The theory of descent is founded mainly on comparative studies, which
have the advantage of affording a broad base and the convincing effect
of concurrent evidence brought together from widely different sources.
The theory of mutation on the other hand rests directly upon
experimental investigations, and facts concerning the actual descent of
one form from another are as yet exceedingly rare. It is always
difficult to estimate the validity of conclusions drawn from isolated
instances selected from the whole range of contingent phenomena, and
this is especially true of the present case. Systematic and physiologic
facts seem to indicate the existence of universal laws, and it is not
probable that the process of production of new species would be
different in the various parts of the animal and vegetable kingdoms.
Moreover the principle of unit-characters, the preeminent significance
of which has come to be more fully recognized of late, is in full
harmony [659] with the theory of sudden mutations. Together these two
conceptions go to strengthen the probability of the sudden origin of all
specific characters.
Experimental researches are limited in their extent, and the number of
cases of direct observation of the process of mutation will probably
never become large enough to cover the whole field of the theory of
descent. Therefore it will always be necessary to show that the
similarity between observed and other cases is such as to lift above all
doubt the assertion of their resulting from the same causes.
Besides the direct comparison of the mutations described in our former
lectures, with the analogous cases of the horticultural and natural
production of species and varieties at large, another way is open to
obtain the required proof. It is the study of the phenomena, designated
by Casimir de Candolle by the name of taxonomic anomalies. It is the
assertion that characters, which are specific in one case, may be
observed to arise as anomalies or as varieties in other instances. If
they can be shown to be identical or nearly so in both, it is obviously
allowable to assume the same origin for the specific character and for
the anomaly. In other terms, the specific marks may be considered as
having originated according to the laws [660] that govern the production
of anomalies, and we may assume them to lie within reach of our
experiments. The experimental treatment of the origin of species may
also be looked upon as a method within our grasp.
The validity and the significance of these considerations will at once
become clear, if we choose a definite example. The broadest and most
convincing one appears to me to be afforded by the cohesion of the
petals in gamopetalous flowers. According to the current views the
families with the petals of their flowers united are regarded as one or
two main branches of the whole pedigree of the vegetable kingdom.
Eichler and others assume them to constitute one branch, and therefore
one large subdivision of the system. Bessey, on the other hand, has
shown the probability of a separate origin for those groups which have
inferior ovaries. Apart from such divergencies the connation of the
petals is universally recognized as one of the most important systematic
characters.
How may this character have originated? The heath-family or the
Ericaceae and their nearest allies are usually considered to be the
lowest of the gamopetalous plants. In them the cohesion of the petals is
still subject to reversionary exceptions. Such cases of atavism may
[661] be observed either as specific marks, or in the way of anomalies.
_Ledum_, _Monotropa_ and _Pyrola_, or the Labrador tea, the Indian pipe
and wintergreen are instances of reversionary gamopetalism with free
petals. In heaths (_Erica Tetralix_) and in rhododendrons the same
deviation is observed to occur from time to time as an anomaly, and even
the common _Rhododendron ponticum_ of our gardens has a variety in which
the corolla is more or less split. Sometimes it exhibits five free
petals, while at other times only one or two are entirely free, the
remaining four being incompletely loosened.
Such cases of atavism make it probable that the coherence of the petals
has originally arisen by the same method, but by action in the opposite
direction. The direct proof of this conclusion is afforded by a curious
observation, made by Vilmorin upon the bright and large-flowered
garden-poppy, _Papaver bracteatum_. Like all poppies it has four petals,
which are free from one another. In the fields of Messrs. Vilmorin,
where it is largely cultivated for its seeds, individuals occur from
time to time which are anomalous in this respect. They exhibit a
tendency to produce connate petals. Their flowers become monopetalous,
and the whole strain is designated by the name of _Papaver_ [662]
_bracteatum monopetalum_. Henry de Vilmorin had the kindness to send me
some of these plants, and they have flowered in my garden during several
years. The anomaly is highly variable. Some flowers are quite normal,
exhibiting no sign of connation; others are wholly gamopetalous, the
four petals being united from their base to the very margin of the cup
formed. In consequence of the broadness of the petals however, this cup
is so wide as to be very shallow.
