New species and varieties occur from time to time in the wild state.
Setting aside all theoretical conceptions as to the common origin of
species at large, the undoubted fact remains that new forms are
sometimes met with. In the case of the peloric toad-flax the mutations
are so numerous that they seem to be quite regular. The production of
new species of evening-primroses was observed on the field and
afterwards duplicated in the garden. There is no reason to think that
these cases are isolated instances. Quite on the contrary they seem to
be the prototypes of repeated occurrences in nature.
If this conception is granted, the question at once arises, how are we
to deal with analogous cases, when fortune offers them, and what can we
expect to learn from them?
A critical study of the existing evidence seems to be of great
importance in order to ascertain the best way of dealing with new facts,
and of estimating the value of the factors concerned. [577] It is
manifest that we must be very careful and conservative in dealing with
new facts that are brought to our attention, and every effort should be
made to bring additional evidence to light. Many vegetable anomalies are
so rare that they are met with only by the purest chance, and are then
believed to be wholly new. When a white variety of some common plant is
met with for the first time we generally assume that it originated on
that very spot and only a short time previously. The discovery of a
second locality for the same variety at once raises the question as to a
common origin in the two instances. Could not the plants of the second
locality have arisen from seeds transported from the first?
White varieties of many species of blue-bells and gentians are found not
rarely, white-flowering plants of heather, both of _Erica Tetralix_ and
_Calluna vulgaris_ occur on European heaths; white flowers of _Brunella
vulgaris_, _Ononis repens_, _Thymus vulgaris_ and others may be seen in
many localities in the habitats of the colored species. Pelories of
labiates seem to occur often in Austria, but are rare in Holland; white
bilberries (_Vaccinium Myrtillus_) have many known localities throughout
Europe, and nearly all the berry-bearing species in the large heath
family are recorded as having white varieties.
[578] Are we to assume a single origin for all the representatives of
such a variety, as we have done customarily for all the representatives
of a wild species? Or can the same mutation have been repeated at
different times and in distant localities? If a distinct mutation from a
given species is once possible, why should it not occur twice or thrice?
A variety which seems to be new to us may only appear so, because the
spot where it grows had hitherto escaped observation. _Lychnis preslii_
is a smooth variety of _Lychnis diurna_ and was observed for the first
time in the year 1842 by Sekera. It grew abundantly in a grove near
Munchengratz in southern Hungary. It was accompanied by the ordinary
hairy type of the species. Since then it has been observed to be quite
constant in the same locality, and some specimens have been collected
for me there lately by Dr. Nemec, of Prague. No other native localities
of this variety have been discovered, and there can be no doubt that it
must have arisen from the ordinary campion near the spot where it still
grows. But this change may have taken place some years before the first
discovery, or perhaps one or more centuries ago. This could only be
known if it could be proved that the locality had been satisfactorily
investigated previously, and that the variety had not [579] been met
with. Even in this case only something would be discovered about the
time of the change, but nothing about its real nature.
So it is in many cases. If a variety is observed in a number of
specimens at the time of its first discovery, and at a locality not
studied previously, it takes the aspect of an old form of limited
distribution, and little can be learned as to the circumstances under
which it arose. If on the contrary it occurs in very small numbers or
perhaps even in a single individual, and if the spot where it is found
is located so that it could hardly have escaped previous observation,
then the presumption of a recent origin seems justified.
What has to be ascertained on such occasions to give them scientific
value? Three points strike me as being of the highest importance. First,
the constancy of the new type; secondly, the occurrence or lack of
intermediates, and last, but not least, the direct observation of a
repeated production.
