INHERITANCE
Phenomena which a complete theory of inheritance must explain—In the
present state of our knowledge it is not possible to formulate a
complete theory of inheritance—Different kinds of inheritance—Mendel’s
experiments and theory—The value and importance of Mendelism has been
exaggerated—Dominance sometimes imperfect—Behaviour of the nucleus of
the sexual cell—Chromosomes—Experiments of Delage and Loeb—Those of
Cuénot on mice and Castle on guinea pigs—Suggested modification of the
generally-accepted Mendelian formulae—Unit characters—Biological
isomerism—Biological molecules—Interpretation of the phenomena of
variation and heredity on the conception of biological
molecules—Correlation—Summary of the conception of biological
molecules.
We have seen that variations may be, firstly, either acquired or
congenital, and, secondly, fluctuating or discontinuous. We have further
seen that acquired variations—at all events in the higher animals—do not
appear to be inherited, and therefore have not played a very important
part in the evolution of the animal world. Discontinuous congenital
variations or mutations are the usual starting points of new species. It
is not unlikely that fluctuating congenital variations, although they do
not appear to give rise directly to new species, may play a considerable
part in the making of new species, inasmuch as they may, so to speak,
pave the way for mutations.
We are now in a position to consider the exceedingly difficult question
of inheritance. We know that offspring tend to resemble their parents,
but that they are always a little different both from either parent and
from one another. How are we to account for these phenomena? What are the
laws of inheritance, whereby a child tends to inherit the peculiarities
of its parents, and what are the causes of variation which make children
differ _inter se_ and from their parents?
Scores of theories of inheritance have been advanced. It is scarcely
exaggerating to assert that almost every biologist who has paid much
attention to the subject has a theory of inheritance which differs more
or less greatly from the theory held by any other biologist.
As regards the phenomena of heredity we may say _Tot homines tot
sententiæ_.
Phenomena of Inheritance
For this state of affairs there is a good and sufficient reason. We are
not yet in possession of a sufficient number of facts to be in a position
to formulate a satisfactory theory of inheritance. A complete theory of
heredity must explain, among other things, the following phenomena:—
1. Why creatures show a general resemblance to their parents.
2. Why they differ from their parents.
3. Why the members of a family display individual differences.
4. Why the members of a family tend to resemble one another more closely
than they resemble individuals belonging to other families.
5. Why “sports” sometimes occur.
6. Why some species are more variable than others.
7. Why certain variations tend to occur very frequently.
8. Why variations in some directions seem never to occur.
9. Why a female may produce offspring when paired with one male of her
species and not when paired with another male of the species.
10. Why organisms that arise by parthenogenesis appear to be as variable
as those which are sexually produced.
11. Why certain animals possess the power of regenerating lost parts,
while others have not this power.
12. Why most plants and some of the lower animals can be produced
asexually from cuttings.
13. Why mutilations are not inherited.
14. Why acquired characters are rarely, if ever, inherited.
15. Why the ovum puts forth the polar bodies.
16. Why the mother-cell of the spermatozoa produces four spermatozoa.
17. Why differences in the nature of the food administered to the larvæ
of ants determines whether these shall develop into sexual or neuter
forms.
18. Why the application of heat, cold, etc., to certain larvæ affects the
nature of the imago, or perfect insect, to which they will give rise.
19. Why the females in some species lay eggs which can produce young
without being fertilised.
20. Why some species exhibit the phenomena of sexual dimorphism, while
others do not.
21. In addition to all the above, a satisfactory theory of inheritance
must account for all the varied phenomena which are associated with the
name of Mendel. It must explain the various facts with which we have
dealt in the chapter on hybridism, why some species produce sterile
hybrids when intercrossed, while others give rise to fertile hybrids, and
yet others form no offspring when crossed; why the hinny differs in
appearance from the mule, etc.
22. It must explain all the facts which constitute what is known as
atavism.
23. It must account for the phenomenon of prepotency.
24. It must explain the why and the wherefore of correlation.
25. It must tell us the meaning of the results of the experiments of
Driesch, Roux, and others.
26. It must render intelligible the effects of castration on animals.
Existing Theories Unsatisfactory
Now, no existing theory of heredity can give anything approaching a
satisfactory explanation of all these phenomena.
It is for this reason that we refrain from critically examining, or even
naming, any of them.
We are convinced that in the present state of our knowledge it is not
possible to formulate anything more than a provisional hypothesis.
It must not be thought that we consider the various theories that have
been enunciated to be of no value. Erroneous hypotheses are often of the
greatest utility to science, for they set men thinking and suggest
experiments by means of which important additions to knowledge are made.
We now propose to set forth certain facts of inheritance, and from these
to make a few deductions—deductions which seem to be forced upon us.
We would ask our readers to distinguish carefully between the facts we
set forth, and the conclusions we draw therefrom. The former, being
facts, must be accepted.
