can only be called so in a wide sense, for the ancestral characters
are contained _visibly_ in the parent, although mingled with those
of the other parent. In human families, however, there are undoubted
cases in which one or more characters of the grandparent reappear in
the child which were not in any visible way expressed in the parent,
and must therefore have been contained in the parent's germ-plasm in
a latent state. And there are both in animals and plants reversions
to ancestors lying much further back, to characters and groups of
characters which have not been visible for many generations, and the
occurrence of these can only be explained on the assumption that
certain groups of ancestral determinants have been carried on in the
germ-plasm in too small a number to be able to give rise ordinarily
to the relevant character. Such isolated determinants may, however,
in certain circumstances be strengthened by the amphimixis of two
germ-cells both containing small groups of them, and thus augmented
they may gain a controlling influence. In this case the chances of the
reducing divisions have a part to play, since they bring together the
old unvaried ancestral determinants which, as we have seen, may persist
in the germ-plasm of any species through a long series of generations.
This would, of course, only suffice to bring about a reversion if the
determinants of the ancestral species were still contained in the
germ-plasm in comparative abundance. If this is no longer the case,
something more is necessary, and that is the relative weakness of the
more modern determinants.
If two white-flowered species of thorn-apple, _Datura ferox_ and
_Datura lævis_, be crossed, there arises a hybrid with bluish violet
flowers and brown stalks instead of green. This was interpreted by
Darwin as a reversion to violet-flowering ancestral species on both
sides, for there are even now a great number of species of _Datura_
with violet flowers and brown stalks. When the two white forms are
crossed the reversion takes place every time, not merely in some cases,
and we may conclude from this that in both these species there is still
such a strong admixture of the same unvaried ancestral ids that they
always excel the ids of the two modern species crossed, in strength
though certainly not in number. And this superiority must again depend
on the fact that similar determinants of the same part are cumulative
in their effect, while dissimilars are not.
For this reason reversions to remote ancestors occur readily when
species and breeds are crossed, while they are rare in the normal
inbreeding of a species. The reversions of the breeds of pigeon to
their wild ancestral form, the slaty-blue rock-pigeon, never result, as
Darwin showed, and as we have already noticed, from pure breeding of
one race, but only when two or more breeds are repeatedly crossed with
one another. Even then it occurs by no means always, but only now and
again. The germ-plasm of the breeds must therefore still contain ids
of the rock-pigeon, but in a small number, varying from individual to
individual. If by fortunate reducing divisions and the meeting together
of a sperm-cell rich in ancestral ids with a similarly endowed egg-cell
the number of ancestral ids be raised to such a point that it exceeds
the number of modern-breed ids contained in each one of the conjugating
germ-cells, the ancestral ids control the development and reversion
occurs, for the ancestral ids together have a cumulative effect while
the ids of the two parent breeds are different and therefore, as far
as they are so, cannot be co-operative in their influence. But it must
be understood that they need not be different as far as _all_ their
determinants are concerned, but usually only as regards some groups,
and thus it happens that reversion does not occur in regard to all, but
only in regard to particular characters--thus in the _Datura_ hybrids,
chiefly in regard to the colour of the flowers and the stem, and in
the hybrids between different breeds of pigeon mainly in regard to the
colour and marking of the plumage.
The reversions of the horse and ass to striped ancestors, which Darwin
has made famous, go much further back into the ancestral history of the
species, for while we know the ancestral form of the domestic pigeon
in the still living rock-dove (_Columba livia_), the ancestral form
common to the horse and the ass is extinct, and we can only suppose
that it was striped like a zebra, because such striping occasionally
occurs in pure horses and pure asses, at least in their youth, although
now only on the legs, and because this striping is often more marked
in the hybrid between the horse and the ass, the mule. In Italy, where
one sees hundreds of mules, the striping is not exactly frequent,
but it may occur in about two per cent., while in America it is said
to be much more frequent. The germ-plasm of the horse and the ass
must therefore contain, in varying numbers, ids whose skin-colour
determinants represent in part still unmodified ancestral characters.