The first two points are easily ascertained. Whether the new type is
linked with its more common supposed ancestor by intermediate steps is a
query which at once strikes the botanist. It is usually recorded in such
cases, and we may state at once that the general result is, that such
intermediates do not occur. This is [580] of the highest importance and
admits of only two explanations. One is that intermediates may be
assumed to have preceded the existent developed form, and to have died
out afterwards. But why should they have done so, especially in cases of
recent changes? On the other hand the intermediates may be lacking
because they have never existed, the change having taken place by a
sudden leap, such as the mutations described in our former lectures. It
is manifest that the assumption of hypothetical intermediates could only
gain some probability if they had been found in some instance. Since
they do not occur, the hypothesis seems wholly unsupported.
The second point is the constancy of the new type. Seeds should be saved
and sown. If the plant fertilizes itself without the aid of insects, as
do some evening-primroses, the seed saved from the native locality may
prove wholly pure, and if it does give rise to a uniform progeny the
constancy of the race may be assumed to be proved, provided that
repeated trials do not bring to light any exceptions. If the offspring
shows more than one type, cross-fertilization is always to be looked to
as the most probable cause, and should be excluded, in order to sow pure
seeds. Garden-experiments of this kind, and repeated trials, should
always be combined [581] with the discovery of a presumed mutation. In
many instances the authors have realized the importance of this point,
and new types have been found constant from the very beginning. Many
cases are known which show no reversions and even no partial reversions.
This fact throws a distinct light on our first point, as it makes the
hypothesis of a slow and gradual development still more improbable.
My third point is of quite another nature and has not as yet been dealt
with. But as it appeals to me as the very soul of the problem, it seems
necessary to describe it in some detail. It does not refer to the new
type itself, nor to any of its morphologic or hereditary attributes, but
directly concerns the presumed ancestors themselves.
The peloric toad-flax in my experiment was seen to arise thrice from the
same strain. Three different individuals of my original race showed a
tendency to produce peloric mutations, and they did so in a number of
their seeds, exactly as the mutations of the evening-primroses were
repeated nearly every year. Hence the inference, that whenever we find a
novelty which is really of very recent date, the parent-strain which has
produced it might still be in existence on the same spot. In the case of
shrubs or perennials the very parents might yet be found. [582] But it
seems probable, and is especially proved in the case of the
evening-primroses, that all or the majority of the representatives of
the whole strain have the same tendency to mutate. If this were a
general rule, it would suffice to take some pure seeds from specimens of
the presumed parents and to sow and multiply the individuals to such an
extent that the mutation might have a chance to be repeated.
Unfortunately, this has not as yet been done, but in my opinion it
should be the first effort of any one who has the good luck to discover
a new wild mutation. Specimens of the parents should be transplanted
into a garden and fertilized under isolated conditions. Seeds saved from
the wild plant would have little worth, as they might have been partly
fertilized by the new type itself.
After this somewhat lengthy discussion of the value of observations
surrounding the discovery of new wild mutations, we now come to the
description of some of the more interesting cases. As a first example, I
will take the globular fruited shepherd's purse, described by Solms
Laubach as _Capsella heegeri_. Professor Heeger discovered one plant
with deviating fruits, in a group of common shepherd's purses in the
market-place near Landau in Germany, in the fall of 1897. They were
nearly spherical, [583] instead of flat and purse-shaped. Their valves
were thick and fleshy, while those of the ordinary form are
membranaceous and dry. The capsules hardly opened and therefore differed
in this point from the shepherd's purse, which readily loosens both its
valves as soon as it is ripe.
Only one plant was observed; whence it came could not be determined, nor
whether it had arisen from the neighboring stock of C_apsella_ or not.
The discoverer took some seed to his garden and sent some to the
botanical garden at Strassburg, of which Solms-Laubach is the director.
The majority of the seeds of course were sowed naturally on the original
spot. The following year some of the seeds germinated and repeated the
novelty. The leaves, stems and flowers were those of the common
shepherd's purse, but no decision could be reached concerning the type
of this generation before the first flowers had faded and the rounded
capsules had developed. Then it was seen that the _heegeri_ came true
from seed. It did so both in the gardens and on the market-place, where
it was observed to have multiplied and spread in some small measure. The
same was noted the following year, but then the place was covered with
gravel and all the plants destroyed. It is not recorded to have been
seen wild since.