The interpretations we suggest should be rigidly examined, we would say
regarded with suspicion, and all possible objections raised. It is only
by so doing that any advance in knowledge can be made.
By inheritance we mean that which an organism receives from its parents
and other ancestors—all the characteristics, whether apparent or dormant,
it inherits or receives from its parents. Professor Thomson’s
definition—“all the qualities or characters which have their initial
seat, their physical basis, in the fertilised egg cell”—seems to cover
all cases except those where eggs are parthenogenetically developed.
The first fact of heredity which we must notice is that inheritance may
take several forms. This is apparent from what was set forth in the
chapter dealing with hybrids.
Types of Crosses
In considering the phenomena of inheritance it is convenient to deal with
crosses in which the parents do not closely resemble one another, because
by so doing we are able readily to follow the various characters
displayed by each parent. It may, perhaps, be urged that such crosses
occur but rarely in nature. This is true. But we should bear in mind that
any theory of inheritance must explain the various facts of
cross-breeding, so that, from the point of view of a theory of
inheritance, crosses are as important as what we may term normal
offspring. As inheritance is so much easier to observe in the former, it
is but natural that we should begin with them. Our deductions must, if
they be valid ones, fit all cases of ordinary inheritance, _i.e._ all
cases where the offspring results from the union of parents which closely
resemble one another. Now, when two unlike forms inter-breed, their
offspring will fall into one of six classes.
I. They may exactly resemble one parent, or rather the type of one
parent, for, of course, they will never be exactly like either parent;
they must of necessity display fluctuating variations. The cases in which
the offspring exactly resemble one parent type in all respects are
comparatively few. They occur only when the parents differ from one
another in one, two, or at the most three characters. Thus when an
ordinary grey mouse is crossed with a white mouse the offspring are all
grey, that is to say, they resemble the grey parent type. Although they
are mongrels or hybrids, they have all the appearance of pure grey mice.
This is what is known as unilateral inheritance.
II. The offspring may resemble one parent in some characters and the
other in other characters. They may have, for example, the colour of one
parent, the shape of the other, and so on. Thus if a pure, albino,
long-haired, and rough-coated male guinea-pig be crossed with a coloured,
short-haired and smooth-coated female, all the offspring are coloured,
short-haired, and rough-coated. That is to say, they take after the
father in being rough-coated, but after the mother in being pigmented and
short-haired. This form of inheritance is usually seen only in crosses
between two types which differ in but few of their characters.
III. The offspring may display a blend of the characters of the two
parents. They may be intermediate in type. They are not of necessity
midway between the two parents; one of the parents may be prepotent. The
crosses between the horse and the ass show this well. Both the mule,
where the ass is the sire, and the hinny, where the horse is the sire,
are more like the ass than like the horse; but the hinny is less ass-like
than the mule. The offspring between a European and a native of India
furnishes a good case of blended inheritance; Eurasians are neither so
dark as the Asiatic nor so fair as the European.
IV. The offspring may show a peculiarity of one parent in some parts of
the body and the peculiarity of the other parent in other parts of the
body. This is known as particulate inheritance. The piebald foal, which
is the result of a cross between a black sire and a white mare, is a good
example of such inheritance. This does not appear to be a common form of
inheritance.
V. The usual kind of inheritance is perhaps a combination between the
forms II. and III. In such cases the offspring display some paternal
characters and some maternal ones, and some characters in which the
maternal and paternal peculiarities are blended. An example of
inheritance of this description is furnished by a cross between the
golden and the amherst pheasants.
VI. The offspring may be quite unlike either parent. For example, Cuénot
found that sometimes a grey mouse when crossed with an albino produces
black offspring.
Mendel’s Experiments
The first two kinds of inheritance were carefully investigated by Gregor
Johann Mendel, Abbot of Brunn. The results of his experiments were
published in the Proceedings of the Natural History Society of Brunn, in
1854, but attracted very little notice at the time.
Mendel experimented with peas, of which many varieties exist. He took a
number of varieties, or sub-species, which differed from one another in
well-defined characters, such as the colour of the seed coat, the length
of the stem, etc. He made crosses between the various varieties, being
careful to investigate one character only at a time. He found that the
offspring of such crosses resembled, in that particular character, one
only of the parents, the other parent apparently exerting no influence on
it. Mendel called the character that appeared in the off-spring dominant,
and the character which was suppressed, recessive. Thus when tall and
short varieties were crossed the offspring were all tall. Hence Mendel
said that tallness is a dominant character, and shortness a recessive
character. Mendel then bred these crosses among themselves, and found
that some of the offspring resembled one grandparent as regards the
character in question while some resembled the other, and he found that
those that showed the dominant character were three times as numerous as
those that displayed the recessive character. He further found that all
those of the second generation of crosses which displayed the recessive
character bred true; that is to say, when they were bred together all
their descendants exhibited this characteristic. The dominant forms,
however, did not all breed true; some of them produced descendants that
showed only this dominant character, others, when crossed, gave rise to
some forms having the dominant character and some having the recessive
character.