When two germ-cells chance to meet in fertilization, both of which have
received, through a favourable reducing division, a relatively large
number of such ids, a relative majority of these in the fertilized
ovum is opposed to the dissimilar and therefore mutually neutralizing
homologous determinants of horse and ass, and reversion to the
ancestral form occurs.
These cases of reversion are enough to show us that the old unmodified
ancestral determinants may persist in the germ-plasm through long
series of generations. But an even deeper glimpse into the dim ancient
history of our modern species of horses is afforded by the occurrence
of three-toed horses, references to a small number of which the
palæontologist Marsh was able to discover in literature, and one of
which he was able to observe in life. Julius Caesar possessed a horse
whose three-toed feet represented a reversion to the horses of Tertiary
times, _Mesohippus_, _Miohippus_, and _Protohippus_ or _Hipparion_; for
all these genera possessed, in addition to the strong middle toe, two
weaker and shorter lateral toes.
In the germ-plasm of our modern horses there must still persist in
certain ids the determinants of the ancestral foot, which, after a long
succession of favourable reducing divisions combined with favourable
chances of fertilization, may come to be in a majority, and may thus be
able to induce a reappearance of a character which has long been hidden
under the surface of the present-day type of the species.
I do not propose to enter further on the discussion of the phenomena
of inheritance. A more detailed investigation of the phenomena of
reversion is to be found in '_The Germ-plasm_' published ten years
ago; and the discussion could not be resumed here without a critical
consideration of a relatively large series of newly acquired facts
not always harmonizing, and, as yet, not even fully available. The
year 1900 has given us the investigations of three botanists, De
Vries, Correns, and Tschermak, who have sought by experiments in
hybridization between different sorts of peas, beans, maize, and other
plants, to throw light on the phenomena of inheritance, and thus on
the actual processes which occur in the germ-plasm at the reducing
division. This led to the discovery that similar experiments had been
published as far back as 1866 by the Abbot of Brünn, Gregor Mendel, and
that these had been formulated as a law which is now called Mendel's
law. Correns showed, however, that this law, though correct in certain
cases, did not by any means hold good in all, and we must thus postpone
the working of this new material into our theory until a very much
wider basis of facts has been supplied by the botanists. There is less
to be hoped for from the zoologists in regard to this problem owing
to the almost insuperable difficulties in the way of a long series
of experiments in hybridization in animals. I myself have repeatedly
attempted experiments in this direction, and have always had to abandon
them, either because the crossing succeeded too rarely, or because the
hybrids did not reproduce among themselves, or did so defectively, or
because the distinguishing characters of the crossed breeds proved
insufficiently tenacious or diagnostic. But it would be a fine task for
zoological gardens to undertake such experiments from the point of view
of the germ-plasm theory, and their success would afford material for
the criticism of the theory, the more valuable because it is apparent
from the experiments on plants that the processes of heredity are
manifold, and are far from being uniform in different domains[10].
[10] Castle and Allen have recently published the results of
experiments in crossing white mice with grey, and these confirm
Mendel's Law.
I have assumed for my theory that the reducing division took place
according to the laws of chance, and that thus every combination of ids
occurred with equal frequency. This assumption seems to be confirmed,
by the experiments of the botanists I have mentioned, only in so far
that in the crossing of hybrids with one another every combination
of distinctive characters occurred with equal frequency. But, on the
other hand, the splitting of the germ-plasm at the reducing division
seems, as I said before, in many cases to take place in such a way that
the id-groups of the two parents are discretely separated from one
another; this was so in the stocks, peas, beans, and other hybrids.