[584] Intermediate forms have not been met with. Some slight reversions
may occur in the autumn on the smallest and weakest lateral branches.
Such reversions, however, seem to be very rare, as I have tried in vain
to produce them on large and richly branched individuals, by applying
all possible inducements in the form of manure and of cutting, to
stimulate the production of successive generations of weaker side
branches.
This constancy was proved by the experiments of Solms-Laubach, which I
have repeated in my own garden during several years with seed received
from him. No atavists or deviating specimens have been found among many
hundreds of flowering plants.
It is important to note that within the family of the crucifers the form
of the capsule and the attributes of the valves and seeds are usually
considered to furnish the characteristics of genera, and this point has
been elucidated at some length by Solms-Laubach. There is, however, no
sufficient reason to construe a new genus on the ground of Heeger's
globular fruited shepherd's purse; but as a true elementary species, and
even as a good systematic species it has proved itself, and as such it
is described by Solms-Laubach, who named it in honor of its discoverer.
Exactly analogous discoveries have been [586] instead of displaying a
bright yellow cup. _O. cruciata_ grows in the Adirondack Mountains, in
the states of New York and Vermont, and seems to be abundant there. It
has been introduced into botanical gardens and yielded a number of
hybrids, especially with _O. biennis and _O. lamarckiana_, and the
narrow petals of the parent-species may be met with in combination with
the stature and vegetative characteristics of these last named species.
_O. cruciata_ has a purple foliage, while _biennis_ and _lamarckiana_
are green, and many of the hybrids may instantly be recognized by their
purple color.
The curious attribute of the petals is not to be considered simply as a
reduction in size. On anatomical inquiry it has been found that these
narrow petals bear some characteristics which, on the normal plants, are
limited to the calyx. Stomata and hairs, and the whole structure of the
surface and inner tissues on some parts of these petals are exactly
similar to those of the calyx, while on others they have retained the
characteristics of petals. Sometimes there may even be seen by the naked
eye green longitudinal stripes of calyx-like structure alternating with
bright yellow petaloid parts. For these reasons the cruciata character
may be considered as a case of sepalody of the petals, or of the petals
being partly converted into sepals.
[587] It is worth while to note that as a monstrosity this occurrence is
extremely rare throughout the whole vegetable kingdom, and only very few
instances have been recorded.
Two cases of sudden mutations have come to my knowledge, producing this
same anomaly in allied species. One has been already alluded to; it
pertains to the common evening-primrose or _Oenothera biennis_, and one
is a species belonging to another genus of the same family, the great
hairy willow-herb or _Epilobium _hirsutum_. I propose to designate both
new forms by the varietal name of _cruciata_, or _cruciatum_.
_Oenothera biennis cruciata_ was found in a native locality of the _O.
biennis itself. It consisted of only one plant, showing in all its
flowers the _cruciata_ marks. In all other respects it resembled wholly
the _biennis_, especially in the pure green color of its foliage, which
at once excluded all suspicion of hybrid origin with the purple _O.
cruciata_. Moreover in our country this last occurs only in the
cultivated state in botanical gardens.
Intermediates were not seen, and as the plant bore some pods, it was
possible to test its constancy. I raised about 500 plants from its
seeds, out of which more than 100 flowered in the first year. The others
were partly kept through the winter and flowered next year. Seeds saved
in [588] both seasons were sown on a large scale. Both the first and the
succeeding generations of the offspring of the original plant came true
without any exception. Intermediates are often found in hybrid cultures,
and in them the character is a very variable one, but as yet they were
not met with in progeny of this mutant. All these plants were exactly
like _O. biennis_, with the single exception of their petals.