It is thus evident that organisms of totally different ancestry may
resemble one another in external appearance. In other words, part of the
material from which an organism is developed may lie dormant.
Mendelism
From the above results Mendel inferred, in the case of what he called
alternating characters, that only one or other of the pair can appear in
the offspring, that they will not blend. If both parents display one of
the opposing characters, the offspring will of course show it. But if one
parent display one character and the other the opposing character, the
hybrid offspring will display one only, and that which is dominant. The
other character is suppressed for the time being. When, however, these
hybrids are bred _inter se_, their gametes or sexual cells split up into
their component parts, and then the recessives are free to unite with
other recessives and thus produce offspring which show the recessive
character.
His results can be set forth in symbols.
Let T stand for the tall form and D for the dwarf form. Since the
offspring are composed of both the paternal and maternal gamete, we may
represent them as TD. But dwarfness is, as we have seen, recessive, so
that the offspring all look as though they were pure T’s. When, however,
we come to breed these TD’s _inter se_, the gamete or sex-cell of each
individual crossed breaks up into its component parts T and D, which
unite with other free T or D units to form TD’s or TT’s or DD’s. What are
the possible combinations? A D of one parent may meet and unite with a D
of the other parent, so that the resulting cells will be pure D, _i.e._
DD, and will give rise to pure dwarf offspring. Or the D gamete from one
parent may unite with a T gamete from the other parent, and the result
will be a TD cross, but this, as we have seen, will grow up to look like
a pure T, _i.e._ will become a tall organism. Similarly, a T gamete from
one parent may unite with a T gamete of the other, and produce a pure
tall form, or it may unite with a D and produce a hybrid TD, which gives
rise to a tall form. Thus the possible combinations of offspring are DD,
DT, TD, TT, but all these three last contain the dominant T gamete, and
so develop into tall offspring; therefore, _ex hypothesi_, we shall have
three tall forms produced to one dwarf form, but of these three tall
forms two are not pure, and do not breed true. Mendel’s experimental
results accorded with what we should expect to obtain if the above
explanation were correct. Hence the inference that there is such a
splitting of the gametes in the sexual act seems a legitimate one.
Mendel’s experiments are of great importance, for they give us some
insight into the nature of the sexual act. But, as is usual in such
cases, Mendel’s disciples have greatly exaggerated the value and
importance of his work. It is necessary to bear in mind that Mendel’s
results apply only to a limited number of cases—to what we may call
balanced characters. In the case of characters which do not balance one
another, which are, so to speak, not diametrically opposed to one
another, Mendel’s law does not hold. A second important point is, that
the dominance is in many cases not nearly so complete as it should be if
the Mendelian formula correctly represented what actually occurs in
nature. Further, the segregation of the gametes does not appear to be so
complete as the above hypothesis requires it to be. The phenomena of
inheritance seem to be far more complex than the thorough-going Mendelian
would have us believe.
Let it be noted that it is not to the facts of Mendelism, but to some
portions of what we may call the Mendelian theory, that we take
exception.
Maturation of the Germ-cells
Before passing on to consider some of the later developments of
Mendelism, it is necessary for us to set forth briefly certain of the
more important facts regarding the sexual act which the microscope has
brought to light. We propose to state these only in the merest outline.
Those who are desirous of pursuing the subject farther are referred to
Professor Thomson’s _Heredity_.
The germ cells, like all other cells, consist of a nucleus lying in a
mass of cytoplasm. The nucleus is composed of a number of rod-like
bodies, which are called chromosomes, because they are readily stainable.
These chromosomes appear, under ordinary circumstances, to be joined
together end to end, and then look like a rope in a tangle.
When a cell is about to divide into two, these chromosomes become
disjoined and can then be counted, and it is found that each cell of each
species of animal or plant has a fixed number of these chromosomes. Thus
the mouse and the lily have twenty-four chromosomes in each cell, while
the ox is said to have sixteen of them per cell.
When a cell divides into two, each of these chromosomes splits by a
_longitudinal_ fissure into two halves, which appear to be exactly alike.
One-half of every chromosome passes into each of the daughter cells, so
that each of these is furnished with exactly half of each one of the
rod-like chromosomes. In the cell division, which takes place immediately
before the male gamete or generative cell meets the female gamete, the
chromosomes do not divide into equal halves, as is usually the case. In
this division half of them pass into one daughter cell and half into the
other daughter cell, so that, prior to fertilisation both the male and
the female gametes contain only half the normal number of chromosomes. In
the sexual act the male and the female chromosomes join forces and then
the normal number is again made up, each parent contributing exactly one
half.
