ARE MODIFICATIONS ACQUIRED DIRECTLY BY THE BODY INHERITED?
=Which New Characters Are Inherited?--=Any new feature which appears in a
given organism may have had its origin in some change which has come about
in the germ from which it sprang, or it may be merely the product of some
unusual stimulus operating on the body. While the outcome, as far as the
present individual is concerned, is in each case a definite modification,
the matter of inheritance is a very different question. On the first
alternative where the new character is the outcome of germinal change, it
is obvious that the altered germ-plasm will find expression in a similar
way in succeeding generations as long as the new germinal combinations
persist. On the other hand, if the new character has resulted merely from
some influence operating on the body of the individual, then to be
inherited it would also have in some way to be transferred to and
incorporated in the germ-plasm. Inasmuch as the body or soma of any
individual is highly plastic and since various of its ultimate features
may be mere somatic modifications, it is important to decide if possible
whether or not somatic variations which are not of germinal origin can be
inherited.
=Examples of Somatic Modifications.--=For example, the small foot of the
Chinese woman of certain caste is the result of inherent germinal factor
for the production of a foot plus the effects of binding which are in no
wise germinal. The hand of the skilled pianist is a normal hand of
germinal origin and normal environment plus the effects of special
training. Again, the head of the Flathead Indian is a normal head of
germinal origin and environment plus the effects of flattening. Similarly,
almost any malformation of extrinsic origin may be cited, ranging from
mutilations and amputations, scars and the like to monstrosities such as
one-eyed fish which may be produced by subjecting a developing embryo to
adverse conditions of development.
=Use and Disuse.--=Even reactions set up through the organism's own
activities must produce changes. For example, a muscle has a certain
average of normal development in the average man; it comes to this through
the innate nature of its component cells plus a certain average amount of
exercise. It may, however, be developed far beyond this average by
excessive exercise. On the other hand, it is a well-known fact that an
unused organ weakens or may remain but partially developed. Thus either
use or disuse may play an important part in the molding of a given
individual. But whether or not in doing this it similarly affects the germ
is a very different matter.
=The Problem Stated.--=The question is can such enhanced or suppressed
development, or can new or modified characters, produced in an individual
by external agencies be so reflected on the germ-cell of the individual
that they tend to reappear _as such_ in its offspring without requiring
the same external factors for their production?
=Special Conditions Prevail in Mammals.--=Before proceeding further we
must recognize clearly the very special conditions which exist in most
mammals. With them environment is in part an intra-maternal environment
and in part independent of parental influences. Thus the formula for most
non-mammalia would be--
Individual == egg + non-parental environment; but
for most mammals, including man--
Individual == egg + intra-maternal environment + non-parental
environment.
This condition in mammals introduces a complicating factor which is likely
to obscure the whole issue unless we bear it constantly in mind. In other
words, we must discriminate sharply, in the discussion of inheritance in
man, for instance, between two classes of influences which may exist in
the infant at birth, that is, which are _congenital_; namely, those which
were truly inherent--were in the germ-cells--at the very inception of the
young individual, and (2) those which might later have been derived from
either parent by the yet unborn offspring. The latter are not regarded as
truly hereditary. Since certain diseases or their effects belong here we
occasionally find a physician using the term inheritance for such prenatal
influences, but the more careful ones now employ the term _transmission_
to discriminate between such conditions and true inheritance. In its
biological usage inheritance always refers to germinal constitution and
never to any condition that may be thrust on a developing organism before
birth. It is clear, then, that congenital conditions are not all
necessarily cases of inheritance.
=Three Fundamental Questions.--=To get at the question of the inheritance
of body modifications with the least confusion, let us examine it in the
form of three fundamental questions, as follows:
1. Can external influences directly affect the germ-cells?
2. Can external influences, operating through the intermediation of the
parental body, affect the germ-cells? If so, is the effect a specific and
a permanent one which persists in succeeding generations independently of
external influences similar to those which originally produced it? Only
such a condition as this would rank as the inheritance of a somatic
modification.
3. Can the appearance of new characters be explained on any other ground,
or on any more inclusive basis, than through the transmission of somatic
acquirements, or do organisms possess heritable characters which are
inexplicable as inheritance of such modifications?
Obviously the only way the question can be settled is through careful
experimentation in which all possible sources of error have been foreseen
and guarded against. Much experimental work has been undertaken for the
solution of this problem as the goal and we may therefore select typical
ones of these experiments and apply the results toward answering our three
questions.
=External Influences May Directly Affect the Germ-Cells.--=There is
evidence that under special conditions external influences may in certain
organisms affect the germ-cells, but that this occurs commonly is
extremely doubtful. For example, Professor MacDougal, by treating the
germ-cells of the evening primrose with various solutions, such as sugar,
zinc sulphate and calcium nitrate, has apparently succeeded in producing
definite germinal mutations. He injected the solution into the ovary of
the flower the forenoon of the day at the close of which pollination would
occur. He reports that in this way changes were produced in the germ which
found expression in new and permanent characters.
Professor Tower has experimented for a number of years with various
species of _Leptinotarsa_, the potato beetle. By varying the conditions of
temperature, humidity and atmospheric pressure when females were laying
their eggs, he reports having produced variations in the young which came
from these eggs although the mothers themselves were not changed.
According to Professor Tower slight increase or decrease in these
environmental factors stimulated the activity of the color producing
ferments, giving rise to melanic or darker individuals. Greater increase
or decrease, inhibited them and produced albinos. He found also that at
times the same stimulus might show different results in different eggs.
The effect, therefore, is a general and not a specific one. Ordinarily the
eggs of these beetles are laid in batches. When one of these batches was
laid and left under normal conditions, the usual form of young hatched
from it, but other batches from the same female under abnormal conditions
resulted in the production of atypical forms. For example, a normal
two-brooded form became five-brooded. The commonest modification was the
production of various color types. These once established, according to
Professor Tower, behave as independent, inheritable units.
