Santiago de Cuba on the 31st of December 1803, studied at the university
of Havana, and was called to the bar in 1823. In the autumn of 1823 he
was arrested on a charge of conspiracy against the Spanish government,
and was sentenced to banishment for life. He took refuge in the United
States, published a volume of verses at New York in 1825, and then went
to Mexico, where, becoming naturalized, he obtained a post as
magistrate. In 1832 a collection of his poems was issued at Toluca, and
in 1836 he obtained permission to visit Cuba for two months.
Disappointed in his political ambitions, and broken in health, Heredia
returned to Mexico in January 1837, and died at Toluca on the 21st of
May 1839. Many of his earlier pieces are merely clever translations from
French, English and Italian; but his originality is placed beyond doubt
by such poems as the _Himno del desterrado_, the epistle to Emilia,
_Desenganos_, and the celebrated ode to Niagara. Bello may be thought to
excel Heredia in execution, and a few lines of Olmedo's _Canto a Junin_
vibrate with a virile passion to which the Cuban poet rarely attained;
but the sincerity of his patriotism and the sublimity of his imagination
have secured for Heredia a real supremacy among Spanish-American poets.
The best edition of his works is that published at Paris in 1893 with
a preface by Elias Zerolo.
HEREDITAMENT (from Lat. _hereditare_, to inherit, _heres_, heir), in
law, every kind of property that can be _inherited_. Hereditaments are
divided into corporeal and incorporeal; corporeal hereditaments are
"such as affect the senses, and may be seen and handled by the body;
incorporeal are not the subject of sensation, can neither be seen nor
handled, are creatures of the mind, and exist only in contemplation"
(Blackstone, _Commentaries_). An example of a corporeal hereditament is
land held in freehold, of incorporeal hereditaments, tithes, advowsons,
pensions, annuities, rents, franchises, &c. It is still used in the
phrase "lands, tenements and hereditaments" to describe property in
land, as distinguished from goods and chattels or movable property.
HEREDITY, in biological science, the name given to the generalization,
drawn from the observed facts, that animals and plants closely resemble
their progenitors. (That the resemblance is not complete involves in the
first place the subject of variation (see VARIATION AND SELECTION); but
it must be clearly stated that there is no adequate ground for the
current loose statements as to the existence of opposing "laws" or
"forces" of heredity and variation.) In the simplest cases there seems
to be no separate problem of heredity. When a creeping plant propagates
itself by runners, when a _Nais_ or _Myrianida_ breaks up into a series
of similar segments, each of which becomes a worm like the parent, we
have to do with the general fact that growing organisms tend to display
a symmetrical repetition of equivalent parts, and that reproduction by
fission is simply a special case of metamerism. When we try to answer
the question why the segments of an organism resemble one another,
whether they remain in association to form a segmented animal, or break
into different animals, we come to the conclusion, which at least is on
the way to an answer, that it is because they are formed from pieces of
the same protoplasm, growing under similar conditions. It is apparently
a fundamental property of protoplasm to be able to multiply by division
into parts, the properties of which are similar to each other and to
those of the parent.
This leads us directly to the cases of reproduction where there is an
obvious problem of heredity. In the majority of cases among animals and
plants the new organisms arise from portions of living matter, separated
from the parents, but different from the parents in size and structure.
These germs of the new organisms may be spores, reproductive cells,
fused reproductive cells or multicellular masses (see REPRODUCTION). For
the present purpose it is enough to state that they consist of portions
of the parental protoplasm. These pass through an embryological history,
in which by growth, multiplication and specialization they form
structures closely resembling the parents. Now, if it could be shown
that these reproductive masses arose directly from the reproductive
masses which formed the parent body, the problems of heredity would be
extremely simplified. If the first division of a reproductive cell set
apart one mass to lie dormant for a time and ultimately to form the
reproductive cells of the new generation, while the other mass, exactly
of the same kind, developed directly into the new organism, then
heredity would simply be a delayed case of what is called organic
symmetry, the tendency of similar living material to develop in similar
ways under the stimulus of similar external conditions. The cases in
which this happens are very rare. In the Diptera the first division of
the egg-cell separates the nuclear material of the subsequent
reproductive cells from the material that is elaborated into the new
organism to contain these cells. In the _Daphnidae_ and in _Sagitta_ a
similar separation occurs at slightly later stages; in vertebrates it
occurs much later; while in some hydroids the germ-cells do not arise in
the individual which is developed from the egg-cell at all, but in a
much later generation, which is produced from the first by budding.
