THE MUTATION THEORY
When with the thoughts suggested in the last chapter we contemplate
the problem of Evolution at large the hope at the present time of
constructing even a mental picture of that process grows weak almost
to the point of vanishing. We are left wondering that so lately men
in general, whether scientific or lay, were so easily satisfied. Our
satisfaction, as we now see, was chiefly founded on ignorance.
Every specific evolutionary change must represent a definite event in
the construction of the living complex. That event may be a disturbance
in the meristic system, showing itself in a change in the frequency of
the repetitions or in the distribution of differentiation among them, or
again it may be a chemical change, adding or removing some factor from
the sum total.
If an attempt be made to apply these conceptions to an actual series
of allied species the complexity of the problem is such that the mind
is appalled. Ideas which in the abstract are apprehended and accepted
with facility fade away before the concrete case. It is easy to imagine
how Man was evolved from an _Amoeba_, but we cannot form a plausible
guess as to how _Veronica agrestis_ and _Veronica polita_ were evolved,
either one from the other, or both from a common form. We have not even
an inkling of the steps by which a Silver Wyandotte fowl descended from
_Gallus Bankiva_, and we can scarcely even believe that it did. The
Wyandotte has its enormous size, its rose comb, its silver lacing, its
tame spirit, and its high egg production. The tameness and the high egg
production are probably enough both recessives, and though we cannot
guess how the corresponding dominant factors have got lost, it is not
very difficult to imagine that they were lost somehow. But the rose comb
and the silver colour are _dominants_. The heavy weight also appears in
the crosses with Leghorns, but we need not at once conclude that it
depends on a simple dominant factor, because the big size of the crosses
may be a consequence of the cross and may depend on other elements.
Now no wild fowl known to us has these qualities. May we suppose that
some extinct wild species had them? If so, may we again make the same
supposition in all similar cases? To do so is little gain, for we are
left with the further problem, whence did those lost wild species
acquire those dominants? Suppositions of this kind help no more than
did the once famous conjecture as to the origin of living things--that
perhaps they came to earth on a meteorite. The unpacking of an
original complex, the loss of various elements, and the recombination
of pre-existing materials may all be invoked as sources of specific
diversity. Undoubtedly the range of possibilities thus opened up is
large. It will even cover an immense number of actual examples which
in practice pass as illustrations of specific distinction. The Indian
Rock pigeon which has a blue rump may quite reasonably be regarded as
a geographically separated recessive form of our own _Columba livia_,
for as Staples-Browne has shown the white rump of _livia_ is due to
a dominant factor. The various degrees to which the leaves of Indian
Cottons are incised have, as Leake says, been freely used as a means
of classification. The diversities thus caused are very remarkable,
and when taken together with diversities in habit, whether sympodial
or monopodial, the various combinations of points of difference
are sufficiently distinctive to justify any botanist in making a
considerable number of species by reference to them alone. Nevertheless
Leake's work goes far to prove that all of these forms represent the
re-combinations of a very small number of factors. The classical example
of _Primula Sinensis_ and its multiform races is in fact for a long
way a true guide as to the actual interrelations of the species which
systematists have made. That they did make them was due to no mistake
in judgment or in principle, but simply to the want of that extended
knowledge of the physiological nature of the specific cases which we now
know to be a prime necessity.
But will such analysis cover all or even most of the ordinary cases
of specific diversity between near allies? Postponing the problem of
the interrelations of the larger divisions as altogether beyond present
comprehension, can we suppose, that in general, closely allied species
and varieties represent the various consequences of the presence or
absence of allelomorphic factors in their several combinations? The
difficulty in making a positive answer lies in the fact that in most
of the examples in which it has been possible to institute breeding
experiments with a view to testing the question, a greater or less
sterility is encountered. Where, however, no such sterility is met
with, as for instance in the crosses made by E. Baur among the species
of _Antirrhinum_ there is every reason to think that the whole mass of
differences can and will eventually be expressed in terms of ordinary
Mendelian factors. Baur has for example crossed species so unlike as
_Antirrhinum majus_ and _molle_, forms differing from each other in
almost every feature of organisation.[1] The F_{2} generation from this
cross presents an amazingly motley array of types which might easily if
met with in nature be described as many distinct species. Yet all are
fertile and there is not the slightest difficulty in believing that they
can all be reduced to terms of factorial analysis.