But even if this were always the case in these, we could hardly infer
that it must be the same everywhere; we should rather expect that the
relationship of the two parents and their ids would bring into play the
finer attractions and repulsions between the ids of the germ-plasm,
and would thus determine their arrangement and grouping. Further
investigations may clear up this point; in the meantime we can only
say that already--even among hybrids--many deviations from Mendel's Law
have been established, for instance, by Bateson and Saunders (1902).
Before I conclude this lecture I should like to refer briefly to a
phenomenon which Darwin was acquainted with and sought to explain
through his theory of Pangenesis, but which at a later date was
regarded as not sufficiently authenticated to justify any attempt
at a theoretical explanation, since it seemed to contradict all our
conceptions of hereditary substance and its operations. I refer to the
phenomenon to which the botanists have given the pretty name of Xenia
(guest-gifts), and which consists in the fact that in crosses of two
different plants the characters of the male may appear not only in the
young plant but even in the seed, so that a transference of paternal
characters seems to take place from the pollen-tube to the mother, to
the 'tissue of the maternal ovary.' In heads of yellow-grained maize
(_Zea_) it is said that, after dusting with pollen from a blue-seeded
variety, blue seeds appear among the yellow, and similar observations
on other cultivated plants have been on record for more than half a
century. Thus dusting the stigma of green varieties of grape with the
pollen of a dark blue kind is said frequently to give rise to dark blue
fruits.
Darwin accepted these observations as correct, and endeavoured
to explain them as due to a migration of his 'gemmules' from the
fertilized ovum into the surrounding tissue of the mother-plant. His
explanation was not correct, we can say with confidence now, but he
was right so far, for the phenomena of Xenia do occur; they are not
illusory as most modern botanists seem inclined to believe. I myself
was at first inclined to wait for further facts in proof that the
phenomena of Xenia really occurred before attempting to bring them
into harmony with my theory, and this will not be found fault with
when it is remembered that these cases of Xenia seem to stand in
direct contradiction to the fundamental postulates of the germ-plasm
theory. For this depends essentially on a definite stable structure
of the germ-substance, which lies within the nucleus in the form of
chromosomes, and which cannot pass from one cell to another in any
other way than by cell-division and division of the nucleus; how then
could it pass from the fertilized ovum to the cells of the endosperm
which do not derive their origin from it at all, but from other cells
of the embryo-sac? In point of fact some of my opponents have cited
Xenia as an actual refutation of my theory.
[Illustration: FIG. 82. Fertilization in the Lily (_Lilium martagon_),
after Guignard. _A_, the embryo-sac before fertilization; _sy_,
synergidæ; _eiz_, ovum; _op_ and _up_, upper and lower 'polar nuclei';
_ap_, antipodal cells. _B_, the upper part of the embryo-sac,
into which the pollen-tube (_pschl_) has penetrated with the male
sex-nucleus (♂_k_) and its centrosphere; below that is the ovum-nucleus
(♀_k_) with its (also doubled) centrosphere (_csph_). _C_, remains of
the pollen-tube (_pschl_); the two sex-nuclei are closely apposed.
Highly magnified.]
That cases of Xenia really do occur is now established by the
comprehensive and at the same time exceedingly careful experiments
recently made by C. Correns with _Zea Mais_; it is only necessary to
look through the beautiful figures with which his work is adorned to be
convinced that heads of maize whose blossoms have been dusted with the
pollen of a different kind produce more or less numerous seeds of the
paternal kind, usually mingled with those of the maternal. Thus heads
of the variety _Zea alba_ resulting from fertilization with _Z. cyanea_
exhibited a majority of white grains, but among them a smaller number
of blue; and the converse experiment, of dusting _Z. cyanea_ with
the pollen of _Z. alba_, yielded heads in which a minority of white
grains appears among a majority of blue. But it is always only the
nutritive layer surrounding the embryo--the endosperm--which exhibits
the character of the paternal species, and even the capsule surrounding
the seed shows nothing of it, but is purely maternal. Thus the heads
of different species with pale-yellow capsule, when dusted with the
pollen of _Z. rubra_, never have red seeds like those of _Z. rubra_,
but always seeds with a pale-yellow skin, while, in the converse
experiment, dusting of the red-skinned species _Z. rubra_ with pollen
from _Z. vulgata_, all the seeds are red, like those of the maternal
species, and the influence of the paternal species only shows when the
strong red skin has been removed, so that the intense yellow colour of
the endosperm, which in the pure maternal species is white, is exposed
to view. Thus the mysterious influence of the pollen never goes beyond
the endosperm, and the riddle of this influence is solved in the most
unexpected manner, indeed was solved even before Correns had securely
established the genuine occurrence of Xenia. The explanation is due
to recent disclosures in regard to the processes of fertilization in
flowering plants.