The experiments of Doctor Bardeen with X-rays and of others with X-rays,
radium and other agents on the sperm and ova of amphibia show that these
are very susceptible to injurious influence at or near the time of
fertilization.
=Such Effects Improbable in Warm-Blooded Animals.--=However possible it
may be to bring about germinal changes in invertebrata or lower vertebrata
by such external agents as temperature and the like it is obvious that the
probability of such extrinsic influences affecting the germ-cells of
warm-blooded animals is very remote indeed. In the latter the germ-cells
are more or less distant from the exterior and are at practically a
constant temperature. Such experiments, therefore, beyond showing the
possibility of producing changes in germ-cells, do not have very direct
bearing on the problem of how inheritable variations are produced in man.
In his case about the only avenue of approach through which germ-cells
might be influenced is the blood or lymph.
=Poisons in the Blood May Affect the Germ-Cells.--=Any poisonous material
in the latter might injuriously affect the gametes. We know, in fact, that
such poisons as alcohol, lead and various drugs, and also the toxins of
various diseases, do so affect germ-cells. It seems plausible to suppose
that changing conditions of nutrition may affect the constitution of the
germ-cells and thus induce changes in the organism which arise from these
cells, but such nutritional effect is not yet a matter of established
fact.
=Difficulty of Explaining How Somatic Modifications Could be Registered in
Germ-Cells.--=As to our second query concerning the possibility of
affecting the germ-cells through the intermediation of parental tissues,
it is evident at a glance that since the germ-cells are built up along
with the body and are not a product of it (Fig. 2, p. 13), if such effects
are possible they must take place through the agency of some transporting
medium. The germ-cells, being lineal descendants of the original fertile
germ or zygote, already have the same possibilities of developing into an
adult that the zygote had, and so the problem becomes one of modifying a
complete germ already organized rather than of establishing a new germ by
getting together samples of every part of the body. This is all the more
evident when one realizes that usually the germ-cells are set apart long
before the body becomes adult, that is, before the body has developed most
of its characteristics. Moreover, among lower animals many instances are
known where the immature young or even larvae will produce offspring which
nevertheless ultimately manifest all the structures of the adult
condition.
But supposing specific modifications of the germinal mechanism were
possible, it is difficult to comprehend how an influence at a distant
point of the body could reach the germ-cell, to say nothing of the even
greater difficulty of understanding how it could become registered in the
germ in a specific way as affecting a particular part. For it must be
remembered that the organs of the adult do not exist as such in the germ
but are present there only as potentialities. How, for example, can a
change in the biceps muscle of one's arm be registered in a germ-cell in
which there is no biceps muscle, but merely the possibilities of
developing one? Or how can increased mental ability which is contingent on
the elaboration of certain brain-cells be impressed on a germ which has no
brain-cells but only the capacity under certain conditions of producing
such cells? For the brain of a child is not descended from the brain of
his parent, but from a germ-cell carried by that parent.
=Persistence of Mendelian Factors Argues Against Such a Mode of
Inheritance.--=On the face of things, the apparent inviolability of
Mendelian factors which may remain unexpressed in the germ for one or many
generations--indeed the whole matter of genotypical differences in the
gametes of the same individual--shows the improbability of somatic
interference with the germ-plasm. But notwithstanding this, because of the
great importance of the issue, it is well to review in some considerable
detail the various phases and possibilities of the question.
=Experiments on Insects.--=Some of the attempts to secure evidence of the
transmission of personally acquired parental modifications in insects are
very interesting. Many insects in the larval stages, particularly just
after pupation seem to be especially susceptible to external influences.
They have been much used, therefore, for purposes of experiment. It has
long been known that differences in size, in color and even in the shape
of wings can be produced by various agents if applied at this period of
development. From the standpoint of heredity, however, the important
consideration is to determine if these experimentally induced changes have
been reflected on to the germ-cells so that they reappear in the offspring
of the modified individuals.
It has been found that in some cases where male and female are of
different color, the color of the female can be changed to that of the
male by altering the conditions of temperature. In certain cases types can
be changed by cold so that they resemble varieties of the same species
found farther north, and by heat, varieties found farther south. But not
all individuals of a given lot are affected, and often different
individuals of the same kind show different effects. Moreover, in some
cases the same aberrations were produced by heat as by cold. This
indicates that it is not so much a question of specific effects as a
general physiological change, apparently mainly a matter of direct
influence of temperature on the chemical composition of the pigments. The
Countess von Linden in fact has shown that the extracted pigments can be
made to undergo the same changes of color in a test-tube by heat and cold
as in the pupae. But there is no evidence that the germ-cells of the living
insect were affected in a specific way. In a small fraction of the
offspring of such modified individuals abnormalities appeared, but these
were not always of the same kind as those which had been produced in the
parent. That is, there was no evidence of a trait or character having been
acquired by the body and handed on to the germ-cell. Where an effect was
produced on the germ-cell it was probably produced directly as in the
first cases discussed.
Size, colors and markings of butterflies have also been altered by
subjecting the caterpillars or the pupae to such influences as strong
light, electricity, various chemical substances, centrifuging, diminished
oxygen supply, etc., but the results were in the main confined to the
immediate generations. In the few cases where permanent inheritable
changes were seemingly produced they were more reasonably interpreted as
the effects of direct action on the germ-cells than as examples of
inherited somatic modifications.
Starvation experiments which resulted in the dwarfing of adult individuals
have been performed on various insects, and while the dwarf condition may
persist through one or two generations due to a diminished food supply in
the eggs of the dwarf, the stock in question when returned to normal food
conditions soon resumes its original characteristic size.