Experiments of Delage and Loeb
Biologists, with a few exceptions, seem to be agreed that these
chromosomes are the carriers of all that which one generation inherits
from another. Thus the cardinal facts of the sexual act are, firstly,
prior to fertilisation the male and the female gamete each part with half
their chromosomes; and, secondly, the fertilised cell is composed of the
normal number of chromosomes, of which one-half have been furnished by
each parent. Thus the microscope shows that the nucleus of the fertilised
egg is made up of equal contributions from each parent. This is quite in
accordance with the observed phenomena of inheritance.
But Delage has shown that a non-nucleated fragment of the ovum in some of
the lower animals, as, for example, the sea-urchin, can give rise to a
daughter organism with the normal number of chromosomes when fertilised
by a spermatozoon. Conversely, Loeb showed that the nucleus of the
spermatozoon can be dispensed with. Thus it seems that either the egg or
the spermatozoon of the sea-urchin contains all the essential elements
for the production of the perfect larva of a daughter organism. We are,
therefore, driven to the conclusion that the fertilised ovum contains two
sets of fully-equipped units. Only one of these seems to contribute to
the developing organism. If this set happens to be composed of material
derived from one only of the parents, we can see how it is that we get
unilateral inheritance in the case of a cross. Where, however, the units
from the two parents intermingle, although only one set is active in
development, the result will be blended inheritance. Thus, we may regard
the fertilised egg as made up of two sets of characters—a dominant set,
which is active in the production of the resulting organism, and a
recessive set, which appears to take little or no part in the production
of the organism.
This is quite in accordance with Mendelian conceptions.
Let X be an organism having the unit characters A _B_ C D _E_ F _G_, and
let Y be another organism having the unit characters _a_ b _c_ _d_ e _f_
g.
Now suppose that these behave as opposed Mendelian units, and that the
unit characters in italics are dominant ones. Then the resulting
individual will resemble each parent in certain unit characters. It may
be represented by the formula a B c d E f G, but it will contain the
characters A b C D e F g in a recessive form, so that its complete
formula may be written
a B c d E f G}
A b C D e F g
When these hybrids are paired together it will be _possible_ to get such
forms as
A B C D E F G
A B C D E F G
and
a b c d e f g
a b c d e f g
which exactly resemble the
respective grandparents, and these should breed
absolutely true, if the segregation of the
gametes is as pure as the Mendel’s law seems
to require.
Experiments of Cuénot and Castle
There are, however, certain facts, which recent experimenters have
brought to light, that seem to show that the segregation is not so
complete as the law requires. For example, the so-called pure extracted
forms may be found, when bred with other varieties, to have some latent
characters. Thus Cuénot observed that extracted pure albino mice, that is
to say, those derived from hybrid forms, did not all behave alike when
paired with other mice. Those which had been bred from grey × white
hybrids behaved, on being crossed, differently to those that had been
bred from black × white hybrids; and further, those derived from yellow ×
white hybrids yielded yet other results on being intercrossed. Castle
records similar phenomena in the case of guinea-pigs, and accordingly
draws a distinction between recessive and latent characters. Recessive
characters are those which disappear when they come into contact with a
dominant character, but reappear whenever they are separated from the
opposing dominant character. Latency is defined by Castle as “a condition
of activity in which a normally dominant character may exist in a
recessive individual or gamete.”
The ordinary Mendelian pictures a unit character in a cross that obeys
Mendel’s law, as follows:—
D
R,
the dominant character only
showing. It seems to us that each unit character
should be represented as a double entity, thus
D(D), the portion within the bracket being
latent. The cross would appear to be represented
by the formula
D(R)
R(D),
since the union
appears to take the form of the transfer of
the dormant latent characters. Now an extracted
pure recessive will, on this hypothesis,
bear the formula
R(D)
R(D).
When such recessives
are crossed the two dormant portions will
ordinarily change places, and never appear, so
that these extracted recessives will, under
ordinary circumstances, appear to be as pure
as the true pure recessives, which are represented
by the formula
R(R)
R(R).
Now, suppose that, from some cause or other,
it is possible for the latent D to change places
with the visible R, it is obvious that the impure
nature of the extracted and hitherto apparently
pure recessives will become manifest. This
seems to be what happens under certain circumstances
to the extracted albino mice. They
possess latent the character of their dominant
ancestor.
Unit Characters
Mendelian phenomena force upon us the conclusion that organisms display a
number of unit characters, each of which behaves in much the same way as
a radicle does in chemistry, inasmuch as for one or more of these
characters others can be substituted without interfering with the
remaining unit characters. For example, it is possible to replace the
chemical radicle NHࠣ by the radicle Naࠢ; _e.g._ (NHࠣ)ࠢSOࠤ (ammonium
sulphate) may be transformed into NaࠢSOࠤ (sodium sulphate).