However, it is not necessary to dismiss the fertile idea of what Moritz
Nussbaum and August Weismann, who drew attention to it, called
"continuity of the germ-plasm." Weismann has shown that an actual series
of organic forms might be drawn up in which the formation of germ-cells
begins at stages successively more remote from the first division of the
egg-cell. He has also shown evidence, singularly complete in the case of
the hydroids, for the existence of an actual migration of the place of
formation of the germ-cells, the migration reaching farther and farther
from the egg-cell. He has elaborated the conception of the germ-track, a
chain of cell generations in the development of any creature along which
the reproductive material saved over from the development of one
generation for the germ-cells of the next generation is handed on in a
latent condition to its ultimate position. And thus he supposes a real
continuity of the germ-plasm, extending from generation to generation in
spite of the apparent discontinuity in the observed cases. The
conception certainly ranks among the most luminous and most fertile
contributions of the 19th century to biological thought, and it is
necessary to examine at greater length the superstructure which Weismann
has raised upon it.
_Weismann's Theory of the Germ-plasm._--A living being takes its
individual origin only where there is separated from the stock of the
parent a little piece of the peculiar reproductive plasm, the so-called
germ-plasm. In sexless reproduction one parent is enough; in sexual
reproduction equivalent masses of germ-plasm from each parent combine to
form the new individual. The germ-plasm resides in the nucleus of cells,
and Weismann identifies it with the nuclear material named chromatin.
Like ordinary protoplasm, of which the bulk of cell bodies is composed,
germ-plasm is a living material, capable of growing in bulk without
alteration of structure when it is supplied with appropriate food. But
it is a living material much more complex than protoplasm. In the first
place, the mass of germ-plasm which is the starting-point of a new
individual consists of several, sometimes of many, pieces named
"idants," which are either the chromosomes into a definite number of
which the nuclear material of a dividing cell breaks up, or possibly
smaller units named chromomeres. These idants are a collection of "ids,"
which Weismann tentatively identifies with the microsomata contained in
the chromosomes, which are visible after treatment with certain
reagents. Each id contains all the possibilities--generic, specific,
individual--of a new organism, or rather the directing substance which
in appropriate surroundings of food, &c., forms a new organism. Each id
is a veritable microcosm, possessed of an historic architecture that has
been elaborated slowly through the multitudinous series of generations
that stretch backwards in time from every living individual. This
microcosm, again, consists of a number of minor vital units called
"determinants," which cohere according to the architecture of the whole
id. The determinants are hypothetical units corresponding to the number
of parts of the organism independently variable. Lastly, each
determinant consists of a number of small hypothetical units, the
"biophores." These are adaptations of a conception of H. de Vries, and
are supposed to become active by leaving the nucleus of the cell in
which they lie, passing out into the general protoplasm of the cell and
ruling its activities. Each new individual begins life as a nucleated
cell, the nucleus of which contains germ-plasm of this complex structure
derived from the parent. The reproductive cell gives rise to the new
individual by continued absorption of food, by growth, cell-divisions
and cell-specializations. The theory supposes that the first divisions
of the nucleus are "doubling," or homogeneous divisions. The germ-plasm
has grown in bulk without altering its character in any respect, and,
when it divides, each resulting mass is precisely alike. From these
first divisions a chain of similar doubling divisions stretches along
the "germ-tracks," so marshalling unaltered germ-plasm to the generative
organs of the new individual, to be ready to form the germ-cells of the
next generation. In this mode the continuity of the germ-plasm from
individual to individual is maintained. This also is the immortality of
the germ-cells, or rather of the germ-plasm, the part of the theory
which has laid so large a hold on the popular imagination, although it
is really no more than a reassertion in new terms of biogenesis. With
this also is connected the celebrated denial of the inheritance of
acquired characters. It seemed a clear inference that, if the hereditary
mass for the daughters were separated off from the hereditary mass that
was to form the mother, at the very first, before the body of the mother
was formed, the daughters were in all essentials the sisters of their
mother, and could take from her nothing of any characters that might be
impressed on her body in subsequent development. In the later
elaboration of his theory Weismann has admitted the possibility of some
direct modification of the germ-plasm within the body of the individual
acting as its host.
The mass of germ-plasm which is not retained in unaltered form to
provide for the generative cells is supposed to be employed for the
elaboration of the individual body. It grows, dividing and multiplying,
and forms the nuclear matter of the tissues of the individual, but the
theory supposes this process to occur in a peculiar fashion. The nuclear
divisions are what Weismann calls "differentiating" or heterogeneous
divisions. In them the microcosms of the germ-plasm are not doubled, but
slowly disintegrated in accordance with the historical architecture of
the plasm, each division differentiating among the determinants and
marshalling one set into one portion, another into another portion.