If allowance be made for the complicating effects of sterility, is there
anything which prevents us from supposing that such good species as
those of _Veronica_ or of any other genus comprising well-defined forms
may not be similarly related? I do not know any reason which can be
pointed to as finally excluding such a possibility. Nevertheless it has
been urged with some plausibility that good species are distinguished
by _groups_ of differentiating characters, whereas if they were really
related as the terms of a Mendelian F_{2} family are, we should expect
to find not groups of characters in association, but rather series of
forms corresponding to the presence and absence of the integral factors
composing the groups of characters. I am not well enough versed in
systematic work to be able to decide with confidence how much weight
should be attached to this consideration. Some weight it certainly
has, but I cannot yet regard it as forming a fatal objection to the
application of factorial conceptions on the grand scale. It may be
recalled that we are no longer under any difficulty in supposing that
differences of all classes may be caused by the presence or absence of
factors. It seemed at first for example that such characters as those of
leaf shape might be too subtle and complex to be reducible to a limited
number of factors. But first the work of Gregory on _Primula Sinensis_
showed that several very distinct types of leaves were related to each
other in the simplest way. In that particular example, intermediates are
so rare as to be negligible, but subsequently Shull dealing with such
a complicated example as _Capsella_, and Leake in regard to Cottons,
both forms in which intergrades occur in abundance, have shown that a
simple factorial scheme is applicable. We need not therefore, to take
an extreme case, doubt that if it were possible to examine the various
forms of fruit seen in the Squashes by really comprehensive breeding
tests, even this excessive polymorphism in respect of structural
features would be similarly reducible to factorial order.
It must always be remembered also that in a vast number of cases, nearly
allied forms which are distinct, occupy distinct ground. Moreover, by
whatever of the many available mechanisms that end be attained, it is
clear that nature very often does succeed in preventing intercrossing
between distinct forms so far that the occurrence of that phenomenon is
a rarity under natural conditions. The facts may, I think, fairly be
summarized in the statement that species are on the whole distinct and
not intergrading, and that the distinctions between them are usually
such as might be caused by the presence, absence, or inter-combination
of groups of Mendelian factors; but that they are so caused the evidence
is not yet sufficient to prove in more than a very few instances.
The alternative, be it explicitly stated, is not to return to the view
formerly so widely held, that the distinctions between species have
arisen by the accumulation of minute or insensible differences. The
further we proceed with our analyses the more inadequate and untenable
does that conception of evolutionary change become. If the differences
between species have not come about by the addition or loss of factors
one at a time, then we must suppose that the changes have been effected
by even larger steps, and variations including groups of characters,
must be invoked.
That changes of this latter order are really those by which species
arise, is the view with which de Vries has now made us familiar by his
writings on the Mutation Theory. In so far as mutations may consist
in meristic changes of many kinds and in the loss of factors it is
unnecessary to repeat that we have abundant evidence of their frequent
occurrence. That they may also more rarely occur by the addition of a
factor we are, I think, compelled to believe, though as yet the evidence
is almost entirely circumstantial rather than direct. The evidence for
the occurrence of those mutations of higher order, by which new species
characterized by several distinct features are created, is far less
strong, and after the best study of the records which I have been able
to make, I find myself unconvinced. The facts alleged appear capable of
other interpretations.
The most famous and best studied examples are of course the forms
of _Oenothera_ raised by de Vries from _Oenothera Lamarckiana_ in
circumstances well known to all readers of genetic literature. Whatever
be the true significance of these extraordinary "mutations" there can
be no question about the great interest which attaches to them, and the
historical importance which they will long preserve. Apart also from
these considerations it is becoming more and more evident that in their
peculiarities they provide illustrations of physiological phenomena of
the highest consequence in the study of genetics at large.
De Vries found, as is well known, that _Oenothera Lamarckiana_ gives off
plants unlike itself. These mutational forms are of several distinct
and recognizable types which recur, and several of them breed true from
their first appearance. The obvious difficulty, which in my judgment
should make us unwilling at present to accept these occurrences as proof
of the genesis of new species by mutation, is that we have as yet no
certainty that the appearance of the new forms is not an effect of the
recombination of factors, such as is to be seen in so many generations
of plants derived from a cross involving many genetic elements. The
first question is what is _Oenothera Lamarckiana_? Is it itself a plant
of hybrid origin? To this fundamental question no satisfactory answer
has yet been given. All attempts to find it as a wild plant in America
have failed. It existed in Europe in the latter half of the eighteenth
century. Whence it came is still uncertain, but the view that it came
into existence in Europe and perhaps in Paris, seems on the whole the
most probable. The question has been debated by Macdougal, Gates, and
Davis. From historical sources there is little expectation of further
light. Those who favour the notion of a hybrid origin look on _Oenothera
biennis_ as one of the putative parents. It has been conjectured that
a species called _grandiflora_ lately re-discovered on the Alabama
river was the other parent. Experiments have been instituted by Davis
to discover whether _Lamarckiana_ can be made artificially by crossing
these two species. The results so far have shown that while plants
approximating in various respects to _Lamarckiana_ have thus been
produced, none agree exactly with that form. Davis, to whom reference
should be made for a full account of the present state of the enquiry,
points out that there are many strains of _biennis_ in existence and
that it is by no means impossible that by using others of these strains
a still closer approximation can be made. None of Davis's artificial
productions as yet breed at all true, as _Lamarckiana_ on the whole
does. In such a case, however, where several characters are involved,
this is perhaps hardly to be expected.