The conclusion that each organism is composed of a number of unit
characters, which sometimes behave more or less independently of one
another, is one which most biologists who have studied the phenomena of
inheritance appear to have arrived at. Zoologists are mostly of opinion
that these characters, or rather their precursors, exist as units in the
fertilised egg. Very varied have been the conceptions of the nature of
these biological units. Almost every biologist has given a name to his
particular conception of them. Thus we have the gemmules of Darwin, the
unit characters of Spencer, the biophors of Weismann, the micellæ of
Naegeli, the plastidules of Haeckel, the plasomes of Wiesner, the
idioblasts of Hertwig, the pangens of De Vries, and so on. It is
unnecessary to extend this list. It must suffice that almost every
investigator of the phenomena of inheritance believes in these units, and
calls them by a different name. Moreover, each clothes them with
characteristics according to his taste or the fertility of his
imagination.
Chemical Molecules
These units behave in such a way as to suggest to us an analogy between
them and the chemical molecules. The sexual act would appear to resemble
a chemical synthesis in some respects. One of the most remarkable
phenomena of chemistry is that of isomerism. It not infrequently happens
that two very dissimilar substances are found, upon analysis, to have the
same chemical composition, that is to say, their molecules are found to
be composed of the same kind of atoms and the same number of these. Thus
chemists are compelled to believe that the properties of a molecule are
dependent, not only on the nature of the atoms which compose it, but also
on the arrangement of these within the molecule. To take a concrete
example: Analysis shows that both alcohol and ether are represented by
the chemical formula CࠢHࠦO. In other words, the molecule of each of these
compounds is made up of two atoms of the element Carbon, six of the
element Hydrogen, and one of the element Oxygen. Now, every chemical atom
possesses the property which chemists term valency, in other words, the
number of other atoms with which it can directly unite is strictly
limited. All atoms of the same element have the same valency. Monovalent
atoms are those which can, under no circumstances, unite with more than
one other atom. The Hydrogen atom is an example of such an atom. Divalent
atoms, as, for example, that of Oxygen, can unite with one other atom of
similar valency or with two monovalent atoms. Similarly, a trivalent
atom, such as that of Nitrogen, can unite with three monovalent atoms. A
tetravalent atom, such as that of Carbon, can combine with four
monovalent atoms. There are also pentavalent and hexavalent atoms. Now,
by indicating the valency of any given atom by a stroke for each
monovalent atom with which it is able to combine, chemists have been able
to represent the molecule of every compound, or, at any rate, of every
inorganic compound, by what is known as a graphic or structural formula.
Thus, ethylic alcohol is represented by the formula:—
H H
| |
H––C––C––O––H = CࠢHࠦO,
| |
H H
and methylic ether by the structural formula:—
H H
| |
H––C––O––C––H = CࠢHࠦO.
| |
H H
The formulæ indicate a very different arrangement of the nine atoms which
compose the molecule in each case. And to this different arrangement the
differing properties of the two compounds are supposed to be due. A rough
illustration of the phenomenon of isomerism is furnished by written
language. Thus, three different words can be made from the letters t, a,
and r, _e.g._ tar, art, and rat. They also form tra, which does not
happen to be an English word, although it might have been one.
Experiments of Gräfin von Linden
Among organisms we sometimes observe a phenomenon which looks very like
isomerism. The classical example of this is furnished by the butterflies
_Vanessa prorsa_ and _Vanessa levana_.
At one time these were supposed to belong to different species, since
they differ so greatly in appearance. _Vanessa levana_ is red, with black
and blue spots. _Vanessa prorsa_ is deep black, with a broad
yellowish-white band across both wings. It is now known that the _levana_
is the spring form and the _prorsa_ the summer and autumn form of the
same species. The pupæ of _levana_ produce the _prorsa_ form, but
Weismann found that after being placed in a refrigerator they emerged,
not as _prorsa_, but partly as _levana_ and partly as another form
intermediate in many respects between _levana_ and _prorsa_. Weismann
also succeeded, by exposing the winter pupa to a high temperature, in
making it give rise to the _prorsa_ form, and not to the _levana_ form,
as it would ordinarily do.
Similar results have been obtained with the seasonally dimorphic _Pieris
napi_. Standfuss, the Gräfin von Linden, and others have obtained like
results in the case of other seasonally dimorphic butterflies. In some
instances it has been proved that the change in the pigment is a purely
chemical one; a similar transformation can be effected in the extracted
pigment. But, we must bear in mind that the changes which are induced in
this way are not confined to colour; they occur in the marking and shape
of the wing.
Even more remarkable is the fact that in some sexually dimorphic species
a change of temperature alters the female, so as to cause her to have the
outward appearance of the male. For example, it has been found that
warmth changes the colours of the female _Rhodocera rhamni_ and
_Parnassius apollo_ into the colours of the male.