Intermediate states occur, and not infrequently. Sometimes only two or
three petals are united, or the connation does not extend the entire
length of the petals. These cases are quite analogous to the imperfect
splitting of the corolla of the rhododendron. Giving free rein to our
imagination, we may for a moment assume the possibility of a new
subdivision of the vegetable kingdom, arising from Vilmorin's poppy and
having gamopetalous flowers for its chief character. If the character
became fixed, so as to lose its present state of variability, such a
group of supposititious gamopetalous plants might be quite analogous to
the corresponding real gamopetalous families. Hence there can be no
objection to the view, that the heaths have arisen in an analogous
manner from their polypetalous ancestors. Other species of [663] the
same genus have also been recorded to produce gamopetalous flowers, as
for instance, _Papaver hybridum_, by Hoffmann. Poppies are not the sole
example of accidental gamopetaly. Linnaeus observed the same deviation
long ago for _Saponaria officinalis_, and since, it has been seen in
_Clematis Vitalba_ by Jaeger, in _Peltaria alliacea_ by Schimper, in
_Silene annulata_ by Boreau and in other instances. No doubt it is not
at all of rare occurrence, and the origin of the present gamopetalous
families is to be considered as nothing extraordinary. It is, as a
matter of fact, remarkable that it has not taken place in more numerous
instances, and the mallows show that such opportunities have been
available at least more than once.
Other instances of taxonomic anomalies are afforded by leaves. Many
genera, the species of which mainly bear pinnate or palmate leaves, have
stray types with undivided leaves. Among the brambles, _Rubus odoratus_
and _R. flexuosus_ may be cited, among the aralias, _Aralia crassifolia_
and _A. papyrifera_, and among the jasmines, the deliciously scented
sambac (_Jasminum Sambac_). But the most curious instance is that of the
telegraph-plant, or _Desmodium gyrans_, each complete leaf of which
consists of a large terminal leaflet and two little lateral ones. These
latter keep up, [664] night and day, an irregular jerking movement,
which has been compared to the movements of a semaphore. _Desmodium_ is
a papilionaceous plant and closely allied to the genus _Hedysarum_,
which has pinnate leaves with numerous pairs of leaflets. Its place in
the system leaves no doubt concerning its origin from pinnate-leaved
ancestors. At the time of its origination its leaves must have become
reduced as to the number of the blades, while the size of the terminal
leaflet was correspondingly increased.
It might seem difficult to imagine this great change taking place
suddenly. However, we are compelled to familiarize ourselves with such
hypothetical assumptions. Strange as they may seem to those who are
accustomed to the conception of continuous slow improvements, they are
nevertheless in complete agreement with what really occurs. Fortunately
the direct proof of this assertion can be given, and in a case which is
narrowly related, and quite parallel to that of the _Desmodium_, since
it affects a plant of the same family. It is the case of the
monophyllous variety of the bastard-acacia or _Robinia Pseud-Acacia_. In
a previous lecture we have seen that it originated suddenly in a French
nursery in the year 1855. It can be propagated by seed, and exhibits a
curious degree [665] of variability of its leaves. In some instances
these are one-bladed, the blade reaching a length of 15 cm., and hardly
resembling those of the common bastard-acacia. Other leaves produce one
or two small leaflets at the base of the large terminal one, and by this
contrivance are seen to be very similar to those of the _Desmodium_,
repeating its chief characters nearly exactly, and only differing
somewhat in the relative size of the various parts. Lastly real
intermediates are seen between the monophyllous and the pinnate types.
As far as I have been able to ascertain, these are produced on weak
twigs under unfavorable conditions; the size of the terminal leaflet
decreases and the number of the lateral blades increases, showing
thereby the presence of the original pinnate type in a latent condition.
The sudden origin of this "one-leaved" acacia in a nursery may be taken
as a prototype of the ancient origin of _Desmodium_. Of course the
comparison only relates to a single character, and the movements of the
leaflets are not affected by it. But the monophylly, or rather the size
of the terminal blade and the reduction of the lateral ones, may be held
to be sufficiently illustrated by the bastard-acacia. It is worth while
to state, that analogous varieties have also arisen in other genera. The
"one-leaved" [666] strawberry has already been referred to. It
originated from the ordinary type in Norway and at Paris. The walnut
likewise, has its monophyllous variety. It was mentioned for the first
time as a cultivated tree about 1864, but its origin is unknown. A
similar variety of the walnut, with "one-bladed" leaves but of varying
shapes, was found wild in a forest near Dieppe in France some years ago,
and appeared to be due to a sudden mutation.