_Epilobium hirsutum cruciatum_ was discovered by John Rasor near
Woolpit, Bury St. Edmunds, in England. It flowered in one spot,
producing about a dozen stems, among large quantities of the
parent-species, which is very common there, as it is elsewhere in
Europe. This species is a perennial, multiplying itself by underground
runners, and the stems of the new variety were observed to stand so
close to each other that they might be considered as the shoots of one
individual. In this case this specimen might probably be the original
mutant, as the variety had not been seen on that spot in previous years,
even as it has not been found elsewhere in the vicinity.
Intermediates were not observed, though the difference is a very
striking one. In the cruciate flowers the broad and bright purple petals
seem at first sight to be wholly wanting. They are too weak to expand
and to reflex the calyx [589] as in the normal flowers of the species.
The sepals adhere to one another, and are only opened at their summit by
the protruding pistils. Even the stamens hardly come to light. At the
period of full bloom the flowers convey only the idea of closed buds
crowned by the conspicuous white cross of the stigma. Any intermediate
form would have at once betrayed itself by larger colored petals, coming
out of the calyx-sheath. The cruciate petals are small and linear and
greenish, recalling thereby the color of the sepals.
Mr. Rasor having sent me some flowers and some ripe capsules of his
novelty, I sowed the latter in my experimental garden, where the plant
flowered in large numbers and with many thousands of flowers both in
1902 and 1903. All of these plants and all of these flowers repeated the
cruciate type exactly, and not the slightest impurity or tendency to
partial reversion has been observed.
Thus true and constant cruciate varieties have been produced from
accidentally observed initial plants, and because of their very curious
characters they will no doubt be kept in botanical gardens, even if they
should eventually become lost in their native localities.
At this point I might note another observation made on the wild species
of _Oenothera cruciata_ [590] from the Adirondacks. Through the kindness
of Dr. MacDougal, of the New York Botanical Garden, I received seeds
from Sandy Hill near Lake George. When the plants, grown from these
seeds, flowered, they were not a uniform lot, but exhibited two distinct
types. Some had linear petals and thin flower-buds, and in others the
petals were a little broader and the buds more swollen. The difference
was small, but constant on all the flowers, each single plant clearly
belonging to one or the other of the two types. Probably two elementary
species were intermixed here, but whether one is the systematic type and
the other a mutation, remains to be seen.
Nor seem these two types to exhaust the range of variability of
_Oenothera cruciata_. Dr. B.L. Robinson of Cambridge, Mass., had the
kindness to send me seeds from another locality in the same region. The
seeds were collected in New Hampshire and in my garden produced a true
and constant _cruciata_, but with quite different secondary characters
from both the aforesaid varieties. The stems and flower-spikes and even
the whole foliage were much more slender, and the calyx-tubes of the
flowers were noticeably more elongated. It seems not improbable that
_Oenothera cruciata_ includes a group of lesser unities, and may prove
to comprise a [591] swarm of elementary species, while the original
strain might even now be still in a condition of mutability. A close
scrutiny in the native region is likely to reveal many unexpected
features.
A very interesting novelty has already been described in a former
lecture. It is the _Xanthium wootoni_, discovered in the region about
Las Vegas, New Mexico, by T.D.A. Cockerell. It is similar in all
respects to _X. commune_, but the burrs are more slender and the
prickles much less numerous, and mostly stouter at their base. It grows
in the same localities as the _X. commune_, and is not recorded to occur
elsewhere. Whether it is an old variety or a recent mutation it is of
course impossible to decide. In a culture made in my garden from the
seed sent me by Mr. Cockerell, I observed (1903) that both forms had a
subvariety with brownish foliage, and, besides this, one of a pure
green. Possibly this species, too, is still in a mutable condition.