By applying rays of strong light, electric shock, or centrifuge, the
Gräfin von Linden was able to change the colours of the butterflies to
which the caterpillars gave rise. Pictet experimented on twenty-one
species of butterflies, or rather on their caterpillars, and found that
in nearly all cases when the caterpillars ate unusual food, they
developed into butterflies with abnormal colouring. Schmankewitsch made
the discovery that, in the case of the crustacean _Artemia_, he could
produce either of two species according to the amount of salt in the
water in which these creatures were placed. He declared that the
anatomical differences between the species _Artemia salina_ and _Artemia
milhausenii_ depended solely on the percentage of the salt in the
surrounding water. He further stated that by adding still more salt he
could change the _Artemia_ into a new genus—_Branchipus_. More recent
observers have cast doubt upon these results of Schmankewitsch. They,
however, admit that the degree of salinity of the water has some effect
on the form of the _Artemia_, although they suggest that factors other
than concentration affect the result. In any case, it is now well-known
that changes in the environment effect changes in the colouring of many
crustacea. Pictet has shown that the alternating wet and dry seasons in
some tropical countries are the cause of, or stimulus that induces,
seasonal dimorphism in some butterflies. He was able to effect changes in
the colouring of certain species by means of humidity.
The most important cases, from our point of view, are those in which the
application of heat or cold to a pupa has affected the colour, shape,
etc., of the emerging butterfly. Here we have but one factor, that of
temperature. All the material for the formation of the butterfly is
already stored up in the pupa. The unit characters, or their precursors,
are all there, and they take one form or another according to the
stimulus applied.
Biological Isomerism
Phenomena of this kind can, we think, be accounted for only on the
assumption that the unit characters affected are each developed from a
definite portion of the fertilised egg, that each of these portions,
these precursors of the unit characters, is, like a chemical molecule,
made up of a number of particles, and that upon the arrangement of these
particles in its precursor in the egg depends the form that the unit
character derived from it will take. One arrangement of these particles
gives rise to one form of unit character, while another arrangement will
give rise to a totally different form of unit character.
Thus, some organisms seem to display a biological isomerism akin to
chemical isomerism, save that the particles which in organisms take the
place of chemical atoms are infinitely more complex.
In other words, the precursors in the fertilised egg of each of these
unit characters behave in some respects like chemical molecules.
In order to avoid the manufacture of fresh terms we may speak
figuratively of the germ cells as being composed of biological molecules,
which in their turn are built up of biological radicles and atoms. These
behave in some ways like chemical molecules, radicles, and atoms, as the
case may be.
Biological Molecules
It seems legitimate to regard each unit character in the adult as the
result of the development of one or more of the biological molecules
which compose the nucleus of the fertilised egg. These biological
molecules are, of course, a million-fold more complex than chemical
molecules. Each biological atom must contain within itself a number of
the very complex protoplasmic molecules. This view of the structure of
the germ cell seems to force itself upon the observer. Notwithstanding
this, the conception will have no value unless it seems to throw light on
the various phenomena of heredity, variation, etc.
Let us then try to interpret some of these.
Each chemical element is made up of atoms which are all of the same kind,
but no two elements are made up of the same kind of atoms, although
chemists are now inclined to conceive of all the various kinds of atoms
as made up of varying amounts of some primordial substance. In any case,
the molecules of chemical compounds are made up of various kinds of
atoms. With biological atoms the case would seem to be different. All
would appear to be made up of the same kind of substance, and the
differences shown by the various unit characters that go to make up an
organism would seem to be due to the different numbers and the varying
arrangement of the biological atoms which compose the molecules from
which unit characters are derived. This would be quite in accordance with
the chemical notion of allotropy. Thus, the graphite and the diamond
molecules are both made up of the same kind of atoms.
But the biological atoms are living, that is to say, they are continually
undergoing anabolism and katabolism, growth and decay. They exhibit all
the phenomena of life, they must grow and divide, and they must absorb
nourishment; hence it is not surprising that they should differ slightly
among themselves, that they should exhibit the phenomenon of variation.
Although probably all are composed of the same living material, no two
are exactly alike, hence the molecules formed by them will also differ
from one another. Thus we can see why it is that all organisms exhibit
fluctuating variations.
Very different are the discontinuous variations or mutations. These would
seem to be due to either a rearrangement of the biological atoms in the
biological molecule or the splitting up of the latter into two or more
molecules. This, of course, is pure hypothesis. Let us take an imaginary
example. Suppose that a biological molecule contains eighteen biological
atoms, and that these are arranged in the form of an equilateral
triangle, six of them going to each side. Suppose now, that from some
cause or other they rearrange themselves to form an isosceles triangle,
so that only four form the base and seven go to each of the remaining
sides. Such an arrangement would give rise to a mutation. Suppose now
that, from some cause or other, this triangular biological molecule were
to split up into two triangles, each having three atoms to each side, we
should obtain a still more marked mutation. We are far from saying that
the atoms in the organic molecule ever take such forms. We have merely
attempted to give rough but simple illustrations of the kind of processes
which on this hypothesis might be expected to take place in the germ
cells or the fertilised eggs.