One feature of the _Oenotheras_ is very curious. Not only _Lamarckiana_,
but all the allied species so far as I am aware, have a considerable
proportion of bad and shrivelled pollen grains. This is undoubtedly true
of species living in the wild state as well as of those in cultivation.
I have had opportunities of verifying this for myself in the United
States. No one looking at the pollen of an _Oenothera_ would doubt that
it was taken from some hybrid plant exhibiting partial sterility. On the
other hand, it is difficult to suppose that numbers, perhaps all, of
the "species" of the genus are really hybrids, and many of them breed
substantially true. I regard this constant presence of bad pollen grains
as an indication that the genetic physiology of _Oenothera_ is in some
way abnormal, and as we shall presently see, there are several other
signs which point in the same direction.
Discussion of the whole series of phenomena is rendered exceedingly
difficult first, by reason of the actual nature of the material. The
characteristics of many of the types which de Vries has named are
evasive. A few of these types, for instance, _gigas_, _nanella_,
_albida_, _brevistylis_, and perhaps a few more are evidently clear
enough, but we have as yet no figures and descriptions precise enough
to enable a reader to appreciate exactly the peculiarities of the vast
number of forms which have now to be considered in any attempt to gain a
comprehensive view of the whole mass of facts. It is also not in dispute
that the forms are susceptible of great variations due simply to soil
and cultural influences.
The fact that no Mendelian analysis has yet been found applicable to
this group of _Oenotheras_ as a whole is perhaps largely due to the fact
that until recently such analysis has not been seriously attempted.
Following the system which he had adopted before the rediscovery of
Mendelism, or at all events, before the development of that method of
analysis, de Vries has freely applied _names_ to special combinations of
characters and has scarcely ever instituted a factorial analysis. Before
we can get much further this must be attempted. It may fail, but we must
know exactly where and how this failure comes about. There are several
indications that such a recognition of factorial characters, could be
carried some way. For example, the height, the size of the flowers, the
crinkling of the leaves, the brittleness of the stems, perhaps even the
red stripes on stems and fruits, and many more, are all characters which
may or may not depend on distinct factors, but if such characters are
really transmitted in unresolved groups, the limitations of those groups
should be carefully determined. The free use of names for the several
forms, rather than for the characters, has greatly contributed to deepen
the obscurity which veils the whole subject.
I do not mean to suggest that these _Oenotheras_ follow a simple
Mendelian system. All that we know of them goes to show that there
are curious complications involved. One of these, probably the most
important of all, has lately been recognized by de Vries himself,
namely, that in certain types the characters borne by the female and
the male germ-cells of the same plant are demonstrably different. There
can be little doubt that further research will reveal cognate phenomena
in many unsuspected places. The first example in which such a state of
things was proved to exist is that of the Stocks investigated by Miss
Saunders.[2] By a long course of analysis she succeeded in establishing
in 1908 the fact that if a plant of _Matthiola_ is of that eversporting
kind which gives a large proportion of double-flowered plants among
its offspring (produced by self-fertilisation), then the egg-cells of
such a plant are mixed in type, but the pollen of the same plant is
homogeneous. Some of the egg-cells have in them the two factors for
singleness, but some of them are short of one or both of these factors.
The pollen-grains, however, are all recessives, containing neither of
these factors. The egg-cells, in other words, are mixed, "singles"
and "doubles," while the pollen-grains are all "doubles." The same is
true of the factor differentiating "white," or colourless plastids
from cream-coloured plastids in _Matthiola_, the egg-cells being mixed
"whites" and "creams," while the pollen-grains are all "creams," viz:
recessives. Later in the same year (1908) de Vries[3] announced a
remarkable case which will be discussed in detail subsequently. It
relates to certain _Oenotheras_ heterozygous for dwarfness, in which (p.
113) the ovules were mixed, tails and dwarfs, while the pollen is all
dwarf.
Again in _Petunia_ Miss Saunders's[4] work has shown that a somewhat
similar state of things exists, but with this remarkable difference,
that though the egg-cells are mixed, singles and doubles, the
pollen-grains are all _singles_, viz: dominants. All the _Petunias_ yet
examined have been in this condition, including some which in botanic
gardens pass for original species. Whether actual wild plants from
their native habitats are in the same state, is not yet known, but it
is by no means improbable. The case may be compared with that of the
moth _Abraxas grossulariata_ studied by Doncaster and Raynor, in which
the females are all heterozygous, or we may almost say "hybrids" of
_grossulariata_ and the variety _lacticolor_. Similarly we may say that
at least garden Petunias are heterozygous in respect of singleness.