Something more is known concerning the "one-bladed" ashes, varieties of
which are often seen in our parks and gardens. The common form has broad
and deeply serrate leaves, which are far more rounded than the leaflets
of the ordinary ash. The majority of the leaves are simple, but some
produce one or two smaller leaflets at their base, closely corresponding
in this respect to the variations of the "one-bladed" bastard-acacia,
and evidently indicating the same latent and atavistic character. In
some instances this analogy goes still further, and incompletely pinnate
leaves are produced with two or more pairs of leaflets. Besides this
variable type another has been described by Willdenow. It has single
leaves exclusively, never producing smaller lateral leaflets, and it is
said to be absolutely constant from seed, while the more variable types
[667] seem to be also more inconstant when propagated sexually. The
difference is so striking and affords such a reliable feature that Koch
proposed to make two distinct varieties of them, calling the pure type
_Fraxinus excelsior monophylla_, and the varying trees _F. excelsior
exheterophylla_. Some writers, and among them Willdenow, have preferred
to separate the "one-leaved" forms from the species, and to call them
_Fraxinus simplicifolia_.
According to Smith and to Loudon, the "one-leaved" ashes are found wild
in different districts in-England. Intermediate forms have not been
recorded from these localities. This mode of origin is that already
detailed for the laciniate varieties of alders and so many other trees.
Hence it may be assumed that the "one-leaved" ashes have sprung suddenly
but frequently from the original pinnate species. The pure type of
Willdenow should, in this case, be considered as due to a slightly
different mutation, perhaps as a pure retrograde variety, while the
varying strains may only be eversporting forms. This would likewise
explain part of their observed inconstancy.
In this respect the historic dates, as collected by Korshinsky, are not
very convincing. Vicinism has of course, almost never been excluded, and
part of the multiformity of the offspring [668] must obviously be due to
this most universal agency. Indirect vicinism also plays some part, and
probably affords the explanation of some reputed mutative productions of
the variety. So, for instance, in the case of Sinning, who after sowing
the seeds of the common ash, got as large a proportion as 2% of
monophyllous trees in a culture of some thousand plants. It is probable
that his seeds were taken partly from normal plants, and partly from
hybrids between the normal and the "one-bladed" type, assuming that
these hybrids have pinnate leaves like their specific parent, and bear
the characters of the other parent only in a latent condition.
Our third example relates to peltate leaves. They have the stalk
inserted in the middle of the blade, a contrivance produced by the
connation of the two basal lobes. The water-lilies are a well known
instance, exhibiting sagittate leaves in the juvenile stage and changing
in many species, into nearly circular peltate forms, of which _Victoria
regia_ is a very good example, although its younger stages do not always
excite all the interest they deserve. The Indian cress (_Tropaeolum_),
the marsh pennywort or _Hydrocotyle_, and many other instances could be
quoted. Sometimes the peltate leaves are not at all orbicular, but are
elongated, oblong or elliptic, and with only the lobes [669] at the base
united. The lemon-scented _Eucalyptus citriodora_ is one of the most
widely known cases. In other instances the peltate leaves become more or
less hollow, constituting broad ascidia as in the case of the
crassulaceous genus _Umbilicus_.
This connation of the basal lobes is universally considered as a good
and normal specific character. Nevertheless it has its manifest analogy
in the realm of the anomalies. This is the pitcher or ascidium. On some
trees it is of quite common occurrence, as on the lime-tree (_Tilia
parvifolia_) and the magnolia (_Magnolia obovata_ and its hybrids). It
is probable that both these forms have varieties with, and others
without, ascidia. Of the lime-tree, instances are known of single trees
which produce hundreds of such anomalous leaves yearly, and one such a
tree is growing in the neighborhood of Amsterdam at Lage Vuursche. I
have alluded to these cases more than once, but on this occasion a
closer inspection of the structure of the ascidium is required. For this
purpose we may take the lime-tree as an example. Take the shape of the
normal leaves in the first place. These are cordate at their base and
mainly inequilateral, but the general shape varies to a considerable
extent. This variation is closely related to the position of the leaves
on the twigs, and shows [670] distinct indications of complying with the
general law of periodicity. The first leaves are smaller, with more
rounded lobes, the subsequent leaves attain a larger size, and their
lobes slightly change their forms. In the first leaves the lobes are so
broad as to touch one another along a large part of their margins, but
in organs formed later this contact gradually diminishes and the typical
leaves have the lobes widely separated. Now it is easily understood that
the contact or the separation of the lobes must play a part in the
construction of the ascidia, as soon as the margins grow together.