It had long been known that the pollen-tube contains not merely one
generative nucleus but two, which arise from one by division. But what
had till recently remained unknown was that not only one of these
penetrated into the embryo-sac to enter into amphimixis with the
egg-cell, but that the other also makes its way in, and there fuses
with the two nuclei which had long been designated the upper and lower
polar-nuclei (Fig. 82, op. cit.). Nawaschin and Guignard demonstrated
that these two nuclei fuse _with the second male_ nucleus; thus two
acts of fertilization are accomplished in the embryo-sac, and one of
these gives rise to the embryo, while the second becomes nothing less
than the endosperm, the nutritive layer which surrounds the embryo,
whose origin from the polar nuclei had been previously recognized.
Thus the riddle of Xenia is essentially solved. We understand how
paternal primary constituents may find their way into the endosperm,
and indeed must do so regularly; we understand also how the paternal
influence never goes beyond the endosperm. The riddle is thus not only
solved, but at the same time the view which assumes a fixed germ-plasm,
and believes it to lie in the nuclear substance of the germ-cells,
receives further confirmation, if it should need any, for if facts
which are apparently contradictory to a theory can be naturally
brought into harmony with it, this affords a stronger argument for the
correctness of the theory than the power of explaining the facts which
were used in building it up.
There is much more to be said in regard to Xenia, and I am sure
that much that is of interest will be brought to light by deeper
investigation; theoretical difficulties will still have to be overcome,
and I have already pointed out one of these in my '_Germ-plasm_,' but I
must here rest satisfied with what has been already said.
We have now passed in review and attempted to fit into the theory a
sufficiently large number of the phenomena of heredity for the purpose
of these lectures. Although, as is natural, much of this must remain
hypothetical, we may accept the following series of propositions as
well founded: there is a hereditary substance, the germ-plasm; it is
contained in very minimal quantity in the germ-cells, and there in
the chromosomes of the nucleus; it consists of primary constituents
or determinants, which in their diverse arrangement beside or upon
one another form an extremely complex structure, the id. Ids and
determinants are living vital unities. Each nucleus contains several,
often many, ids, and the number of ids varies with the species and is
constant for each. The ids of the germ-plasm of each species have had
a historical development, and are derived from the germ-plasm of the
preceding lineage of species; therefore ids can never arise anew but
only through multiplication of already existing ids.
And now, equipped with this knowledge, let us return to the point
from which we started, and inquire whether the Lamarckian principle
of evolution, the inheritance of functional modifications, must be
accepted or rejected.
LECTURE XXIII
EXAMINATION OF THE HYPOTHESIS OF THE TRANSMISSIBILITY OF FUNCTIONAL
MODIFICATIONS
Darwin's Pangenesis--Alleged proofs of functional
inheritance--Mutilations not transmissible--Brown-Séquard's
experiments on Epilepsy in guinea-pigs--Confusion of infection of the
germ with inheritance, Pebrine, Syphilis, and Alcoholism--Does the
interpretation of the facts require the assumption of the transmission
of functional modifications?--Origin of instincts--The untaught
pointer--Vom Rath's and Morgan's views--Attachment of the dog to his
master--Fearlessness of sea-birds and seals on lonely islands--Flies
and butterflies--Instincts exercised only once in the course of a
lifetime.
