the father and of the
mother; if the determining power of the maternal and paternal ids were
equal a child would result which would maintain the medium between
father and mother.
As is well known this does happen not infrequently, although it is
difficult or impossible to demonstrate it precisely. In plant-hybrids
proof is easier, and it has been established that by far the greater
number of hybrids maintain a medium between the characters of the two
ancestral species. This proves that our assumption of equal strength
of the ids of both species must be correct in general, for we know
definitely in this case, as I shall show later, that the paternal
and the maternal ids are equivalent as regards the characters of the
species. This is the case, for instance, with the hybrids between the
two species of tobacco-plant, _Nicotiana rustica_ and _N. paniculata_,
which were reared by Kölreuter as far back as the eighteenth century,
and which then, as now, maintained a fairly exact medium between the
two ancestral species, and did so in all the individuals. Both species
thus strive to stamp their own character on the young plant, and in
both the hereditary power is equally great; in both it is contained
in the same number of ids, that is, in the half, for both kinds
of sex-cell have undergone reducing division. We have here, then,
strict proof that the half number of ids suffices to reproduce in the
offspring the type of the species, or, more generally, of the parents.
If we apply these results to the inheritance of individual differences
in Man, we may say, that those germ-cells, to which at the reducing
division the same combination of ids has been handed on as that which
already determined the type of the parent, will endeavour to impress
this image again on the child. If a female cell of this kind combine
with a male which likewise contains the facies-combination of the
parent, in this case the father, the same thing will happen which we
described in the case of the plant-hybrids, that is, a medium form
between the type of the two parents will arise.
Not infrequently, however, there is a marked preponderance of the one
parent in the type of the child, and we have to inquire whether the
theory gives us any help with regard to such a case.
One might be inclined to assume a difference in the determining power
of the paternal and maternal ids, but if we cannot show to what extent
and for what reason this power may be different such an assumption
remains rather an evasion than an explanation. Moreover, it would not
always apply to the conditions in Man, for if, for instance, the ids
of a particular mother were in general stronger than those of the
father, all the children of the pair in question would necessarily
take after the mother; but it happens not infrequently that one child
resembles the father preponderantly, and another the mother. Moreover,
the ids pass continually from the male to the female individual, and
conversely, by virtue of the continuity of the germ-plasm, so that the
idea that sex can have anything to do with the relative strength of the
ids is altogether erroneous.
But, as I have already said, unilateral inheritance occurs even in
the mingling of species-characters, and most clearly in the case of
plant-hybrids. Thus, for instance, hybrids between the two species of
pink, _Dianthus barbatus_ and _Dianthus deltoides_, resemble the latter
species much more closely than the former, and the hybrid between the
two species of foxglove which are wild in Germany, _Digitalis purpurea_
and _Digitalis lutea_, is much more like the latter than like the
former.
It might be reasonably asked whether, in these crossings, the normal
number of ids in one species is not greater than in the other. We
know that, among animals at least, differences in the normal number
of chromosomes occur even in very nearly related species. It is not
impossible that this, in many cases, is really the cause of the
diversity of transmitting power in different species. Nevertheless, we
cannot rest satisfied with this, for, in the first place, this cause
could not apply to the apparent unilateral inheritance from one parent
in Man, since the normal number of ids, as far as we know, is strictly
maintained in the same species, and second, this would not explain
certain phenomena of inheritance in plant-hybrids.
It happens not only frequently, but usually, that the different parts
of the hybrid take after one or other parent in different degree, and
this is the case also with children. In the hybrid between the two
species of tobacco-plant, _Nicotiana rustica_ and _N. paniculata_,
which I have already given as an example of a medium form between
the two parents, such diversities occur regularly in all the hybrid
individuals. Thus the corolla-tube of the hybrid is nearer _N.
paniculata_ in regard to its length, but nearer _N. rustica_ in regard
to its breadth. Many hybrids suggest one parent-form in the leaves, the
other in the blossoms. In the same way in the child the form of eye
may be that of the father, the colour of the iris that of the mother,
the nose maternal, the mouth paternal--in short, the preponderance in
heredity swings hither and thither from part to part, from organ to
organ, from character to character, and this is even the rule though
the oscillations may not always be apparent and are often invisible.