Leaves which touch one-another, may be affected by the connation without
any further malformation. They remain flat, become peltate and exhibit a
shape which in some way holds a middle position between the pennyworts
and the lemon-scented eucalyptus. Here we have the repetition of the
specific characters of these plants by the anomaly of another. Whenever
the margins are not in contact, and become connate, notwithstanding
their separation, the blade must be folded together in some slight
degree, in order to produce the required contact. This is the origin of
the ascidium. It is quite superfluous to insist upon the fact that their
width or narrowness must depend upon the corresponding normal form. The
more distant the [671] lobes, the deeper the ascidium will become. It
should be added that this explanation of the different shapes of ascidia
is of general validity.
Ascidia of the snake-plantain or _Plantago lanceolata_ are narrow tubes,
because the leaves are oblong or lanceolate, while those of the broad
leaved species of arrowhead, as for instance, the _Sagittaria japonica_,
are of a conical shape.
From the evidence of the lime-tree we may conclude that normal peltate
leaves may have originated in the same way. And from the fact that
pitchers are one of the most frequent anomalies, we may conclude that
the chance of producing peltate leaves must have been a very great one,
and wholly sufficient to account for all observed cases. In every
instance the previously existing shape of the leaf must have decided
whether peltate or pitcher-like leaves would be formed. As far as we can
judge peltate anomalies are quite uninjurious, while ascidia are forms
which must impede the effect of the light on the leaf, as they conceal
quite an important part of the upper surface. In this way it is easily
conceivable that peltate leaves are a frequent specific character, while
ascidia are not, as they only appear in the special cases of limited
adaptation, as in the instances of the so called pitcher-plants. The
genera _Nepenthes_, [672] _Sarracenia_ and some others are very well
known and perhaps even the bladderworts or _Utricularia_ might be
included here.
The reproduction of specific characters by anomalous ascidia is not at
all limited to the general case as described above. More minute details
may be seen to be duplicated in the same way. Proofs are afforded on one
side by incomplete ascidia, and on the other by the double cups.
Incomplete ascidia are those of the _Nepenthes_. The leaf is divided
into three parts, a blade, a tendril and the pitcher. Or in other words,
the limb produces a tendril at its summit, by means of which the plant
is enabled to fasten itself to surrounding shrubs and to climb between
their branches. But the end of this tendril bears a well-formed urn,
which however, is produced only after the revolving and grasping
movements of the tendril have been made. Some species have more rounded
and some more elongated ascidia and often the shape is seen to change
with the development of the stem. The mouth of the urn is strengthened
by a thick rim and covered with a lid. Numerous curious contrivances in
these structures to catch ants and other insects have been described,
but as they have no relation to our present discussion, we shall abstain
from dealing with them. [673] Likewise we must refrain from a
consideration of the physiologic qualities of the tendril, and confine
our attention to the combination of a limb, a naked midvein and an
ascidium. This combination is to be the basis of our discussion. It is
liable to be produced all of a sudden. This assertion is proved by its
occurrence as a varietal mark in one of our most ordinary cultivated
plants. It is the group known as _Croton_, belonging to the genus
_Codiaeum_. A variety is called _interruptum_ and another
_appendiculatum_, and these names both relate to the interruption of the
leaves by a naked midvein. The leaves are seen to be built up of three
parts. The lower half retains the aspect of a limb; it is crowned by a
vein without lateral nerves or blade-like expansions, and this stalk in
its turn bears a short limb on its summit. The base of this apical limb
exhibits two connate lobes, forming together a wide cup or ascidium. It
should be stated that these _interruptum_ varieties are highly variable,
especially in the relative size of the three principal parts of the
leaf. Though it is of course conceded that the ascidium of _Nepenthes_
has many secondary devices which are lacking in _Croton_, it seems
hardly allowable to deny the possibility of an analogous origin for
both. Those of the _Croton_, according to our knowledge regarding
similar cases, must [674] have arisen at once, and hence the conclusion
that the ascidia of _Nepenthes_ are also originally due to a sudden
mutation. Interrupted leaves, with an ascidium on a naked prolongation
of the midvein, are by no means limited to the _Croton_ varieties. As
stray anomalies they have often been observed, and I myself had the
opportunity of collecting them on magnolia, on clover and on some other
species. They are additional evidence in support of the explanation
given above.