The proof of this is of course that when fertilised with the pollen of
doubles they throw a mixture of doubles and singles. The statements
which de Vries has published regarding the behaviour of several of the
_Oenotheras_ go far to show that they must have a somewhat similar
organisation. On the present evidence it is still quite impossible to
construct a coherent scheme which will represent all the phenomena in
their interrelations, and among the facts are several which, as will
appear, seem mutually incompatible. The first indication that the
_Oenotheras_ may have either mixed ovules or mixed pollen appears in the
fact that _Lamarckiana_ and several of its "mutants" used as males, with
several other forms as females, give a mixed offspring. For example, de
Vries (1907) found that
_biennis_ [F] × _Lamarckiana_ [M]
_biennis cruciata_ [F] × _Lamarckiana_ [M]
_muricata_ [F] × _Lamarckiana_ [M]
_biennis_ [F] × _rubrinervis_ [M]
_biennis cruciata_ [F] × _rubrinervis_ [M]
all give a mixture of two distinct types which he names _laeta_ and
_velutina_, consisting of about equal numbers of each. On account of the
fact that the two forms are produced in association de Vries has called
these forms "twin hybrids," a designation which is not fortunate, seeing
that it is impossible to imagine that any kind of twinning is concerned
in their production. The distinction between these two seems to be
considerable, _laeta_ having leaves broader, bright green in colour, and
flat, with pollen scanty, while _velutina_ has leaves narrower, grayish
green, more hairy, and furrow-shaped, with pollen abundant.
We next meet the remarkable fact that these two forms, _laeta_
and _velutina_ breed true to their respective types, and do not
reproduce the parent-types among their offspring resulting from
self-fertilisation. This statement must be qualified in two respects.
When _muricata_ [M] is fertilised by _brevistylis_ the forms _laeta_
and _velutina_ are produced, but each of them subsequently throws the
short-styled form as a recessive (de Vries, 1907, p. 406). It may be
remembered that de Vries's previous publications had already shown that
the short style of _brevistylis_, one of the _Lamarckiana_ "mutants,"
behaves as a recessive habitually (_Mutationstheorie_, II, p. 178, etc.).
Also when _nanella_, the dwarf "mutant" of _Lamarckiana_ is used as
male on _muricata_ as female, _laeta_ and _velutina_ are produced, but
one only of these, namely, _velutina_, subsequently throws dwarfs on
self-fertilisation. The dwarfs thus thrown are said to form about 50 per
cent. of the families in which they occur (de Vries, 1908, p. 668). The
fact that the two forms, _laeta_ and _velutina_, are produced by many
matings in which _Lamarckiana_ and its mutant _rubrinervis_ are used as
males is confirmed abundantly by Honing, who has carried out extensive
researches on the subject. After carefully reading his paper, I have
failed to understand the main purport of the argument respecting the
"double nature" of _Lamarckiana_ which he founds on these results, but
I gather that in some way _laeta_ is shown to partake especially of the
nature of _Lamarckiana_, while _velutina_ is a form of _rubrinervis_.
The paper contains many records which will be of value in subsequent
analysis of these forms.
Before considering the possible meaning of these facts we must have
in our minds the next and most novel of the recent extensions of
knowledge as to the genetic properties of the _Oenotheras_. In the
previous statement we have been concerned with the results of using
either _Lamarckiana_ itself or one of its "mutants" _rubrinervis_,
_brevistylis_, or _nanella_ as male, on one of the species _biennis_
or _muricata_. The new experiments relate to crosses between the two
species _biennis_ and _muricata_ themselves.
De Vries found:
1. That the reciprocal hybrids from these two species differed,
_biennis_ × _muricata_ producing one type of F_{1} and _muricata_ ×
_biennis_ producing another. Each F_{1} resembled the father more than
the mother.
2. That each of the hybrids so produced breeds true on
self-fertilisation.
3. That if we speak of the hybrid from _biennis_ × _muricata_ as _BM_
and of the reciprocal as _MB_, then
_BM_ × _MB_
gives exclusively offspring of _biennis_ type but that
_MB_ × _BM_
gives exclusively offspring of _muricata_ type. Evidently, apart
from all controversy as to the significance of the "mutants" of
_Lamarckiana_, we have here a series of observations of the first
importance.
The fact that reciprocal crossings give constantly distinct results
must be taken to indicate that the male and female sides of one, if not
of both, of the parents are different in respect of characters which
they bear. This is de Vries's view, and he concludes rightly, I think,
that the evidence from all the experiments shows that both _biennis_
and _muricata_ are in this condition, having one set of characters
represented in their pollen-grains and another in their ovules. The
plants breed true, but their somatic structures are compounded of the
two sets of elements which pass into them from their maternal and
paternal sides respectively. This possibility that species may exist of
which the males really belong to one form and the females to another, is
one which it was evident from the first announcement of the discovery of
Mendelian segregation might be found realised in nature.[5]
_Oe. biennis_ and _muricata_ were crossed reciprocally with each other
and with a number of other species, and the behaviour of each, when
used as mother, was consistently different from its behaviour when
used as father. De Vries is evidently justified by the results of
this series of experiments in stating that the "Bild," as he terms
it, or composition of the male and female sides of these two species,
_biennis_ and _muricata_, are distinct. On the evidence before us it
is not, however, possible to form a perfectly clear idea of each, and
until details are published, a reader without personal knowledge of the
material cannot do more than follow the general course of the argument.