=Experiments on Plants.--=Many experiments have been performed with
plants, inasmuch as they are particularly prone to become modified by
changes of food supply, or climate. For example, plants which grow
luxuriantly in a warm moist climate or a rich soil may become stunted and
markedly changed if transplanted to a cold climate or a poor soil.
Naturally, their progeny will exhibit the same behavior as long as they
are kept under the new conditions. Experiments carried on through numerous
generations, however, practically all show that the germinal constitution
of the plants remains unchanged, for when their seeds are planted under
the original favorable conditions of soil or climate, the plants resume
their former habits of growth. Naegeli, for instance, who made a study of
many varieties of Alpine plants, and who carried on experiments with many
of them for years in the Garden of Munich, concluded that no permanent
effects had been produced by the Alpine climate and conditions in plants
from other regions which had come under its influence. A few botanists
have claimed to have found that the changes produced by the Alpine climate
have persisted for a generation or two and have then worn off. More recent
experiments on various of our field grains which have been stunted and cut
down in productivity by growing for a number of generations under adverse
conditions show that they have not been permanently modified by such
treatment, for they resume normal productivity and size when grown again
under favorable conditions.
On the other hand, Lederbaur found that a common weed, _Capsella_, when
transplanted from an Alpine habitat to the lowlands did not return to the
lowland type of the weed, but retained certain of its Alpine
characteristics. It is not clear, however, that this particular species
during its long sojourn of many generations in Alpine conditions may not
have undergone a series of germinal variations and have developed into a
new variety or species quite independently of changes wrought in the germ
by reflected somatic effects. Indeed, in face of the preponderance of
other cases to the contrary, this interpretation would seem to be the more
plausible one.
=Experiments on Vertebrates.--=In the vertebrates we may also find
examples of various somatic modifications experimentally produced, but
evidence of their inheritance is as difficult to establish as in the
invertebrates. Let us examine a few of the more significant of these which
are alleged by some to bear evidence of such inheritance.
By decreasing the amount of water in an aquarium Marie von Chauvin was
able to transform the aquatic, gill-breathing salamander _Axolotl_ into
the gill-less land form _Ambystoma_, heretofore regarded by systematists
as a different species. Either of these forms when sexually mature
produces its like. The salamanders in question have both lungs and gills,
but after a time the ones which are to be land forms lose their gills and
become exclusively lung-breathers. What seems to have been accomplished
then is the accelerating or forcing of normal natural tendencies already
inherent in the organism instead of introducing something new into the
inheritance by way of the soma. _Axolotl_ is in all probability merely a
larval form of _Ambystoma_ which with high temperature and an abundance of
water reproduces without advancing to the final possible stage of its life
cycle.
=Epilepsy in Guinea-Pigs.--=Perhaps the most frequently cited case and the
one in which the defenders of the idea of somatic inheritance usually take
final refuge is that of Doctor Brown-Sequard's guinea-pigs,
notwithstanding the fact that no one has had convincing success in
repeating the experiments and that the original results are apparently
open to more than one interpretation. This experimenter rendered
guinea-pigs epileptic by certain injuries to the nervous system. Epilepsy
appeared in some of the offspring of these operated animals. He regarded
this as an example of the inheritance of an artificially induced epilepsy.
An indirect loss of toes occurred in some of the parents as a result of
the operations on the nervous system. Some of their young also had missing
toes. However, as has been pointed out by various critics, guinea-pigs are
strongly predisposed toward epileptic-like seizures, and the epilepsy in
the young may have been merely a coincidence. Voison and Peron believe
they have shown that in epilepsy a toxin is produced that may affect the
unborn fetus. That is, the result might have been due to a poison derived
directly from the mother. The experiments in fact show that it was mainly
in the offspring of affected mothers that the condition appeared. Others
maintain that we do not know the exact nature of epilepsy, that in some
cases it may be the result of infection by disease-germs, and that
Brown-Sequard's cases may, therefore, have been merely the communication
of a disease from parent to child. As to the disappearance of toes it is a
well-known fact that rodents in particular are likely to gnaw off the toes
of their young very soon after birth, and little credence can be put in a
lack of toes in such young as cases of inheritance except under conditions
of much more careful observation than existed in Brown-Sequard's
experiments. A fuller account of these experiments will be found in
Romanes' _Darwin and After Darwin_, Vol. II, Chap. 6.
=Effects of Mutilations Not Inherited.--=Many experiments have been
performed by investigators to determine whether or not the results of
mutilation are transferred to succeeding generations, but so far only with
negative results. Many such experiments have been unwittingly carried on
for many generations, in fact, by breeders and fanciers, in the docking of
horses, dogs and sheep, the dehorning of cattle and the like, yet no
satisfactory evidence of the transmission of such conditions in any degree
has ever been forthcoming. The mutilations or distortions of the human
body through various rites or social customs also fails to yield any
convincing examples. Foot-binding, head-binding, or waist-binding must be
repeated in each successive generation to produce the particular type of
"beauty" that results from such deformities. And lucky it is for man that
injuries do not persist in subsequent generations, otherwise the modern
human being would be but a maimed relic of past misfortunes.
=Transplantation of Gonads.--=An interesting experimental test regarding
the effect of the body on the germ was made recently by Castle and
Phillips with guinea-pigs. It will be recalled from the discussion on
Mendelism that when a black guinea-pig is mated with a white one the
offspring are always black. These experimenters transplanted the ovaries
from a young black guinea-pig to a young white female whose own ovaries
had been previously removed. This white female was later mated to a white
male. Although she produced three different litters of young, six
individuals in all, the latter were all black. That is, not a trace of
coat-color of the white father or of the white foster-mother was impressed
on the transplanted germ-cells or the developing young. Later experiments
of the same kind by Castle and Phillips, with other varieties of
guinea-pigs, have yielded the same results. The body of the mother,
indeed, seems to serve merely as a protective envelope and a source of
nutrition.