In the same way double ascidia may be made use of to explain the foliar
cups of the teasels and some other plants, as for instance, some
European snakeroots (_Eryngium maritimum_ and _E. campestre_), or the
floral leaves of the honeysuckle. The leaves on the stems of the teasels
are disposed in pairs, and the bases of the two leaves of each pair are
connate so as to constitute large cups. We have already mentioned these
cups, and recall them in the present connection to use them as a
prototype of the double ascidia. These are constituted of two opposite
leaves, accidentally connated at their base or along some part of their
margins. If the leaves are sessile, the analogy with the teasels is
complete, as shown, for instance, in a case of _Cotyledon_, a
crassulaceous plant which is [675] known to produce such cups from time
to time. They are narrower than those of the teasel, but this depends,
as we have seen for the "one-leaved" ascidium, on the shape of the
original leaf. In other respects they exactly imitate the teasel cups
showing thereby how these cups may probably have originated.
In numerous cases of anomalies some accidental structures are parallel
to specific characters, while others are not, being obviously injurious
to their bearers. So it is also with the double ascidia. In the case of
stalked leaves the two opposite stalks must, of course, constitute a
long and very narrow tube, when growing together. This tube must bear at
its summit the conical ascidium produced by the two connate limbs. At
its base however, it includes the terminal bud of the stem, and
frequently the tube is so narrow as to impede its further development.
By this contrivance the double ascidium assumes a terminal position.
Instances have been observed on magnolia, in _Boehmeria_ and in other
cases.
Flowers on leaves are of rare occurrence. Notwithstanding this, they
constitute specific characters in some instances, accidental anomalies
in others. _Helwingia rusciflora flora_ is the most curious and best
known instance. It is a little shrub, belonging to the Cornaceae, and
[676] has broad elliptical undivided leaves. On the middle of the
midvein these leaves are seen to bear small clusters of flowers; indeed
this is the only place where flowers are produced. Each cluster has from
13-15 flowers, of which some are staminate and borne on stalks, while
others are pistillate and nearly sessile. These flowers are small and of
a pale greenish color and yield small stone-fruits, with a thin coating
of pulpy tissue. As the name indicates, this mode of flowering is
closely similar to that of _Ruscus_, which however, does not bear its
flowers and berries on real leaves, but on leaflike expansions of the
twigs. _Phyllonoma ruscifolia_, a saxifragaceous plant, bears the same
specific name, indicating a similar origin of the flowers. Other
instances have been collected by Casimir de Candolle, but their number
is very small.
As a varietal mark, flowers on leaves likewise rarely occur. One
instance however, is very remarkable, and we have already dealt with it,
when treating of constant varieties, and of the lack of vicinism in the
case of species with exclusive self-fertilization.
It is the "Nepaul-barley" or _Hordeum trifurcatum_. The leaves, which in
this case bear the adventitious flowers, are the inner scales of the
spikelets, and not on green leaves as in the [677] cases already alluded
to. But this of course makes no real difference. The character is
variable to a high degree, and this fact indicates its varietal nature,
though it should be recalled that at least with the _Helwingia_, the
majority of the leaves are destitute of flowers, and that in this way
some degree of variability is present in this normal case too.
All in all there are three sorts of "Nepaul-barley." They have the same
varietal mark, but belong to different species of barley. These are
differentiated according to the number of the rows in which the grains
are seen on the spikes. These numbers may be two, four or six, giving
rise to the specific names of _Hordeum distichum_, _tetrastichum_ and
_hexastichum_. Whether these three varieties are of independent, but
parallel origin, or are to be considered as due to a single mutation and
subsequent crosses is not known, all of them being of ancient origin.