For fuller comprehension a proper analysis of the characters with a
clear statement of how they are distributed among the several types and
crosses is absolutely necessary. According to de Vries the female of
_biennis_ possesses a group of characters which he defines as "_conica_"
in allusion to the shape of the flower-buds. Besides the conical buds,
this group of features includes imperfect development of wood, rendering
the plant very liable to attacks of _Botrytis_, and comparatively narrow
leaves.
The female of _muricata_ carries a group of features which he
calls "_frigida_," and, though this is not quite explicitly stated
in a definition of that type, it is to be inferred[6] that its
characteristics are regarded as greater height, strong development of
wood with comparative resistance to _Botrytis_, and broad leaves.
The characters borne by the male parts of the two species are in general
those by which they are outwardly distinguished. For example, the
leaves of _Oe. biennis_ are comparatively broad and are bright green,
while those of _muricata_ are much narrower and of a glaucous green,
and I understand that de Vries regards these properties as contributed
by the male side in each case and to be carried by the male cells of
each species. The suggestion as regards _biennis_ and _muricata_ comes
near the conception often expressed by naturalists in former times (_e.
g._, Linnaeus) and not rarely entertained by breeders at the present
day, that the internal structure is contributed by the mother and the
external by the father.
On the other hand, the offspring of each species when used as mother is
regarded as possessing in the main the features of the maternal "Bild,"
but the matter is naturally complicated by the introduction of features
from the father's side, and it is here especially that the account
provided is at present unsatisfactory and inconclusive. There seems,
however, to be no serious doubt that _biennis_ and _muricata_ each in
their outward appearance exhibit on the whole the features which their
pollens respectively carry, and that the features borne by their ovules
are in many respects distinct.
The _types_ are thus "hybrids" which breed true. The results of
intercrossing them each way are again "hybrids" which breed true. It
will be remembered that on former occasions de Vries has formulated a
general rule that _species_-hybrids breed true, but that the cross-breds
raised by interbreeding _varieties_ do not. One of these very cases
was quoted[7] as an illustration of this principle, viz: _muricata_ ×
_biennis_. The grounds for this general statement have always appeared
to me insufficient, and with the further knowledge which the new
evidence provides we are encouraged to hope that when a proper factorial
analysis of the types is instituted we shall find that the phenomenon
of a constant hybrid will be readily brought into line with the systems
of descent already worked out for such cases as that of the Stocks, and
others already mentioned.
In further discussion of these facts de Vries makes a suggestion which
seems to me improbable. Since the egg-cells of _muricata_, for instance,
bear a certain group of features which are missing on the male side,
and conversely the pollen bears features absent from the female side,
he is inclined to regard the _bad pollen grains_ as the bearers of the
missing elements of the male side and to infer that there must similarly
be defective ovules representing the missing elements of the female
side. No consideration is adduced in support of this view beyond the
simple fact that the characters borne by male and female are dissimilar,
whereas it would be more in accord with preconception if the same sets
of combinations were represented in each--as in a normal Mendelian case.
There is as yet no instance in which the absence of any particular
class of gametes has been shown with any plausibility to be due to
defective viability, though there are, of course, cases in which certain
classes of zygotes do not survive owing to defective constitution (_e.
g._, the albinos of _Antirrhinum_ studied by Baur, and the homozygous
yellow mice). I am rather inclined to suppose that in these examples of
hybrids breeding true we shall find a state of things comparable with
that to which we formerly applied the terms "coupling" and "repulsion."
In these cases certain of the possible combinations of factors occur
in the gametic series with special frequency, being in excess, while
the gametes representing other combinations are comparatively few.