=Effects of Body on Germ-Cells General, Not Specific.--=As far as the
evidence regarding the modification of the germ-plasm by the body is
concerned, we must conclude then that while under special circumstances
the germ-cells may be affected, the effect is general rather than specific
and the result as seen in the offspring has no discoverable correlation
with any particular part or structure of the parental soma. The effect is
presumably of much the same nature as where the germ is directly affected
by external agents. Where a new character or a modification of one already
existing is produced by a given condition of environment, in our
experience so far to have the same repeated in the offspring, a similar
evocative condition must prevail in the environment of the latter. Or in
other words the new character is not a permanent one which persists in
succeeding generations independently of external influences similar to
those which originally produced it.
=Certain Characters Inexplicable as Inherited Somatic Acquirements.--=It
would require remarkable credulity, in fact, to believe that some of the
most striking features about certain plants or animals could have been
developed by means of the inheritance of somatic modifications. For
example, many animals such as the quail, the rabbit, or the leaf-butterfly
are protectively colored. That is, they harmonize in color-pattern with
their surroundings so closely that they are overlooked by their enemies.
But how can this oversight on the part of an enemy so affect the bodies
and through them the germ-cells of such individuals as to develop so high
a degree of protective coloration? Or how, indeed, could any of numerous
adaptive structures which one can think of, such as the color or scent of
flowers to lure insects for cross-pollenation, the various grappling
devices on many seeds to secure wide distribution by animals, or the like,
have been directly produced by use or disuse or by any variation produced
in them by the agents to which they are adapted?
=The Case of Neuter Insects.--=A very instructive example of the
improbability that great skill, highly specialized structures, or certain
instincts are first developed in the parental body as the result of use
and then passed on to the offspring, is seen in the case of neuter
insects. In bees, for example, there are three classes of individuals: the
drones or males; the queens or functional females; and the workers, which
are neuter, that is, take no part in reproduction. The latter are really
sexually undeveloped females. The queen can lay either fertilized or
unfertilized eggs. The latter always give rise to males. The workers
gather the food, attend the queen, wait on the young, construct the comb,
and in short perform all the ordinary functions of the colony except the
reproductive. They have many highly specialized structures on various
parts of their bodies for carrying on their many activities, as well as
the very highly specialized instincts necessary to the maintenance of the
colony. But now, complex and highly developed as these workers are, since
they do not give rise to offspring, no matter how much experience or
structural modifications they may acquire during their lifetime, it can
not be handed on to another generation. Nor can they have come to their
present highly organized state through such a form of transmission since
they are not the descendants of workers but of a queen. Any new
modifications that appear in the workers of a colony must therefore have
their origin in changes which have taken place in the germ-cells of the
queen, and not in the soma of some other worker. It has been argued that
the worker has not always been infertile; that at a more primitive stage
of the evolution of the bee colony every female was both worker and
mother, and that individual somatic acquirements might therefore have been
transmitted, but this argument can not hold for many of the instincts or
features of the modern bee because these have to do only with the
conditions of life which exist in the colony in its present form. It is
obviously absurd to maintain, for instance, that all the highly
specialized instincts incident to queen production, queen attendance and
the like were functionally produced through usage before there was any
queen to produce or attend, while on the other hand, the very necessity of
queen production and maintenance is the outcome of the infertility of the
workers. Some workers have been known to lay eggs, but as these are few in
number and are never fertilized, which means if they develop they can only
produce males, they can play no considerable part in inheritance.
ORIGIN OF NEW CHARACTERS
=Origin of New Characters in Germinal Variation.--=This brings us to our
last query as to whether the appearance of new characters can be explained
on any other or any more inclusive ground than that which infers that
changes undergone by the parent-body are in some way registered in the
germ-cells so as to be repeated in a certain measure in the body of the
offspring. The answer to the question of how inheritable variations do
come to appear in offspring if not through changes produced in the body of
the parent, is uncertain; nevertheless most biologists believe that they
do not have such a somatic origin but arise directly as germinal
variations. Some would attribute them to the fluctuating nature of living
substance in general. The instability of protoplasm is one of its striking
characteristics. It is constantly being broken down and built up, or, in
other words, undergoing waste and repair. Like all other protoplasm, that
of the germ-cells must also undergo these metabolic changes and it is
possible though not proved that in this give and take of substances small
changes occur in their constitution which find expression in the offspring
as variations. As already seen, substances in the blood other than food
may also affect the constitution of the germ-cells.
=Sexual Reproduction in Relation to New Characters.--=Some biologists
attribute great importance to sexual reproduction as a basis of variation
and the origin of new characters. They argue that the mingling of
determiners from two different lines must produce many new combinations
and expressions of germinal potentialities. Plausible as the argument
seems at first sight no one has succeeded as yet in securing proof that
absolutely new characters can be originated in this way. What seems to
occur under such circumstances is merely a reshuffling or sorting of old
unit-characters. Although innumerable permutations and combinations of
these may be made which find new expression outwardly, this is obviously
not creating determiners of new unit-characters in the germ-plasm. While
many biologists would not deny the possibility or even the probability
that the determiners of unit-characters may sometimes combine or influence
one another so as to form actual permanent new characters, the proof of
such performance is wholly lacking. On the other hand, there are not a few
biologists who argue that sexual reproduction accomplishes just the
reverse of increasing the extent of variation or creating new characters;
according to them it tends to annul exceptional peculiarities of either
parent by throwing the offspring back to the average racial type. It is
thus looked on by these advocates as a stabilizer which reduces the
amplitude of variations instead of increasing them. As a matter of fact
the two ideas are not mutually exclusive; sexual reproduction may
accomplish both of these ends. A limited number of observations and
experiments have been made to test out the correlation between sexual
reproduction and variation, but they have so far been too few or too
inconclusive to enable us to come to a satisfactory conclusion.