Perhaps the same may be asserted concerning the beautiful shrub,
_Hibiscus Moscheutos_, observed in quite a number of divergent types by
John W. Harshberger. They grew in a small meadow at Seaside Park, New
Jersey, in a locality which had been undisturbed for years. They
differed from each other in nearly all the [592] organs, in size, in the
diameter of the stems, which were woody in some and more fleshy in
others, in the shape of the foliage and in the flowers. More than twenty
types could be distinguished and seeds were saved from a number of them,
in order to ascertain whether they are constant, or whether perhaps a
main stem in a mutating condition might be found among them. If this
should prove to be the case, the relations between the observed forms
would probably be analogous to those between the _O. lamarckiana_ and
its derivatives.
Many other varieties have sprung from the type-species under similar
conditions from time to time. A fern-leaved mercury, _Mercurialis annua
laciniata_, was discovered in the year 1719 by Marchant. The type was
quite new at the time and maintained itself during a series of years.
The yellow deadly nightshade or _Atropa Belladonna lutea_ was found
about 1850 in the Black Forest in Germany in a single spot, and has
since been multiplied by seeds. It is now dispersed in botanical
gardens, and seems to be quite constant. A dwarf variety of a bean,
_Phaseolus lunatus_, was observed to spring from the ordinary type by a
sudden leap about 1895 by W.W. Tracy, and many similar cases could be
given.
The annual habit is not very favorable for [593] the discovery of new
forms in the wild state. New varieties may appear, but may be crowded
out the first year. The chances are much greater with perennials, and
still greater with shrubs or trees. A single aberrant specimen may live
for years and even for centuries, and under such conditions is pretty
sure to be discovered sooner or later. Hence it is no wonder that many
such cases are on record. They have this in common that the original
plant of the variety has been found among a vast majority of
representatives of the corresponding species. Nothing of course is
directly known about its origin. Intermediate links have as a rule been
wanting, and the seeds, which have often been sown, have not yielded
reliable results, as no care was taken to preserve the blossoms from
intercrossing with their parent-forms.
Stress should be laid upon one feature of these curious occurrences.
Relatively often the same novelty has been found twice or thrice, or
even more frequently, and under conditions which make it very improbable
that any relation between such occurrences might exist. The same
mutation must have taken place more than once from the same main stem.
The most interesting of these facts are connected with the origin of the
purple beech, which [594] is now so universally cultivated. I take the
following statements from an interesting historical essay of Prof.
Jaggi. He describes three original localities. One is near the Swiss
village, Buch am Irchel, and is located on the Stammberg. During the
17th century five purple beeches are recorded to have grown on this
spot. Four of them have died, but one is still alive. Seedlings have
germinated around this little group, and have been mostly dug up and
transplanted into neighboring gardens. Nothing is known about the real
origin of these plants, but according to an old document, it seems that
about the year 1190 the purple beeches of Buch were already enjoying
some renown, and attracting large numbers of pilgrims, owing to some old
legend. The church of Embrach is said to have been built in connection
with this legend, and was a goal for pilgrimages during many centuries.
A second native locality of the purple beech is found in a forest near
Sondershausen in Thuringen, Germany, where a fine group of these trees
is to be seen. They were mentioned for the first time in the latter half
of the eighteenth century, but must have been old specimens long before
that time. The third locality seems to be of much later origin. It is a
forest near Roveredo in South Tyrol, where a new [595] university is
being erected. It is only a century ago that the first specimens of the
purple beech were discovered there.
As it is very improbable that the two last named localities should have
received their purple beeches from the first named forest, it seems
reasonable to assume that the variety must have been produced at least
thrice.
The purple beech is now exceedingly common in cultivation. But Jaggi
succeeded in showing that all the plants owe their origin to the
original trees mentioned above, and are, including nearly all cultivated
specimens with the sole exception of the vicinity of Buch, probably
derived from the trees in Thuringen. They are easily multiplied by
grafting, and come true from seed, at least often, and in a high
proportion. Whether the original trees would yield a pure progeny if
fertilized by their own pollen has as yet not been tested. The young
seedlings have purple seed-leaves, and may easily be selected by this
character, but they seem to be always subjected in a large measure to
vicinism.