In a recent paper on these cases Professor Punnett and I have shown
that these curious results vary according to the manner in which the
factors are grouped in the parents. If _A_ and _B_ are two factors which
exhibit these phenomena we find that the gametic series of the double
heterozygote differs according as the combination is made by crossing
_AB × ab_, or by crossing _AB × aB_. In a normal Mendelian case the
F_{1} form, _AaBb_, produces gametes _AB_, _Ab_, _aB_, _ab_, in equal
numbers; but in these peculiar cases those gametes which contain
Gametic series Number of Number of
--------------------- gametes zygotes
AB Ab aB ab in series formed
Partial repulsion { 1 (n-1) (n-1) 1 2n 4n^{2}
from zygote { 1 31 31 1 64 4096
of form { 1 15 15 1 32 1024
Ab×aB { 1 7 7 1 16 256
{ 1 3 3 1 8 64
1 1 1 1 4 16
Partial coupling { 3 1 1 3 8 64
from zygote { 7 1 1 7 16 256
of form { 15 1 1 15 32 1024
AB×ab { 31 1 1 31 64 4096
{ 63 1 1 63 128 16384
{ (n-1) 1 1 (n-1) 2n 4n^{2}
Nature of zygotic series
---------------------------------------
AB Ab aB ab
Partial repulsion { 2n^{2}+1 n^{2}-1 n^{2}-1 1
from zygote { 2049 1023 1023 1
of form { 513 255 255 1
Ab×aB { 129 63 63 1
{ 33 15 15 1
9 3 3 1
{ 41 7 7 9
Partial coupling { 177 15 15 49
from zygote { 737 31 31 225
of form { 3009 63 63 961
AB×ab { 12161 127 127 3969
{ 3n^{2}-(2n - 1) 2n-1 2n-1 n^{2}-(2n-1)
the _parental combinations_ are in excess. This excess almost certainly
follows the system indicated by the accompanying table. In the general
expressions _n_ is half the number of gametes required to express the
whole system. Now if we imagine that sex-factors are involved with the
others concerned in such a relationship as this we have a system of
distribution approximating to that found in _biennis_ and _muricata_.
The difference in reciprocals is represented in a not improbable way.
It cannot yet be said that the rarer terms in the series are formed at
all, and perhaps they are not. As we pointed out in our discussion of
these phenomena, the peculiar distribution of factors in these cases
must be taken to mean that the planes of division at some critical stage
in the segregation are determined with reference to the parental groups
of factors, or in other words, that the whole system has a polarity,
and that the distribution of factors with reference to this polarity
differs according to the grouping of factors in the gametes which united
in fertilization to produce the plant. Subsequent proliferation of
cells representing certain combinations would then lead to excess of
the gametes bearing them. It is on similar lines that I anticipate we
shall hereafter find the interpretation of the curious facts discovered
by de Vries, though it is evident that a long course of experiment and
analysis must be carried through before any certainty is reached. The
work must be begun by a careful study of the descent of some single
factor, for example, that causing the broader leaf of _biennis_, and we
may hope that the study of _Oenothera_ by proper analytical methods will
no longer be deferred.
We have now to return to the relations of _laeta_ and _velutina_.
These two forms, it will be remembered are frequently produced when
_Lamarckiana_ or one of its derivatives is used as male, and the most
unexpected feature in their behaviour is that _both breed true as
regards their essential characteristics, on self-fertilisation_. If one
only bred true the case might, in view of the approximate numerical
equality of the two types, be difficult to interpret on ordinary lines,
but as both breed true it must be clear that some quite special system
of segregation is at work. What this may be cannot be detected on the
evidence, but with the results from the _biennis-muricata_ experiments
before us, it is natural to suspect that we may here again have to
recognise a process of allocation of different factors to the male and
female sides in _laeta_ and _velutina_. That some such system is in
operation becomes the more probable from the new fact which de Vries
states in describing the group of characters which he calls _conica_,
namely that this type is the same as that of _velutina_.
There are many collateral observations recorded both by de Vries and
others which have a bearing on the problems, but they do not yet fall
into a coherent scheme. For example, we cannot yet represent the
formation of _laeta_ and _velutina_ from the various species fertilised
by _Lamarckiana_ [M]. That this is not due to any special property
associated with the pollen of _Lamarckiana_ is shown by the fact that
a species called _Hookeri_ gives _laeta_ and _velutina_ in both its
reciprocal crosses with _Lamarckiana_ (de Vries, 1909, p. 3), and also
by the similar fact that _Lamarckiana_ [F] fertilised by the pollen of
a peculiar race of _biennis_ named _biennis Chicago_ throws the same
types. Before these very complicated phenomena can be usefully discussed
particulars must be provided as to the individuality of the various
plants used. This criticism applies to much of the work which de Vries
has lately published, for, as we now know familiarly, plants to which
the same name applies can be quite different in genetic composition.
Attention should also be called to one curiously paradoxical series of
results. When the dwarf "mutant" of _Lamarckiana_ which de Vries names
"_nanella_" is used as father on _muricata_, F_{1} consists of _laeta_
and _velutina_ in approximately equal numbers. Both forms breed true to
their special characteristics, but _velutina_ throws dwarfs of its own
type, while _laeta_ does not throw dwarfs. Subsequent investigation of
the properties of these types has led to some remarkable conclusions,
and it was in a study of these plants that de Vries first came upon the
phenomena of dissimilarity between the factors borne by the male and
female cells of the same plant, a condition which had been recently
detected in the Stocks as a result of Miss Saunders's investigations.
The details are very remarkable. We have first the fact that _muricata_
[F] × dwarf _nanella_ [M] gives about 50 per cent. _laeta_ and about 50
per cent. of _velutina_.
As regards _Velutina_ it was shown that:
Talls, Dwarfs,
per cent. per cent.