While we are uncertain about the method of origin of new characters the
fact remains that they do arise in abundance as abrupt mutations or
otherwise and become a part of the permanent heritage of a stock. It is
clear that sexual reproduction may be one important means by which a given
new character which has arisen in one or a few individuals may become
incorporated in the species at large. Through Mendelian combinations and
segregations it would by cross-breeding be spread and gradually introduced
into more and more strains of the general population.
=Why So Many Features of an Organism Are Characterized by
Utility.--=Germinal variations are seemingly at first more or less hit or
miss affairs as far as utility to the organism is concerned. Useless
variations, so long as they are not actually harmful, may persist and
apparently be indefinitely inherited. However, a special premium is put on
variations which happen to be useful for they help the organism to succeed
in its struggle for life and since success in the world of life means not
only mere individual survival but also the production of progeny, through
this very means insured transmission to subsequent generations. It is
probable that the very many useful features of any organism, that is, its
_adaptations_, have thus been established. It is possible also that many
variations which at their inception are indifferent may wax in strength in
successive generations until they reach a point where they must become
either useful or harmful. In the former case they would mean increased
insurance of survival for their possessors, in the latter, elimination.
With such an automatic process as this operative in nature it is not
astonishing that the main features of any organism are characterized by
their utility to it.
=Germinal Variation a Simpler and More Inclusive Explanation.--=The gist
of the whole matter regarding the source of new characters in offspring
seems to be that the explanation based on the idea of germinal variation
is in last analysis the simpler and more inclusive, and there is no
alleged case of inheritance of parental modification, which can not be
equally well explained as the result of a germinal variation. There are
numerous cases which can not be explained as transmissions of somatic
acquirements even if this transmission could be established in certain
cases. So, many biologists argue, why have two explanations when one is
sufficient, especially when the other has never been conclusively
established as true in any case and is obviously untrue in certain test
cases? The attitude of most investigators is that of the open mind. While
feeling that the weight of probability is very decidedly against the
theory of the inheritance of somatic modifications, they still stand ready
and willing to accept any evidence in its favor which when weighed in the
balance is not found wanting.
ANALYSIS OF CASES
While space will not permit extended discussion, in order further to fix
the nature of the problem in mind as well as to exemplify the conditions
that must be satisfied to form convincing evidence of inherited somatic
acquirements, it will be well perhaps to analyze a few typical cases as
they are frequently cited.
=Are the Effects of Training Inherited?--=Breeders and trainers very
commonly believe that the offspring of trained animals inherit in some
measure the effects of the training. Thus the increased speed of the
American trotting horse is often pointed to as strong evidence of such
transmission. According to W. H. Brewer, the earliest authentic record of
a mile in three minutes was made in 1818. The improvement, approximately
by decades, from that time was as follows:
During 1st decade after 1818, improved to 2:34
2nd " " " " " 2:31-1/2
3rd " " " " " 2:29-1/2
4th " " " " " 2:24-1/2
5th " " " " " 2:17-1/2
6th " " " " " 2:13-1/2
7th " " " " " 2:08-1/2
By 1892, the date of Professor Brewer's publications (See _Agricultural
Science_, Vol. 4, 1892) the record had reached 2:08-1/2. Since then it has
been lowered still further.
On the face of it this looks like a good case of inheritance of training,
and Brewer himself believed it such. If so this would mean that colts of a
highly trained trotter would be faster than they would have been if their
parent had remained untrained. It is impossible to get positive proof in
the case of any trained horse since there is no way of establishing just
how speedy the progeny would have been had the parent remained untrained.
If it could be shown that colts sired by a trotter late in life were on
the whole faster than those sired by the same father when younger and as
yet not highly exercised in trotting, then the facts might give some
evidence of value, but unfortunately no such records are available.
On the other hand, even ignoring the fact that improvement in track and
sulky are probably the biggest items in the shortening of records in
recent times, _selection_ instead of inheritance of the effects of
training will equally well account for any innate progress in trotting.
And since, as pointed out by Professor Ritter, there are even more
striking cases of similar improvements in other fields, such as college
athletics, where the factor of use-inheritance is entirely precluded, it
is wholly unnecessary to postulate it in the case of the trotter.
For example an inspection of the records of college athletics for the last
thirty-five years in running, hurdling, pole-vaulting, jumping, putting
the shot, etc., shows on the whole a steady advance year by year.
Moreover, the greatest improvement has occurred in those events in which
skill and practise count for most together with selection of the
inherently ablest candidate for the events. But in the case of athletics
the improvements shown in thirty-five years have all come within a single
generation and hence the inheritance of the effects of training is ruled
out as a factor. Selection and improved training are the only factors
operative.
In the case of the trotter inheritance undoubtedly has also been a factor,
but inheritance based on selection of what the race-track has shown to be
the speediest individual, not inheritance of the effects of training. In
other words, horses which have shown the capacity for being trained to the
highest degree of speed have naturally been selected as sires and dams
and so through selection generation after generation a speedier strain has
gradually been established.