Many other instances of trees and shrubs, found in accidental specimens
constituting a new variety in the wild state, might be given. The
oak-leaved beech has been found in a forest of Lippe-Detmold in Germany
and near Versailles, [596] whence it was introduced into horticulture by
Carriere. Similarly divided and cleft leaves seem to have occurred more
often in the wild state, and cut-leaved hazels are recorded from Rouen
in France, birches and alders from Sweden and Lapland, where both are
said to have been met with in several forests. The purple barberry was
found about 1830 by Bertin, near Versailles. Weeping varieties of ashes
were found wild in England and in Germany, and broom-like oaks, _Quercus
pedunculata fastigiata_, are recorded from Hessen-Darmstadt, Calabria,
the Pyrenees and other localities. About the real origin of all these
varieties nothing is definitely known.
The "single-leaved" strawberry is a variety often seen in botanical
gardens, as it is easily propagated by its runners. It was discovered
wild in Lapland at the time of Linnaeus, and appeared afterwards
unexpectedly in a nursery near Versailles. This happened about the year
1760 and Duchesne tested it from seeds and found it constant. This
strain, however, seems to have died out before the end of the 18th
century. In a picture painted by Holbein (1495-1543), strawberry leaves
can be seen agreeing exactly with the monophyllous type. The variety may
thus be assumed to have arisen independently [597] at least thrice, at
different periods and in distant localities.
From all these statements and a good many others which can be found in
horticultural and botanical literature, it may be inferred that
mutations are not so very rare in nature as is often supposed. Moreover
we may conclude that it is a general rule that they are neither preceded
nor accompanied by intermediate steps, and that they are ordinarily
constant from seed from the first.
Why then are they not met with more often? In my opinion it is the
struggle for life which is the cause of this apparent rarity; which is
nothing else than the premature death of all the individuals that so
vary from the common type of their species as to be incapable of
development under prevailing circumstances. It is obviously without
consequence whether these deviations are of a fluctuating or of a
mutating nature. Hence we may conclude that useless mutations will soon
die out and will disappear without leaving any progeny. Even if they are
produced again and again by the same strain, but under the same
unfavorable conditions, there will be no appreciable result.
Thousands of mutations may perhaps take place yearly among the plants of
our immediate vicinity without any chance of being discovered. [598] We
are trained to the appreciation of the differentiating marks of
systematic species. When we have succeeded in discerning these as given
by our local flora lists, we rest content. Meeting them again we are in
the habit of greeting them with their proper names. Such is the
satisfaction ensuing from this knowledge that we do not feel any
inclination for further inquiry. Striking deviations, such as many
varietal characters, may be remarked, but then they are considered as
being of only secondary interest. Our minds are turned from the
delicately shaded features which differentiate elementary species.
Even in the native field of the evening-primroses, no botanist would
have discovered the rosettes with smaller or paler leaves, constituting
the first signs of the new species. Only by the guidance of a distinct
theoretical idea were they discovered, and having once been pointed out
a closer inspection soon disclosed their number.
Variability seems to us to be very general, but very limited. The limits
however, are distinctly drawn by the struggle for existence. Of course
the chance for useful mutations is a very small one. We have seen that
the same mutations are as a rule repeated from time to time by the same
species. Now, if a useful mutation, [599] or even a wholly indifferent
one, might easily be produced, it would have been so, long ago, and
would at the present time simply exist as a systematic variety. If
produced anew somewhere the botanist, would take it for the old variety
and would omit to make any inquiry as to its local origin.
Thousands of seeds with perhaps wide circles of variability are ripened
each year, but only those that belong to the existing old narrow circles
survive. How different would Nature appear to us if she were free to
evolve all her potentialities!
Darwin himself was struck with this lack of harmony between common
observations and the probable real state of things. He discussed it in
connection with the cranesbill of the Pyrenees (_Geranium pyrenaicum_).