Let us now consider the sexual act from this aspect. The various
molecules (we speak, of course, of biological molecules) of the male
parent meet those of the female parent, and a synthesis occurs, which
results in the formation of a new organism. When these two sets of
gametes meet one another, one of several events may happen. The gametes
may refuse to combine. This will occur whenever they are of very
different constitution; thus it is that widely differing species will not
interbreed. But it may even happen that gametes of individuals of the
same species may refuse to coalesce on account of some peculiarity in the
composition of one or other of them. Secondly, they may be able to form
some sort of a union, but, owing to their diverse nature, the resulting
molecules may be so complex that they cannot be broken up into equal
halves, and as this seems to be necessary for the sexual act, the
resulting organism will be sterile. Thirdly, the two sets of gametes may
enter into a proper union, that is to say, form new molecules, but these
may be of such different structure to the molecules of the gametes, that
the resulting offspring will be quite unlike their parents in appearance.
Fourthly, some or all the groups of radicles in each gamete may be united
so closely that in the sexual act they do not break up, but enter bodily
into the new resulting organism. In these circumstances the inheritance
of the offspring will follow Mendel’s law. Fifthly, there may be some
slight disturbance of the molecule, perhaps one or only a few atoms will
be replaced by those of the other gamete. This would give us impure
dominance.
Thus this hypothesis appears to be compatible with the various modes of
inheritance.
The curious phenomenon known as prepotency would seem also to be quite in
accordance with the conception.
In chemical reactions the tendency is for the most stable combinations to
be formed, so in nature.
We may probably go farther and say, not only will the most stable
biological molecules be formed, but the most stable radicles will
dominate the molecule. Hence, if any two animals are crossed and the
offspring show alternate inheritance, the resulting organism will, in the
case of each unit character, display the most stable of the pair; in
other words, it will take after the parent which happens to have the
greater stability as regards that particular character. The difference
between the mule and the hinny would seem to be explicable on this
supposition. If the union were like a simple chemical synthesis it should
not make any difference which way the cross were made. But if the species
crossed are of varying stability, and if their respective degrees of
stability vary with the sex, it is easy to see that it will make a
difference how the animals are crossed.
In the cases of creatures that obey Mendel’s law, the most stable form of
a unit character will presumably be the dominant one.
One of the most curious of the phenomena of inheritance is that of
correlation. We shall deal with this more fully in Chapter VIII. It will
suffice here to say that certain characters appear to be linked together
in organisms. Such seem to be transmitted in pairs. The offspring never
exhibits one of such a correlated couple without exhibiting the other
also.
It would thus seem that certain combinations of biological atoms, certain
molecules, can only exist in conjunction with certain other combinations.
This is quite in accordance with the teaching of physiologists regarding
the interdependence of the various organs of the body. We have now
reached the stage of the fertilised ovum. According to our conception it
is a series or conglomeration of the precursors of the unit characters of
the adult. These precursors we call biological molecules. Each is of a
very complex nature. Each seems to be composed of several portions, only
one of which will take part in the building up of the body of the
offspring, the other portions remaining latent. We further conceive that
it is possible for the various radicles which compose these molecules to
arrange themselves in various manners, and with each new arrangement a
different form of unit character will be developed. These molecules,
then, are built up from radicles derived from both parents, the most
stable combinations being formed and one portion of the molecule
dominating the whole. Under normal circumstances this dominant portion of
the molecule will give rise to a character of a definite type. But it
seems that other factors may come into play and cause a rearrangement of
the radicles which compose it, and this will result in the formation of a
unit character different from that to which it would ordinarily give
rise.
But, it may be objected, if the colour of an organism be derived from one
of these so-called biological molecules, how is it that it affects the
whole organism, or, at any rate, several of the other unit characters?
The objection may be met in several ways. In the first place, the
colour-forming molecules may split up into as many portions as there are
units which it affects, and each portion may attach itself to a unit. Or
the property which we call colouration may not be derived from a
molecule, it may be an expression in the relative positions of the
various molecules in the fertilised egg. Or the colour-determining
molecule may secrete a ferment or a hormone, and this may be the cause of
the particular colouring of the resulting organism. We do not pretend to
say which (if any) of these alternative suppositions is the correct one.
But it seems to us that some such conception as that which we have set
forth is forced upon us by observed facts. This conception should be
regarded not as a theory, but rather as an indication of the lines along
which we believe the study of inheritance could best be made.
The fertilised ovum has nothing of the shape of the creature to which it
will give rise. It is merely a potential organism, a something which
under favourable conditions will develop into an organism.