=Instincts.--=When we turn to the realm of mental traits, particularly of
instincts, we meet with a whole host of activities which are frequently
pointed to by transmissionists as examples of inherited acquirements. Thus
according to them, habits at first acquired through special effort
ultimately become instinctive, or according to some, instinct is "lapsed
intelligence." Instances often cited are the pointing of the bird-dog, the
extraordinary crop-inflation of the pouter-pigeon, or the tumbling of the
tumbler pigeon. We can not stop to discuss these cases beyond pointing out
as many others have done that practically all dogs have more or less of an
impulse to halt suddenly, crouch slightly and lift up one fore-foot when
they scent danger or prey, that all pigeons pout more or less, and that
practically all show more or less instincts of tumbling when pursued by a
hawk. Thus in all of these cases the fundamental germinal tendency is
already at hand for the fancier to base his choice on and thus through
selection build up the type desired. Just as in the fan-tailed pigeon, by
repeatedly selecting for breeding purposes individuals which showed an
unusual number of tail-feathers he has built up a type with an upright,
fan-like tail having many more feathers than the twelve found in the tail
of the ordinary pigeon, so by similar procedure in the case of other forms
he has markedly enhanced certain features. The idea of instincts being
"lapsed intelligence" is so clearly and concisely criticized in an
article by the late Professor Whitman[4] that I can not do better than
quote an excerpt. His views to the contrary are as follows:
"The view here taken places the primary roots of instinct in the
constitutional activities of protoplasm and regards instinct in every
stage of its evolution as action depending essentially upon
organization. It places instinct before intelligence in order of
development, and is thus in accord with the broad facts of the present
distribution and relations of instinct and intelligence, instinct
becoming more general as we descend the scale, while intelligence
emerges to view more and more as we ascend to the higher orders of
animal life. It relieves us of the great inconsistencies involved in
the theory of instinct as "lapsed intelligence." Instincts are
universal among animals, and that can not be said of intelligence. It
ill accords with any theory of evolution, or with known facts, to make
instinct depend upon intelligence for its origin; for if that were so,
we should expect to find the lowest animals free from instinct and
possessed of pure intelligence. In the higher forms we should expect
to see intelligence lapsing more and more into pure instinct. As a
matter of fact, we see nothing of the kind. The lowest forms act by
instinct so exclusively that we fail to get decided evidence of
intelligence. In higher forms not a single case of intelligence
lapsing into instinct is known. In forms that give indubitable
evidence of intelligence we do not see conscious reflection
crystallizing into instinct, but we do find instinct coming more and
more under the sway of intelligence. In the human race instinctive
actions characterize the life of the savage, while they fall more and
more into the background in the more intellectual races."
For further discussion of this field the reader is referred to an
excellent chapter on "Are Acquired Habits Inherited?" in C. Lloyd Morgan's
book, _Habit and Instinct_.
=Disease.--=Perhaps in the realm of disease more than in any other has an
interest in the inheritance of somatic acquirements been manifested. The
problem arising here is not essentially different from other questions of
inheritance but since it is a matter of such practical importance to man,
we may well give it special attention. We have to deal simply with the old
questions of what is constitutionally in the germ, what is acquired by the
body, and lastly, whether the somatically acquired is inherited. While we
all know in a general way what is meant by disease, especially if some
specific disorder such as scarlet fever, malaria or tuberculosis is
mentioned, an attempt to give an accurate definition is much like trying
to define a weed, inasmuch as what is functionally all right at one time
or place may be all wrong at another, or what is normal in one animal may
be abnormal in another. In general we may say that disease is derangement
or failure of physiological function.
=Reappearance of a Disorder in Successive Generations Not Necessarily
Inheritance.--=In attempting to study the inheritance of diseases we must
recognize clearly at the outset that reappearance of a disease in
successive generations by no means necessarily signifies inheritance.
Before it can be pronounced such we must make sure that it is not a case
of reimpressing similar modifications on the individuals of successive
generations. For example, in England there is a well-recognized condition
known as collier's lung which results from constant working in coal mines.
And while both father and son may exhibit it, because of their similar
occupations, there is nothing hereditary about the malady. Likewise there
is what is known as emery grinder's lung, and practically every large
manufacturing city with soot-laden atmosphere leaves its impress on the
lungs of the inhabitants. This will occur, of course, generation after
generation, as long as such pollutions of the atmosphere continue to
exist. It is clear that any unhealthy occupation is likely to cause the
reappearance of an associated typical disease generation after generation
as long as the children follow the calling of their parents. The common
misconception that deformities or postures associated with a trade, such
as a shoemaker's or tailor's, is genetically stamped on offspring by the
end of the third or fourth generation results from failure to discriminate
between real inheritance and mere reappearances under similar conditions
of environment.
=Prenatal Infection Not Inheritance.--=Again, we must recognize that
prenatal infection is not inheritance. We have already seen that the young
mammal undergoes a certain period of intra-maternal development, but
influences operating on it during this period of gestation must be
reckoned with as environmental, not germinal. For example, it is said that
an unborn child may take smallpox from its mother but this and all similar
occurrences are cases of contagion. We find the great pathologist,
Virchow, who with many others of his time was a believer in the
inheritance of acquired characters, saying nevertheless regarding such
instances that, "What operates on the germ after the fusion of the
sex-nuclei, modifying the embryo, or even inducing an actual deviation in
the development, can not be spoken of as inherited. It belongs to the
category of early acquired deviations which are therefore frequently
congenital."
=Inheritance of a Predisposition Not Inheritance of a Disease.--=We must
discriminate sharply also between the inheritance of a predisposition and
the inheritance of a disease itself.
We often hear the statement made that tuberculosis is inherited and have
cited in evidence certain consumptive families or strains. But
tuberculosis is a bacterial disease and children of tuberculous parents
are never born with the disease except in the rarest of instances.
=Tuberculosis.--=What is really inherited is a constitutional
susceptibility to this particular germ. While almost any individual may
contract tuberculosis when in a state of depressed vitality, or under
stress of adverse surroundings, there is no doubt that certain families
are more easily infected than others and much less resistant to the
ravages of the disease when once it gains a foothold. However, a
predisposition is a vastly different thing from the inheritance of the
actual disease. For just as we are born with a nose well adapted to
eye-glasses but not with eye-glasses on our nose, so many of us are born
tuberculizable though not tuberculous, and every sanitary advance we make
toward lessening the chances of infection is just so much more insurance
for the susceptible.