As I have already said in an earlier lecture, Darwin adhered to
Lamarck's assumption of the transmission of functional adaptations,
and perhaps the easiest way to make clear the theoretical difficulties
which stand in the way of such an assumption is to show how Darwin
sought to present this principle as theoretically conceivable and
possible.
Darwin was the first to think out a theory of heredity which was worthy
of the name of theory, for it was not merely an idea hastily suggested,
but an attempt, though only in outline, at elaborating a definite
hypothesis. His theory of 'Pangenesis' assumes that cells give rise to
special gemmules which are infinitesimally minute, and of which each
cell brings forth countless hosts in the course of its existence. Each
of these gemmules can give rise to a cell similar to the one in which
it was itself produced, but it cannot do this at all times, but only
under definite circumstances, namely, when it reaches 'those cells
which precede in order of development' those that it has to give rise
to. Darwin calls this the 'elective affinity' of each gemmule for
this particular kind of cell. Thus, from the beginning of development
there arises in every cell a host of gemmules, each of which virtually
represents a specific cell. These gemmules, however, do not remain
where they originated, but migrate from their place of origin into the
blood-stream, and are carried by it in myriads to all parts of the
body. Thus they reach also the ovaries and testes and the germ-cells
lying within these, penetrate into them, and there accumulate, so that
the germ-cells, in the course of life, come to contain gemmules from
all the kinds of cells which have appeared in the organism, and, at
the same time, all the variations which any part may have undergone,
whether due to external or internal influences, or through use and
disuse.
In this manner Darwin sought to attribute to the germ-cells the power
of giving rise, in the course of their development, to the same
variations as the individual had acquired during its lifetime in
consequence of external conditions or functional influences.
I abstain from analysing the assumptions here made; their improbability
and their contradictions to established facts are so great that it
is not necessary to emphasize them; the theory shows plainly that it
is necessary to have recourse to very improbable assumptions, if an
attempt is to be made to find a theoretical basis for the transmission
of acquired (somatogenic) characters. Even when Darwin formulated
his theory of Pangenesis his assumptions were hardly reconcileable
with what was known of cell-multiplication; now they are above all
irreconcileable with the fact that the germ-substance never arises
anew, but is always derived from the preceding generation--that is,
with the continuity of the germ-plasm.
If we were now to try to think out a theoretical justification we
should require to assume that the conditions of all the parts of
the body at every moment, or at least at every period of life, were
reflected in the corresponding primary constituents of the germ-plasm
and thus in the germ-cells. But, as these primary constituents are
quite different from the parts themselves, they would require to vary
in quite a different way from that in which the finished parts had
varied; which is very like supposing that an English telegram to China
is there received in the Chinese language.
In spite of this almost insuperable theoretical obstacle, various
authors have worked out the idea that the nervous system, which
connects all parts of the body with the brain and thus also with each
other, communicates these conditions to the reproductive organs, and
that thus variations may arise in the germ-cells corresponding to those
which have taken place in remote parts of the body.
Even supposing it were proved that every germ-cell in ovary or testis
was associated with a nerve-fibre, what could be transmitted to it by
the nerves, except a stronger or weaker nerve-current? There is no such
thing as _qualitative_ differences in the current; how then could the
primary constituents of the germ be influenced by the nerve-current,
either individually or in groups, in harmony with the organs and parts
of the body corresponding to them, much less be caused to vary in a
similar manner? Or are we to imagine that a particular nerve-path leads
to every one of the countless primary constituents? Or does it make
matters more intelligible if we assume that the germ-plasm is without
primary constituents, and suppose that, after each functional variation
of a part, telegraphic notice is sent to the germ-plasm by way of the
brain as to how it has to alter its 'physico-chemical constitution,'
so that the descendants may receive some benefit from the acquired
improvement?