There are differences in the observed facts of nuclear division which
tend to support the theoretical possibility of two sorts of division,
but as yet these have not been correlated definitely with the divisions
along the germ-tracks and the ordinary divisions of embryological
organogeny. The theoretical conception is, that when the whole body is
formed, the cells contain only their own kind of determinants, and it
would follow from this that the cells of the tissues cannot give rise to
structures containing germ-plasm less disintegrated than their own
nuclear material, and least of all to reproductive cells which must
contain the undisintegrated microcosms of the germ-plasm. Cases of
bud-formation and of reconstructions of lost parts (see REGENERATION OF
LOST PARTS) are regarded as special adaptations made possible by the
provision of latent groups of accessory determinants, to become active
only on emergency.
It is to be noticed that Weismann's conception of the processes of
ontogeny is strictly evolutionary, and in so far is a reversion to the
general opinion of biologists of the 17th and 18th centuries. These
supposed that the germ-cell contained an image-in-little of the adult,
and that the process of development was a mere unfolding or evolution of
this, under the influence of favouring and nutrient forces. Hartsoeker,
indeed, went so far as to figure the human spermatozoon with a mannikin
seated within the "head," and similar extremes of imagination were
indulged in by other writers for the spermatozoon or ovum, according to
the view they took of the relative importance of these two bodies. C. F.
Wolff, in his _Theoria generationis_ (1759), was the first distinguished
anatomist to make assault on these evolutionary views, but his direct
observations on the process of development were not sufficient in bulk
nor in clarity of interpretation to convince his contemporaries.
Naturally the improved methods and vastly greater knowledge of modern
days have made evolution in the old sense an impossible conception; we
know that the egg is morphologically unlike the adult, that various
external conditions are necessary for its subsequent progress through a
slow series of stages, each of which is unlike the adult, but gradually
approaching it until the final condition is reached. None the less,
Weismann's theory supposes that the important determining factor in
these gradual changes lies in the historical architecture of the
germ-plasm, and from the theoretical point of view his theory remains
strictly an unfolding, a becoming manifest of hidden complexity.
_Hertwig's View._--The chief modern holder of the rival view, and the
writer who has put together in most cogent form the objections to
Weismann's theory, is Oscar Hertwig. He points out that there is no
direct evidence for the existence of differentiating as opposed to
doubling divisions of the nuclear matter, and, moreover, he thinks that
there is very generally diffused evidence as to the universality of
doubling division. In the first place, there is the fundamental fact
that single-celled organisms exhibit only doubling division, as by that
the persistence of species which actually occurs alone is possible. In
the case of higher plants, the widespread occurrence of tissues with
power of reproduction, the occurrence of budding in almost any part of
the body in lower animals and in plants, and the widespread powers of
regeneration of lost parts, are easily intelligible if every cell like
the egg-cell has been formed by doubling division, and so contains the
germinal material for every part of the organism, and thus, on the call
of special conditions, can become a germ-cell again. He lays special
stress on those experiments in which the process of development has been
interfered with in various ways at various stages, as showing that the
cells which arise from the division of the egg-cell were not predestined
unalterably for a particular role, according to a predetermined plan. He
dismisses Weismann's suggestion of the presence of accessory
determinants which remain latent unless they happen to be required, as
being too complicated a supposition to be supported without exact
evidence, a view in which he has received strong support from those who
have worked most at the experimental side of the question. From
consideration of a large number of physiological facts, such as the
results of grafting, transplantations of tissues and transfusions of
blood, he concludes that the cells of an organism possess, in addition
to their patent microscopical characters, latent characters peculiar to
the species, and pointing towards a fundamental identity of the germinal
substance in every cell.
_The Nuclear Matter._--Apart from these two characteristic protagonists
of extreme and opposing views, the general consensus of biological
opinion does not take us very far beyond the plainest facts of
observation. The resemblances of heredity are due to the fact that the
new organism takes its origin from a definite piece of the substance of
its parent or parents. This piece always contains protoplasm, and as the
protoplasm of every animal and plant appears to have its own specific
reactions, we cannot exclude this factor; indeed many, following the
views of M. Verworn, and seeing in the specific metabolisms of
protoplasm a large part of the meaning of life, attach an increasing
importance to the protoplasm in the hereditary mass. Next, it always
contains nuclear matter, and, in view of the extreme specialization of
the nuclear changes in the process of maturation and fertilization of
the generative cells, there is more than sufficient reason for believing
that the nuclear substance, if not actually the specific germ-plasm, is
of vast importance in heredity. The theory of its absolute dominance
depends on a number of experiments, the interpretation of which is
doubtful. Moritz Nussbaum showed that in Infusoria non-nucleated
fragments of a cell always died, while nucleated fragments were able to
complete themselves; but it may be said with almost equal confidence
that nuclei separated from protoplasm also invariably die--at least, all
attempts to preserve them have failed. Hertwig and others, in their
brilliant work on the nature of fertilization, showed that the process
always involved the entrance into the female cell of the nucleus of the
male cell, but we now know that part of the protoplasm of the
spermatozoon also enters. T. Boveri made experiments on the
cross-fertilization of non-nucleated fragments of the eggs of
_Sphaerechinus granularis_ with spermatozoa of _Echinus
microtuberculatus_, and obtained dwarf larvae with only the paternal
characters; but the nature of his experiments was not such as absolutely
to exclude doubt. Finally, in addition to the nucleus and the
protoplasm, another organism of the cell, the centrosome, is part of the
hereditary mass. In sum, while most of the evidence points to a
preponderating importance of the nuclear matter, it cannot be said to be
an established proposition that the nuclear matter is the germ-plasm.