Phenomenon of Sex
In the higher animals each individual is either of the male or the female
sex. A vast amount of ingenuity has been expended by zoologists in the
attempt to ascertain what it is that determines sex. Many theories have
been advanced, but no one of them has obtained anything like general
acceptance, because its opponents are able to adduce facts which appear
to be incompatible with it.
It is tempting to try to interpret the phenomenon of sex on the
assumption that the female-producing biological molecule or unit is an
isomeride of the male-producing cell. Certain facts, however, seem to
negative the idea, as, for example, the occasional appearance in an
individual of one sex of characteristics of the other sex.
Possibly the attempts to explain the phenomena of sex-production on a
Mendelian basis may prove to be more successful. It seems not impossible
that each fertilised egg contains material which is capable of developing
into male generative organs and material which is capable of developing
into female generative organs, but that only one kind of material, that
which dominates, succeeds in developing. The number of what are known as
“X-elements” that happen to be present in the fertilised egg appear to
decide which kind of material is to be dominant.
But the problem of the determination of sex, fascinating though it be, is
not one that can be discussed adequately in a general work on evolution.
Those interested in the subject are referred to Professor Thomson’s
_Heredity_, and to the address given by Professor E. B. Wilson, of
Columbia University, before the American Association for the Advancement
of Science, which was fully reported in the issue of _Science_, dated
January 8, 1909.
Stated briefly, then, our conception is, that the fertilised egg is
composed of a number of entities, to which we have given the name
“biological molecules,” because in certain respects their behaviour is
not unlike that of chemical molecules.
The units which compose these molecules, being made up of protoplasm, are
endowed with all the properties of life, including the inherent
instability which characterises all living matter.
We suggest that the continuous or fluctuating variations that appear in
the adult organism may be the result of individual differences in the
biological “atoms” that compose the molecule.
Discontinuous variations, or mutations, on the other hand, may be the
result of a rearrangement of the atoms within the biological molecule.
Upon what causes this rearrangement it would not be very profitable to
speculate in the present state of our knowledge. To do this would be to
inquire into the cause of a re-grouping of entities of the existence of
which we are not certain! For aught we know there may be an intracellular
struggle for nourishment among the various molecules and among the atoms
which compose the molecules. If one molecule enjoys any special advantage
over the others the result may be an unusual degree of development of the
resulting unit character; in other words, the result will be a variation
in the organism. This variation may prove favourable or unfavourable to
its possessor.
Struggle for Nourishment
Certain phenomena seem to point to a struggle for nourishment between the
germinal and the somatic portions of the egg, between the parts from
which the sexual cells of the resulting organism are produced and those
which give rise to the body of the organism. Each molecule may strive, so
to speak, to increase at the expense of the others. Thus, great size in
an organism is likely to be produced at the expense of the germinal
cell-forming molecules. In other words, great size in an organism would
be incompatible with excessive fecundity. This is what we observe in
nature. On the other hand, poor development of bodily tissue, as in the
case of intestinal parasites, would be correlated with great fecundity.
Some organisms are mere sacs full of eggs.
Success in the struggle for nourishment of one molecule might be shared
by the other molecules near to it, hence the phenomena of correlation.
It is thus conceivable that, in a brood consisting of several
individuals, a particular molecule or set of molecules in one of the
individuals may receive more than its share of nourishment, and this will
result in the organs of that individual which spring from the
well-nourished molecules being exceptionally well developed. Thus arises
the phenomenon of differences between the members of a litter or brood.
Natural selection will tend to eliminate those individuals in which the
resulting variation is an unfavourable one. If the environment is such,
as in the case of an internal parasite, that the production of germ cells
is the most necessary function of the organism, then those individuals in
which the germ-forming molecules increase at the expense of the
body-forming ones will tend to be preserved. This would cause the
phenomenon which biologists term degeneration. The nourishment of the
various biological molecules may possibly depend on their relative
positions in the egg. Those in a favourable position will then tend to
develop at the expense of the others. This will result in variation along
definite lines. Each succeeding generation will tend to an increased
development of that particular organ to which the favourably-situated
molecule gives rise. This process may continue, as in the case of the
horns of the Irish elk, until the development of that particular organ
becomes so excessive as to be positively injurious; then natural
selection will step in and eliminate the species. But before this
happens, something may cause a rearrangement of the biological molecules
in the fertilised egg, and thus a mutation may arise, which, so to speak,
strikes out a new line.
Origin of Mutations
Finally, on this conception there may be some sort of connection between
fluctuating variations and mutations. We can picture the fluctuating
variations being piled up, one upon the other, until there results a
rearrangement of the atoms in one or more of the biological molecules
which, in turn, causes a mutation.
Occasionally this remodelling, as it were, of one biological molecule may
affect certain of the other molecules, and thus lead to correlated
mutations.