Historic evidence concerning their birth is wholly wanting. From analogy
it would seem probable that the character had arisen by a mutation in
one of the three named species, and had been transferred to others by
means of accidental crosses, even as it has been artificially
transmitted of late to quite a number of other sorts. But however
admissible this conception may seem, there is of course no real
objection [678] to the assumption of independent and parallel mutations.
For the purpose of a comparison with the _Helwingia_ type we are
however, not at all concerned with the species to which the
_trifurcatum_ variety belongs, but only with the varietal mark itself.
The spikelets may be one-, two- or three-flowered, according to the
species. If we choose for further consideration the _hexastichum_ type,
each spikelet produces three normal flowers and afterwards three normal
grains. Morphologically however, the spikelet is not homologous to those
parts of other grasses which have the same name. It is constituted of
three real spikelets, and thus deserves the name of a triple
construction. Each of these three little organs has its normal pair of
outer scales or glumae. These are linear and short, ending in a long and
narrow spine. Those of the middle-most spikelets stand on its outer
side, while those of the lateral part are placed transversely. In this
way they form a kind of involucre around the central parts. The latter
consist of the inner and outer palets or scales, each two of which
include one of the flowers. The outer palet is to be considered as the
metamorphosed leaf, in the aril of which the flower is produced. In the
common sorts of barley it bears a long awn, giving thereby its typical
aspect to the [679] whole spike. The axillary flower is protected on the
opposite side by a two-keeled inner palet. Each flower exhibits three
stamens and an ovary. In the six-rowed barley all the three flowers of a
triple spikelet are fertile, and each of them has a long awn on the top
of the outer palet. But in the two-rowed species only the middle-most
flower is normal and has an awn, the two remaining being sterile and
more or less rudimentary and with only very short awns. From this
description it is easily seen that the species of barley may be
distinguished from one another, even at a casual glance, by the number
of the rows of the awns, and therefore by the shape of the entire
spikes. This striking feature, however, does not exist in the
"Nepaul-barley." The awns are replaced by curiously shaped appendices,
which are three-lobed. The central lobe is oblong and hollow, and forms
a kind of hood, which covers a small supernumerary floret. The two
lateral lobes are narrower, often linear and extended into a smaller or
longer awn. These awns are mostly turned away from the center of the
spike. The central lobe may sometimes bear two small florets, but
ordinarily only one is to be found, and this is often incomplete, having
only one or two stamens, or is different in some other way. [680] These
narrow lateral lobes heighten the abnormal aspect of the whole spike.
They are only produced at a somewhat advanced stage of the development
of the palet, are united to one another and to the central part by
strong veins, which form transversal anastomoses at their insertion. The
length of these awns is very variable, and this quality is perhaps the
most striking of the whole variety. Often they reach only 1-2 mm., or
the majority may become longer and attain even 1 cm., while here and
there, between them, longer ones are inserted, extending in some
instances even as far as 3 cm. from the spike. Their transverse position
in such cases is strikingly contrasted with the ordinary erect type of
the awns.
These lateral lobes are to be regarded, from the morphologic point of
view, as differentiated parts of the blade of the leaf. Before they are
formed, or coincidently with the beginning of their development, the
summit of the central lobe becomes hollow, and the development of the
supernumerary flower commences. In different varieties, and especially
in the most recent crosses of them, this development is excessively
variable.
The accidental flower arises at some distance beneath the summit of the
scale, on its middle [681] vein. The development begins with the
protrusion of a little scale, and the flower itself is situated beneath
this scale, and is to be protected by it and by the primary scale, but
is turned upside down at the same time. Opposite to this organ, which
represents the outer palet of the adventitious flower, two little
swollen bodies are evolved. In the normal flowers of barley and other
grains and grasses their function is to open the flowers by swelling,
and afterwards collapse and allow them to close.
In the adventitious flowers of the "Nepaul-barley," however, this
function is quite superfluous. The stamens occur in varying numbers;
typically there are three, but not rarely less, or more, are seen. In
some instances the complete double whorl of six, corresponding to the
ancestral monocotyledonous type, has been found. This is a very curious
case of systematic atavism, quite analogous to the _Iris pallida
abavia_, previously alluded to, which likewise has six stamens, and to
the cases given in a previous lecture. But for our present discussion it
is of no further interest. The ovary is situated in the middle of the
flower, and in some instances two have been observed. This is also to be
considered as a case of atavism.