Nor are we yet definitely in a position to say that the germinal mass
(nuclear matter, protoplasm, &c., of the reproductive cells) differs
essentially from the general substance of the organism--whether, in
fact, there is continuity of _germ-plasm_ as opposed to continuity of
living material from individual to individual. The origin of sexual
cells from only definite places, in the vast majority of cases, and such
phenomena as the phylo-genetic migration of their place of origin among
the Hydro-medusae, tell strongly in favour of Weismann's conception.
Early experiments on dividing eggs, in which, by separation or
transposition, cells were made to give rise to tissues and parts of the
organism which in the natural order they would not have produced, tell
strongly against any profound separation between germ-plasm and
body-plasm. It is also to be noticed that the failure of germ-cells to
arise except in specific places may be only part of the specialized
ordering of the whole body, and does not necessarily involve the
interpretation that reproductive material is absolutely different in
kind.
_Amphimixis._--Hitherto we have considered the material bearer of
heredity apart from the question of sexual union, and we find that the
new organism takes origin from a portion of living matter, forming a
material which may be called germ-plasm, in which resides the capacity
to correspond to the same kind of surrounding forces as stimulated the
parent germ-plasm by growth of the same fashion. In many cases (e.g.
asexual spores) the piece of germ-plasm comes from one parent, and from
an organ or tissue not associated with sexual reproduction; in other
cases (parthenogenetic eggs) it comes from the ovary of a female, and
may have the apparent characters of a sexual egg, except that it
develops without fertilization; here also are to be included the cases
where normal female ova have been induced to develop, not by the
entrance of a spermatozoon, but by artificial chemical stimulation. In
such cases the problem of heredity does not differ fundamentally from
the symmetrical repetition of parts. In most of the higher plants and
animals, however, sexual reproduction is the normal process, and from
our present point of view the essential feature of this is that the
germ-plasm which starts the new individual (the fertilized egg) is
derived from the male (the spermatozoon) and from the female parent (the
ovum). Although it cannot yet be set down sharply as a general
proposition, there is considerable evidence to show that in the
preparation of the ovum and spermatozoon for fertilization the nuclear
matter of each is reduced by half (reducing division of the
chromosomes), and that fertilization means the restoration of the normal
bulk in the fertilized cell by equal contributions from male and female.
So far as the known facts of this process of union of germ-plasms go,
they take us no farther than to establish such a relation between the
offspring and two parents as exists between the offspring and one parent
in the other cases. Amphimixis has a vast importance in the theory of
evolution (Weismann, for instance, regards it as the chief factor in the
production of variations); for its relation to heredity we are as yet
dependent on empirical observations.
_Heredity and Development._--The actual process by which the germinal
mass slowly assumes the characters of the adult--that is, becomes like
the parent--depends on the interaction of two sets of factors: the
properties of the germinal material itself, and the influences of
substances and conditions external to the germinal material. Naturally,
as K. W. von Nageli and Hertwig in particular have pointed out, there is
no perpetual sharp contrast between the two sets of factors, for, as
growth proceeds, the external is constantly becoming the internal; the
results of influences, which were in one stage part of the environment,
are in the next and subsequent stages part of the embryo. The
differences between the exponents of evolution and epigenesis offer
practical problems to be decided by experiment. Every phenomenon in
development that is proved the direct result of epigenetic factors can
be discounted from the complexity of the germinal mass. If, for
instance, as H. Driesch and Hertwig have argued, much of the
differentiation of cells and tissues is a function of locality and is
due to the action of different external forces on similar material, then
just so much burden is removed from what evolutionists have to explain.
That much remains cannot be doubted. Two eggs similar in appearance
develop side by side in the same sea-water, one becoming a mollusc, the
other an _Amphioxus_. Hertwig would say that the slight differences in
the original eggs would determine slight differences in metabolism and
so forth, with the result that the segmentation of the two is slightly
different; in the next stage the differences in metabolisms and other
relations will be increased, and so on indefinitely. But in such cases
_c'est le premier pas qui coute_, and the absolute cost in theoretical
complexity of the germinal material can be estimated only after a
prolonged course of experimental work in a field which is as yet hardly
touched.