If we think of the proposition we arrived at earlier, and which was
proved chiefly by the case of identical twins, that the facies or
'type' of the descendant is determined at fertilization, we may be
inclined to regard such an oscillation of the hereditary tendencies
as almost impossible, for it means that, with the given mingling of
parental germ-plasms, the potency of inheritance from the two parents
in every part of the offspring is determined once for all in advance.
But the case of identical twins corroborates these oscillations, for
in them, too, the father predominates in one part, the mother in
another, and it proves, at the same time, that these oscillations do
not depend on any chances whatever in development, but that they are
exactly predetermined in the mingling of the hereditary substances in
the germ-plasm of the fertilized ovum, and are strictly adhered to
throughout development.
This fact can only be explained thus: the primary constituents of
the different parts and characters of the body are contained in the
parental germ-plasm in varying degrees of hereditary or transmissive
strength, and this can be understood very well from our point of view
without putting anything new into the 'portmanteau' of our theory
(Delage).
But I must digress a little in order to make this plain.
When, in speaking of plant-hybrids, I said that the collective ids
of the germ-plasm of a species must be equivalent in regard to the
characters of the species, I did not speak quite precisely; in the
majority of ids, in many cases in an overwhelming majority, this must
be the case, but not actually in all, at least not on the assumption
we make that the transformation of species is accomplished under the
control of natural selection.
Let us recall what we have already established in regard to the
evolutionary power of natural selection, namely, that the changes which
it controls can never transcend the range of their utility, and it will
be clear to us that, of the many ids which make up the germ-plasm of
the species, only so many will be modified as are necessary to evoke
the character which has varied. Just as the protective resemblance
of an insect to a leaf may be raised to a very high pitch, but can
never become perfect, because an imperfect resemblance is already
sufficient to deceive the persecutor, and the selective process comes
to a standstill because individuals which possessed a still greater
resemblance to a leaf would be no better protected from destruction
than the others, so in the modification of a species the whole of the
ids need not at once be modified, if a majority is sufficient to stamp
the great majority of individuals with the desired variation. But it
may happen that, at the reduction of ids during the development of the
germ-cells, an id-combination with wholly or almost wholly unchanged
ids may come together in one germ-cell, and if another sperm-cell of
this kind meets with an egg-cell similarly constituted, an individual
of the old species must arise. But this must--on our assumption--be
at a disadvantage as compared with the transformed individuals in the
struggle for existence, and will perish in it, and therefore the number
of unmodified ids in the germ-plasm of the species will gradually
diminish. It is obvious, however, that this will take place very
slowly, as we may conclude from the phenomena of reversion, of which I
shall have to speak later on.
But what is true of the ids is true also of their constituent parts,
the determinants, and that--if I mistake not--is fundamental in the
interpretation of the alternation of hereditary succession in the parts
of the child.
According to our theory, the ids do not collectively exert a
controlling influence on the cells, not even on the germ-cells, whose
histological differentiation into spermatic or egg-cells can only
depend on control through specific sex-cell determinants. It is the
different determinants of the ids that control; transformations of
the species will, it is true, depend on transformations of the ids,
but this need not necessarily consist in a variation of _all_ the
determinants of the id. If, for instance, two species of butterfly,
_Lycæna agestis_ in Germany and _Lycæna artaxerxes_ in Scotland, only
differ from each other in that the black spot in the middle of the wing
in _L. agestis_ is milk-white in _L. artaxerxes_, no other determinants
in the id of the germ-plasm can be different except those which control
this particular spot. In a majority of the ids in _L. artaxerxes_ the
determinants of this spot must have been modified, let us say, to the
production of 'milk-white.' This majority will increase very slowly if
the white colour has no pronounced advantage for the persistence of
the species, but it will increase gradually, as we have already seen,
though extremely slowly, through the elimination of those individuals
whose germ-plasm at the reducing division has chanced to receive a
majority of ids with the old, unmodified determinants, and which have
therefore reverted to the ancestral form. This will happen whenever the
new character has any use, however small, in maintaining the species.