All these parts of the adventitious flower are more or less subject to
arrest of development, [682] in a later stage. They may even sometimes
become abnormal. Stamens may unite into pairs, or carpels bear four
stigmas. The pollen-sacs are as a rule barren, the mother-cells
undergoing atrophy, while normal grains are seen but rarely. Likewise
the ovaries are rudimentary, but Wittmack has observed the occasional
production of ripe grains from these abnormal florets.
The scale is seldom seen to extend any farther upwards than the
supernumerary flower. But in the rare instances where it does prolong
its growth, it may repeat the abnormality and bear a second floret above
the first. This of course is generally much weaker, and more
rudimentary.
Raciborsky, who has lately given a full and very accurate description of
this anomaly, lays great stress upon the fact that it is quite useless.
It is perhaps the most obviously useless structure in the whole
vegetable kingdom. Notwithstanding this, it has come to be as completely
hereditary as any of the most beautiful adaptations in nature. Therefore
it is one of the most serious objections to the hypothesis of slow and
gradual improvements on the sole ground of their usefulness. The
struggle for life and natural selection are manifestly inadequate to
give even the slightest indication of [683] an explanation of this case.
It is simply impossible to imagine the causes that might have produced
such a character. The only way out of this difficulty is to assume that
it has arisen at once, in its present apparently differentiated and very
variable condition, and that, being quite uninjurious and since it does
not decrease the fertility of the race, it has never been subjected to
natural selection, and so has saved itself from destruction.
But if we once grant the probability of the origin of the
"Nepaul-barley" by a sudden mutation, we obviously must assume the same
in the case of the _Helwingia_ and other normal instances. In this way
we gain a further support for our assertion, that even the strangest
specific characters may have arisen suddenly.
After having detailed at some length those proofs which seem to be the
most striking, and which had not been previously described with
sufficient detail, we may now take a hasty survey of other contingent
cases. In the first place the cruciate flowers of some onagraceous
plants should be remembered. Small linear petals occur as a specific
character in _Oenothera cruciata_ of the Adirondacks, but have been seen
to arise as sudden mutations in the common evening-primrose (_O.
biennis_) in Holland, and in the willow-herb (_Epilobium hirsutum_) in
England. [684] Leaves placed in whorls of three are very rare. The
oleander, juniper and some few other plants have ternate whorls as a
specific character. As an anomaly, ternate whorls are far more common,
and perhaps any plant with opposite leaves may from time to time produce
them. Races rich in this abnormality are found in the wild state in the
yellow loosestrife or _Lysimachia vulgaris_, in which it is a very
variable specific character, the whorls varying from two to four leaves.
In the cultivated state it is met with in the myrtle or _Myrtus
communis_, where it has come to be of some importance in Israelitic
ritual. Crisped leaves are known in a mallow, _Malva crispa_, and as a
variety in cabbages, parsley, lettuce and others. The orbicular fruits
of Heeger's shepherd's purse (_Capsella heegeri_) recall similar fruits
of other cruciferous genera, as for instance, _Camelina_. Screw-like
stems with wide spirals are specific in the flower-stalks of _Cyclamen_
and _Vallisneria_, varietal in _Juncus effusus spiralis_ and accidental
in _Scirpus lacustris_. Dormant buds or small bulbs in inflorescences
are normal for wild onions, _Polygonum viviparum_ and others, varietal
in _Poa alpina vivipara_ and perhaps in _Agave vivipara_, and accidental
in plantains (_Plantago lanceolata_), _Saxifraga umbrosa_ and others.
[685] Cleft leaves, one of the most general anomalies, are typical in
_Boehmeria biloba_. The adnation of the peduncles of the inflorescences
to the stem is typical in _Solanum_ and accidental in many other cases.
It seems quite superfluous to add further proof. It is a very general
phenomenon that specific characters occur in other genera as anomalies,
and under such circumstances that the idea of a slow evolution on the
ground of utility is absolutely excluded. No other explanation remains
than that of a sudden mutation, and once granted for the abnormal cases,
this explanation must obviously likewise be granted for the analogous
specific characters.
Our whole discussion shows that mutations, once observed in definite
instances, afford the most probable basis for the explanation of
specific characters at large.
[686]
LECTURE XXIV