The whole problem of tuberculosis is an extremely complex one. We do not
know just the measure of the inheritance of the predisposition. Some
writers in the past have maintained that tuberculosis is mainly a question
of infection and not of inherent susceptibility, but steadily increasing
evidence all points the other way.
Where the predisposition exists the chances of infection are still, even
under the conditions of present-day sanitation, very great. The close
association between a consumptive and other members of the family through
a prolonged period of time, of course, renders the latter likely to
infection unless unusual care is exercised. Very often where a parent is
consumptive a child contracts the malady shortly after birth and is
particularly likely to do so if the mother, who nurses it and cares for it
most intimately, is the tubercular member of the family. Where the mother
is tubercular, indeed, the probabilities are that the child has already
before birth had its vitality lowered through the toxins circulating in
her blood or through defective nutrition, and in consequence does not
resist well any diseases.
Undoubtedly a large proportion of our infant mortality is of tubercular
origin. It is now a well-established fact that much tuberculosis in
children is attributable to drinking milk from tuberculous cows, yet we
find individuals so uninformed and dairymen so mercenary that they fight
all attempts of the commonwealth to test out cattle for tuberculosis so as
to condemn the infected individuals and thus save our babies. Recent
investigations made in some of our large pork-packing establishments also
indicate that hogs, especially such as have been around tubercular
cattle, are often shot through and through with tuberculosis and that such
flesh when used as food, if not thoroughly cooked, may become a serious
menace to our health.
With the wide prevalence of bovine and human tuberculosis it is little
wonder that nearly every human being becomes more or less infected at some
period of life. Autopsies on large numbers of individuals in some of our
great hospitals have shown that as many as ninety-nine per cent. of the
subjects show tubercular lesions of some kind. While it is true that the
class of people who would come to autopsy in such public hospitals would
perhaps be more likely to be tubercular than the average of the community,
still it can not be denied that a very large degree of infection exists.
Pearson, from statistics gathered in Europe, has shown that about eighty
to ninety per cent. of the population have tubercular lesions before the
age of eighteen. Hamburger found that in Vienna ninety-five per cent. of
the children of the poor, between twelve and thirteen years of age, were
infected with tubercular bacilli and he estimates that all would be before
maturity. According to Doctor Mott, pathologist to the London County
Asylums, the insane between the ages of fifteen and thirty-five are about
fifteen times as likely to acquire tuberculosis as the sane are.
Yet the mortality from tuberculosis, great though it be, is obviously not
in proportion to the enormous degree of infection. The crux of the
situation is mainly the matter of resistance. From the standpoint of
heredity, therefore, the question largely resolves itself into one of the
inheritance or non-inheritance of constitutional resistance. Some are
predisposed to be non-resistant and hence succumb.
The work of Karl Pearson[5] and other recent researches forcibly indicate
that hereditary constitutional predisposition is one of the chief factors
concerned in subjects who develop well defined attacks of the disease. Yet
we must not forget that there are degrees of susceptibility and that
therefore a constitutional predisposition which might be of little
significance under good average conditions of nutrition and sanitation
might be insufficient under unfavorable conditions.
Before we can make any relatively accurate estimate of the exact degree to
which the malady is based on inheritance we must have more data. Many
difficulties beset the path of the investigator. In the first place, when
one gets back a generation or two he finds that diagnosis was crude and
uncertain; a given malady may or may not have been tuberculosis. The main
error however was probably on the side of not recognizing it in mild or
obscure cases. Then again the questions of virulence of the infection, of
size and frequency of the dose, etc., are also complicating factors.
Moreover, in very many cases the infection is a mixed one and hence we are
dealing with other factors than straight tuberculosis.
=Two Individuals of Tubercular Stocks Should Not Marry.--=However,
sufficient is now known of the inheritance of susceptibility to the
disease that we can have little conscience toward the welfare of the race
if we in any way countenance the marriage of two individuals who come each
of tubercular strains, and marriage of even a normal person into a badly
tainted strain, where the one married is tubercular, is extremely
hazardous looked at from the standpoint of the children likely to be born
of such a union. The Supreme Court of New York recently held that the
fraudulent concealment of tuberculosis by a person entering into a
marriage relation is ground for the annulment of the marriage.
=Special Susceptibility Less of a Factor in Many Diseases.--=With some
diseases such as leprosy, typhoid fever, smallpox and cholera there seems
to be less a question of special susceptibility since nearly all persons
are vulnerable. Yet in cases of typhoid, at least, there are some
indications that certain families are more likely to take the disease than
others under similar exposure. We know of no inherited effects of such
diseases, however. For instance, children of lepers do not inherit leprosy
and if kept out of leper districts remain normal.
=Deaf-Mutism.--=In certain abnormal states there is danger of confusing
similar conditions which may have two entirely different sources of
origin. Deafness, for example, may be strictly inborn as the outcome of a
germinal variation or it may result from extraneous influences such as
accidents, infective diseases, neglected tonsils and the like. The former
is inheritable, the latter not. Bell in 1906 in a special census report to
the United States government showed that deaf-mutism is markedly
hereditary, particularly where deaf-mutes intermarry as they are prone to
do. Fay's extensive studies on _Marriage of the Deaf in America_ also
demonstrate the hereditary nature of the congenital forms of deafness. Cut
off as such individuals are from communication with normal people, the
association of the two sexes in special schools and institutions is of
course highly conducive to such marriages. The defect seems to behave in
the manner of a Mendelian recessive. Two deaf-mutes should not have
children and yet such marriages are occurring every day. Even if two
persons marry from families which tend to become hard of hearing the
evidence indicates that their children are likely also to develop this
partial deafness as they grow older, although it seems safe for a person
of such tendency to marry into a family without it.