1. Velutina selfed gave 38 62
{Velutina [F] × dwarf nanella [M] gave 39 61
2.{ do. × do. gave 49 51
{ do. × dwarf [M] derived from
velutina gave 43 57
3. Dwarfs × velutina [M] gave -- all dwarfs
The three experiments taken together prove, as de Vries says, that the
ovules of _velutina_ are mixed, talls and dwarfs, and that the pollen is
all dwarf. The condition is almost the same as that of the Stocks. It
may be noted also that in the Stocks the egg-cells of the "double" type
are in excess, being approximately 9 to 7 of the "single" type, but de
Vries regards the two types in _velutina_ as probably equal in number.
The figures (169:231) rather suggest some excess of the recessives,
perhaps 9:7, and the point would be worth a further investigation.
As regards _laeta_, by self-fertilisation _no dwarfs were produced_, but
in all other respects it behaved almost exactly like _velutina_. The
ovules are evidently mixed talls and dwarfs, and whether fertilised by
dwarfs or by the pollen of _velutina_, which is already proved to be all
dwarf, the result was a steady 50 per cent. of talls and 50 per cent. of
dwarfs. The pollen of _laeta_ used on dwarfs gives nothing but dwarfs,
and in three series of such experiments 226 dwarfs were produced.
We are thus faced with this difficulty. Since the egg-cells of _laeta_
are evidently mixed, talls and dwarfs, and the pollen used on dwarfs
gives all dwarfs, why does not self-fertilisation give a mixed result,
talls and dwarfs, instead of _all talls_? De Vries regards the result
of self-fertilisation as showing the real nature of the pollen, and
declares it to be all talls, while he represents the behaviour of
the same pollen used on dwarfs by stating that in these combinations
the dwarf character dominates. This does not seem to me a natural
interpretation. I should regard the pollen of _laeta_ as identical
with that of _velutina_, namely dwarf, and I suspect the difficulty
is really created by the behaviour of _laeta_ on self-fertilisation.
Until a proper analysis is made in which the identity of the different
individuals used is recorded, no further discussion is possible.[8]
Other results of a complicated kind involving production of _laeta_ and
_velutina_ together with a third form have been published by de Vries in
his paper on "Triple Hybrids." To these also the same criticism applies.
Some of the observations seem capable of simple factorial representation
and others are conflicting.
Taking the work on _Oenothera_ as a whole we see in it continually
glimpses of order which further on are still blocked by difficulties
and apparent inconsistencies. Through such a stage all the successful
researches in complicated factorial analysis have passed and I see
no reason for supposing that with the application of more stringent
methods this more difficult set of problems will be found incapable
of similar solutions. To return to the original question whether in
_Oenothera_ we can claim to see a special contemporaneous output of new
species in actual process of creation, it will be obvious that while
the interrelation of the several types is still so little understood,
such a claim has no adequate support. It is true that many of the
"mutants" of _Lamarckiana_ can well pass for species, but this is
equally true of many new combinations of pre-existing factors as we
have seen in _Primula Sinensis_ and other cases. Still less can it be
admitted that these facts of uncertain import supply a justification for
the conception which has played a prominent part in the scheme of the
_Mutationstheorie_, namely that there are special periods of Mutation,
when the parent-species has peculiar genetic properties. To conclude:
The impression which the evidence leaves most definitely on the mind is
that further discussion of the bearing which the _Oenotheras_ may have
on the problem of evolution should be postponed until we have before
us the results of a searching analysis applied to a limited part of
the field. In such an analysis it is to be especially remembered that
we have now a new clue in the well-ascertained fact that the genetic
composition of the male and female germ-cells of the same individual
may be quite different. When with this possibility in view the behaviour
of the types is re-examined I anticipate that many of the difficulties
will be removed.
Outside the evidence from _Oenothera_, which, as we have seen, is
still ambiguous, I know no considerable body of facts favourable to
that special view of Mutation which de Vries has promulgated. Of
variation, or if we will, Mutation, in respect of some one character,
or resulting from recombination, there is proof in abundance; but of
that simultaneous variation in several independent respects to which de
Vries especially attributes the origin of new specific types I know only
casual records which have yet to undergo the process of criticism.
* * * * *
Besides de Vries's "_Mutationstheorie_" and "Species and Varieties"
the chief publications relating to the subject of the behaviour of
_Oenothera_ are the following: (Many other papers relating especially to
the cytology of the forms have appeared.)
Davis, B. M. Genetical Studies on _Oenothera_,
I. _Amer. Nat._, XLIV, 1910, p. 108. Genetical Studies on
_Oenothera_, II. _Ibid._, XLV, 1911, p. 193.
Gates, R. R. An Analytical Key to some of the Segregates of
Oenothera. _Twentieth Annual Report of the Missouri Botanical
Garden_, 1909.
Studies on the Variability and Heritability of Pigmentation in
_Oenothera_. _Ztsch. f. Abstammungslehre_, 1911, IV, p. 337.