But in most modifications of species quite a number of parts and
characters have undergone variation either simultaneously or in
rapid succession; in many cases nearly all the details of structure,
and therefore almost all the determinants of the germ-plasm, must
have varied. We must not, however, assume that all the equivalent
determinants, for instance, all the determinants _K_ in all the ids,
have varied[9], and above all we must not take for granted that
the determinants of different characters or parts of the body, for
instance, the determinants _L_, _M_, or _N_, must all undergo variation
in an equal number of ids. It will depend on two factors whether a new
character is implicit, in the form of varied determinants, in a small
or in a very large majority of ids: first, on the relative age of the
character, and, secondly, on its value in relation to the persistence
of the species. The more important a character is for the species the
more frequently is it decisive for the life or death of the individual,
and the more sharply will individuals not possessing it be eliminated,
and the more rapidly, therefore, will those whose germ-plasm still
contains a majority of the unvaried determinants of this character
tend to disappear. In this way these determinants will tend to sink
down from generation to generation to an ever smaller minority in the
germ-plasm of those that survive.
[9] By equivalent or homologous determinants I mean the determinants of
different ids which determine _similar parts_, e.g. the scales of that
wing-spot in _Lycæna agestis_ which is alluded to above, and to which
we must refer again in more detail.
Thus in the ids of any species which has been in some way
transformed--and that is as much as to say, in every species--the
equivalent or homologous determinants are modified in a very varied
percentage. A very modern and at the same time not very important
character _K´_ will only be contained in a small majority of ids,
while in the remainder the original homologous ancestral determinant
_K_ is contained; an older but not very much more important character
_M´_ must have its determinants in a larger majority in the ids, while
a character _V´_ of decisive importance for the preservation of the
species, if it has been in existence long enough, must be represented
in almost all the ids, so that the homologous unvaried determinants of
the ancestral species _V_ can only have persisted in an id here and
there.
If this argument be correct, many phenomena of inheritance become
intelligible, especially the variability in the expression of the
inheritance in the parts of the offspring, which is more or less
rigidly predetermined at fertilization. For the germ-plasm thus
contains in advance every kind of determinant in diverse _nuances_,
and in definite numerical proportions. In a plant _N´_, for instance,
_Ba´_ may be the determinant of the modern leaf-form, and may occur
in twenty-two out of twenty-four ids of the germ-plasm, while the two
remaining ids still contain unmodified the old leaf-form determinants
_Ba_, which the ancestral form _N_ possessed. But the flower of _N´_
may be of still more recent origin, and contain the modern flower
determinants _Bl´_ only in sixteen out of twenty-four ids, while in the
other eight the old flower-determinant _Bl_ of the ancestral form has
persisted. Let us now suppose that another nearly related species _P´_
has, conversely, a recently changed leaf-form but a very ancient form
of flower, so that the former is represented only in sixteen ids by the
determinants of the leaf _ba´_ and the latter in twenty-two ids by the
flower-determinants _bl´_: it is obvious that when the two species are
crossed, notwithstanding the equal number of ids in the germ-plasm, the
leaves of the hybrid will resemble more closely those of the ancestral
form _N_, and the flowers those of the form _P_; it is even conceivable
that in such a case the numerically preponderating leaf-determinants
_N_, and the equally preponderating flower-determinants of _P_ may form
a close phalanx, so to speak, against the much less numerous homologous
determinants of the other species, and that against this power working
in a definite direction the others can make no headway and are simply
condemned to inaction.