He described how this fine little plant, which has never been
extensively cultivated, had escaped from a garden in Staffordshire and
had succeeded in multiplying itself so as to occupy a large area.
In doing so it had evidently found place for an uncommonly large number
of plantlets from its seeds and correspondingly it had commenced to vary
in almost all organs and qualities and nearly in all imaginable
directions. It displayed under these exceptional circumstances a
capacity which never had been exceeded and [600] which of course would
have remained concealed if its multiplication had been checked in the
ordinary way.
Many species have had occasion to invade new regions and cover them with
hundreds of thousands of individuals. First are to be cited those
species which have been introduced from America into Europe since the
time of Columbus, or from Europe into this country. Some of them have
become very common. In my own country the evening-primroses and Canada
fleabane or are examples, and many others could be given. They should be
expected to vary under these circumstances in a larger degree. Have they
done so? Manifestly they have not struck out useful new characters that
would enable their bearers to found new elementary species. At least
none have been observed. But poor types might have been produced, and
periods of mutability might have been gone through similar to that which
is now under observation for Lamarck's evening primrose in Holland.
From this discussion we may infer that the chances of discovering new
mutating species are great enough to justify the utmost efforts to
secure them. It is only necessary to observe large numbers of plants,
grown under circumstances which allow the best opportunities for [601]
all the seeds. And as nature affords such opportunities only at rare
intervals, we should make use of artificial methods. Large quantities of
seed should be gathered from wild plants and sowed under very favorable
conditions, giving all the nourishment and space required to the young
seedlings. It is recommended that they be sown under glass, either in a
glass-house or protected against cold and rain by glass-frames. The same
lot of seed will be seen to yield twice or thrice as many seedlings if
thus protected, compared with what it would have produced when sown in
the field or in the garden. I have nearly wholly given up sowing seeds
in my garden, as circumstances can be controlled and determined with
greater exactitude when the sowing is done in a glasshouse.
The best proof perhaps, of the unfavorable influence of external
conditions for slightly deteriorated deviations is afforded by
variegated leaves. Many beautiful varieties are seen in our gardens and
parks, and even corn has a variety with striped leaves. They are easily
reproduced, both by buds and by seeds, and they are the most ordinary of
all varietal deviations. They may be expected to occur wild also. But no
real variegated species, nor even good varieties with this attribute
occurs in nature. [602] On the other hand occasional specimens with a
single variegated leaf, or with some few of them, are actually met with,
and if attention is once drawn to this question, perhaps a dozen or so
instances might be brought together in a summer. But they never seem to
be capable of further evolution, or of reproducing themselves
sufficiently and of repeating their peculiarity in their progeny. They
make their appearance, are seen during a season, and then disappear.
Even this slight incompleteness of some spots on one or two leaves may
be enough to be their doom.
It is a common belief that new varieties owe their origin to the direct
action of external conditions and moreover it is often assumed that
similar deviations must have similar causes, and that these causes may
act repeatedly in the same species, or in allied, or even systematically
distant genera. No doubt in the end all things must have their causes,
and the same causes will lead under the same circumstances to the same
results. But we are not justified in deducing a direct relation between
the external conditions and the internal changes of plants. These
relations may be of so remote a nature that they cannot as yet be
guessed at. Therefore only direct experience may be our guide. Summing
up the result of our facts and discussions [603] we may state that wild
new elementary species and varieties are recorded to have appeared from
time to time. Invariably this happened by sudden leaps and without
intermediates. The mutants are constant when propagated by seed, and at
once constitute a new race. In rare instances this may be of sufficient
superiority to win a place for itself in nature, but more often it has
qualities which have led to its introduction into gardens as an
ornamental plant or into botanical gardens by reason of the interest
afforded by their novelty, or by their anomaly.
Many more mutations may be supposed to be taking place all around us,
but artificial sowings on a large scale, combined with a close
examination of the seedlings and a keen appreciation of the slightest
indications of deviation seem required to bring them to light.
[604]
LECTURE XXI