Honing, J. A. Die Doppelnatur der _Oenothera Lamarckiana_.
_Ztsch. f. Abstammungslehre_, 1911, IV, p. 227.
Macdougal, D. T. (with A. M. Vail, G. H. Shull, and J. K.
Small). Mutants and Hybrids of the _Oenotheras_. _Carnegie
Institution's Publication_, No. 24, 1905.
Macdougal, D. T., Vail, A. M., Shull, J. H. Mutations,
Variations and Relationships of the _Oenotheras_. _Carnegie
Institution's Publication_, No. 81, 1907.
de Vries, H. On Atavistic Variation in _Oenothera cruciata_.
_Bull. Torrey Club_, 1903, Vol. 30, p. 75.
On Twin Hybrids, _Bot. Gaz._, Vol. 44, 1907, p. 401.
Ueber die Zwillingsbastarde von _Oenothera nanella_. _Ber.
Deut. Bot. Ges._, 1908, XXVI, _a_, p. 667.
Bastarde von _Oenothera gigas_. _Ibid._, p. 754.
On Triple Hybrids. _Bot. Gaz._, 1909, Vol. 47, p. 1.
Ueb. doppeltreziproke Bastarde von _Oenothera biennis_ L. und
_Oenothera muricata_ L. _Biol. Cbltt._, 1911, XXXI, p. 97.
Zeijlstra, H. H. _Oenothera nanella_ de Vries, eine krankhafte
Pflanzenart. _Biol. Cbltt._, 1911, XXXI, p. 129.
NOTE.
Since this chapter was written two contributions of special importance
have been made to the study of the _Oenothera_ problems. The first is
that of Heribert-Nilsson.[9] The author begins by giving a critical
account of the evidence for de Vries's interpretation of the nature of
the mutants. In general this criticism pursues lines similar to those
sketched in the foregoing chapter, concluding, as I have done, that the
chief reason why factorial analysis has been declared to be inapplicable
to the _Oenothera_ mutants is because no one has hitherto set about this
analysis in the right way. He has also himself made a valuable beginning
of such an analysis and gives good evidential reasons for the belief
that at least the red veining depends on a definite factor which also
influences the size of certain parts of the plant. He argues further
that many of the distinctions between the mutants are quantitative
in nature. With great plausibility he suggests that the system of
cumulative factors which Nilsson-Ehle discovered in the case of wheat
(subsequently traced by East in regard to maize) may be operating also
in these _Oenotheras_. According to this system several factors having
similar powers may coexist in the same individual, and together produce
a cumulative effect. Scope would thus be given for the production of
the curious and seemingly irregular numbers so often recorded in the
"mutating" families.
Another remarkable observation relating to the crosses of _muricata_
and _biennis_ has been published by Goldschmidt.[10] He finds that in
the formation of this cross the female pronucleus takes no part in the
development of the zygotic cell, but that when the male pronucleus
enters, the female pronucleus is pushed aside and degenerates. As de
Vries observed, the reciprocal hybrids are in each case very like the
father ("_stark patroklin_"), a consequence which finds a natural
explanation in the phenomenon witnessed by Goldschmidt. The results
of the subsequent matings can also be readily interpreted on the same
lines. Indications of maternal characters are nevertheless mentioned
by de Vries, and if Goldschmidt's account of the cytology is confirmed,
these must presumably be referred to the influence of the maternal
cytoplasm. Clearly this new work opens up lines of exceptional interest.
The interpretation I have offered above must probably be reconsidered.
The distinction between the male and female cells of the types may no
doubt be ultimately factorial, but it is difficult to regard such a
distinction as created by a differential distribution of the ordinary
factors.
FOOTNOTES:
[1] See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426.
Compare Lecoq on _Mirabilis jalapa_ × _longiflora_, Fécondation des
Végétaux, 1862, p. 311.
[2] _Rep. Evol. Ctee. R. S._, IV, 1908, p. 38.
[3] _Ber. Deut. Bot. Ges._, 1908, XXVI, _a_, p. 672.
[4] _Jour. Genetics_, 1, 1910, p. 57.
[5] In Rep. 1 to Evol. Committee, 1902, p. 132, attention was called to
this possibility, though of course at that date it was in sexual animals
alone that it was supposed to exist. It had not occurred to me that even
a hermaphrodite plant might be in this condition.
[6] From the description of the offspring of _muricata_ used as mother.
[7] de Vries, _Species and Varieties_, 1905, p. 259.
[8] Zeijlstra in a recent paper announces that many _nanella_ plants
are the subject of a bacterial disease to which he attributes their
dwarfness. I gather that this does not apply to all _nanella_ plants
and that some are dwarfs apart from disease. The matter may no doubt be
further complicated from this cause.
[9] _Zts. f. Abstamm._, 1912, VIII.
[10] _Arch. f. Zellforschung_, 1912, IX, p. 331.