=Gout.--=In such disorders as gout there is little question but that a
tendency to it runs in families. On the other hand it may also be acquired
without special susceptibility. There is no evidence, however, that
because a father has gout the effect of the gout is reflected on his
germ-cells and the son has gout as a result. Indeed, often a son who
becomes gouty was born long before the father became gouty. Son and father
both have gout then, because each has innate germinal tendencies which
when subjected to certain evocative stimuli become expressed as gout.
=Nervous and Mental Diseases.--=Inasmuch as the question of nervous and
mental diseases has become one of such overshadowing importance at the
present day, a discussion of the subject at some length will be presented
in a separate chapter. I shall merely point out here that the general
verdict of experts in nervous and mental disorders is to the effect that
externally induced mental disorders are of rare occurrence except as the
result of general poisoning or enfeeblement of the system in some way, or
by traumatic conditions such as a blow on the head, and that there is no
evidence of the transmission of the effects of such conditions. In most
cases of insanity, supposedly caused by fright or worry, a close study of
the family stock will reveal nervous instability of some kind. The
supposed cause has been merely the precipitating stimulus which has
brought to expression a dormant weakness of germinal origin. The stress
and strain of modern life is particularly likely to test out and reveal
such neurally unstable individuals.
=Other Disorders Which Have Hereditary Aspects.--=Space will not permit
discussion of various other specific disorders which are known to have
important hereditary aspects, although none shows any convincing evidence
of having become hereditary in nature through first affecting the soma.
Some of these, such as epilepsy and other nervous affections,
tuberculosis, color-blindness, cataract and various malformations, have
already been mentioned. Others that may be listed are cancer,
arterio-sclerosis, obesity and certain forms of rheumatism, and of heart
and kidney diseases. In practically all of these cases in which heredity
enters as a factor the condition is one of inheriting a special
susceptibility and not the disease itself. Which means simply that the
disorder in question is much more easily called forth in such persons by
appropriate bacterial or other stimulus, than in the case of the normal
individual.
=Induced Immunity Not Inherited.--=Lastly, it is well known that various
animals, including man, after recovery from an attack of any one of
certain diseases, become more or less immune from further attacks of the
same disease. Moreover in some instances as in inoculation against typhoid
or diphtheria, immunity may be artificially induced by means of
anti-toxins. The question arises as to whether such immunity is
transmitted to offspring. Experiments have been made (see _Bulletin No.
30, U. S. Hygienic Laboratory_) to test this and it has been found that
the condition is not inherited. Young guinea-pigs, for instance, born of
mothers immunized during pregnancy are immune at birth but they lose their
immunity in the course of a few weeks. The effect is clearly one of direct
transference from the blood of the mother. The same temporary immunity can
be produced in the young, in fact, by merely having them nurse from an
immunized mother.
=Non-Inheritance of Parental Modifications Has Social, Ethical and
Educational Significance.--=Like many other biological conclusions these
relative to the non-inheritance of parental modifications are of extreme
importance to humanity. It is clear that they have not only physical but
social, ethical and educational significance. For if the education which
we give our children of to-day, or the desirable moral conduct which we
inculcate does not affect the offspring of succeeding generations through
inheritance, then the actual progress of the race is much slower than is
commonly supposed, and the advance of modern over ancient times lies more
in an improvement in extraneous conditions through invention and the
accumulation and rendering accessible of knowledge, than in an actual
innate individual superiority. And when we face the issue squarely we have
to admit that there is no more indication of the inheritance of parentally
acquired characters as regards customs, knowledge, habits and moral
traditions than there is of physical features. In fact, if such
acquirements were inherited then we should soon have a race which would
naturally, spontaneously as it were, do what its ancestors did with
effort. Yet we do not find the children in our schools reading, doing sums
and developing proper social relations without ceaseless prompting and
urging on the part of the teacher. Indeed I can testify that this
necessity carries over even into a university. In short, the habits and
standards of each generation have to be instilled into the succeeding
generation.
=No Cause for Discouragement.--=At first glance when we realize that
notwithstanding our individual advancement, that in spite of all our
painstaking efforts toward self-improvement, we can not add one jot or
tittle to the native ability of our children, that, aside from possible
advantageous germinal variations, they will have to start in at
approximately the same level as we did, and like us will have to struggle,
or be coaxed, pulled or spurred up to the higher reaches of attainments,
we are apt to feel discouraged and to look on heredity as the hand of fate
which irrevocably bars progress. But there is another side to the picture.
This very fact of heredity which can not be altered at will is the
conservative factor which maintains the excellence of our standard strains
of plants and animals, and sustains man himself at his present level of
accomplishment. While we are denied advancement through the efforts of the
flesh, we are also largely protected from our misfortunes and follies, as
witness the non-inheritance of mutilations, of various maladies of
extrinsic origin, or of personally acquired bad habits.
=Improved Environment Will Help Conserve the Superior Strains When They Do
Appear.--=If we can not hand on to our descendants a personally enhanced
blood heritage, we at least can do our share toward building up a social
heritage of established truth, of efficient institutions and of
stimulating ideals, through which their dormant capacities may be led to
expand more surely and more effectively to their uttermost limits. Each
advance in such social heritage will tend more and more to create an
atmosphere which will make it sure that the occasional real progressive
and permanent variations which occur from time to time will find adequate
expression and preservation in future lines of descendants. It will reduce
the numbers of our "mute, inglorious Miltons" by more certainly disclosing
the individual of exceptional talents and insuring for him an opportunity
of revealing them to the best advantage. Above all, since surrounding
influences are especially powerful on young and developing organisms, we
should realize that great care must be exercised in behalf of the young
child to secure an environment which is saturated with wholesome
influences. For it is a rule of development that if the environment is
faulty the organism is impaired.