How we may or can picture this as occurring is a question which of
course admits only of being answered very hypothetically, and it leads
us, moreover, into the region of the fundamental phenomena of life,
with the interpretation of which we are not here concerned. For the
present we have assumed that life is a chemico-physical phenomenon, and
we have postponed the deeper explanation of it to the remote future,
that we may confine ourselves in the meantime to the solution of the
problem of inheritance on the basis of the forces resident in the vital
elements. But we may, nevertheless, make the supposition that a kind of
struggle between the different kinds of biophors may take place within
the cell, if the homologous determinants of all the ids for the control
of the cell have entered into it.
In many cases this struggle will be decided by the numerical
preponderance of one kind of determinant over the other, but it is
certainly conceivable that dynamic differences may also have something
to do with it.
Let us, however, abstain from trying to penetrate further into the
obscurity of these processes, and let us content ourselves with
establishing that the preponderance of one parent in some or many
parts of the child may be almost if not quite complete, and that this
compels us to assume that the hereditary substance of the other parent
is in such cases rendered inoperative--for we know it is present--since
the ids of both parents all go through the whole ontogeny, and are
contained in every somatic cell.
Upon this struggle between homologous determinants depends the
possibility of the entire suppression or inhibition of the influence
of one parent, the whole diversity in the mingling of paternal and
maternal character in the body of the offspring. It is in this that we
must seek the explanation of the fact that not only whole bodily parts
of the child, such as arms, legs, the nature of the skin, the form of
the skull, may take after sometimes the father, sometimes the mother
wholly or predominantly, but that the small separate subdivisions of
a complex organ may sometimes turn out more maternal, sometimes more
paternal. Thus intelligence from the mother and will from the father,
musical talent from the father and a talent for drawing from the
mother, may be inherited by the same child. I do not doubt that genius
depends in great part on a happy combination of such mental endowments
of the ancestors in one child. Of course something more is necessary,
namely, the strengthening of certain of these hereditary endowments,
but of this we shall speak later.
It is, however, not only the immediate ancestors, that is to say,
the parents, that have to be taken account of in this mingling of
hereditary contributions, but also those more remote. Not a few
characters in the child do not occur in either parent, but were present
in the grandparents, and their reappearance is called 'atavism' or
'reversion.' Let us consider this phenomenon in more detail, and try
to find out whether and how far it can be interpreted by means of our
theory.
The simplest and clearest cases are again found among plant-hybrids.
It may happen, for instance, that a hybrid between two species, when
dusted with its own pollen, gives rise to descendants, some of which
resemble only one of the ancestral forms: thus we have reversion to
one of the grandparents. The explanation of this lies in the different
modes in which the reducing divisions are effected; if they take place
in such a manner that all the paternal ids of the hybrid are separated
from the maternal ids, then the result is germ-cells which are like
those of the grandparents, that is, those of the parent species, and
these, if they happen to combine in amphimixis, must give rise to a
pure seedling of one or other of the two ancestral species. This case
occurs less rarely than was formerly supposed, and than it could do if
absolutely free combination of the idants took place at reduction. If
combination were quite unrestricted, all other possible combinations
would be likely to occur as frequently as these. But recent experiments
have shown that, in many plant-hybrids, the germ-cells of the hybrids
which are fertilized by their own pollen are either purely paternal or
purely maternal. There cannot, therefore, be free combination of the
idants at the reducing divisions; the idants of the two parent-forms
separate from one another and do not combine. It is doubtful, however,
whether the same thing occurs within a race, for instance, in the case
of reproduction within a human race.
In Man reversion to a grandparent occurs not infrequently, and we
may explain it thus: the id-group which controlled the type of the
grandfather was also contained in the germ-cell which gave rise to
the existence of the father, but it did not dominate the type in
that case because a more powerful id-group was opposed to it in the
germ-cell of the grandmother. When, later on, at the reducing divisions
of the germ-cells of the father, this id-group again arrived in the
sperm-cells of the father, it would predominately control the type of
the child, that is, of the third generation, provided that the egg-cell
with which it combined contained a weaker id-group.