The steady cooperation of progression and retrogression is one of the
important principles of organic evolution. I have dwelt upon this point
more than once in previous lectures. I have tried to show that both in
the more important lines of the general pedigree of the vegetable
kingdom, and in the numerous lateral branches ending in the genera and
species within the families, progression and retrogression are nearly
always at work together. Your attention has been directed to the
monocotyledons as an example, where retrogression is everywhere so
active that it can almost be said to be the prevailing movement.
Reduction in the vegetative and generative organs, in the anatomical
structure and growth of the stems, and in sundry other ways is the
method by which the monocotyledons have originated as a group from their
supposed ancestors among the lower dicotyledonous families.
Retrogression is the leading idea in the larger families of the group,
[631] as for instance in the aroids and the grasses. Retrograde
evolution is also typical in the highest and most highly differentiated
family of the monocotyledons, the orchids, which have but one or two
stamens. In the second place I have had occasion more than once to
assert that retrogression, though seemingly consisting in the
disappearance of some quality, need not, as a rule, be considered as a
complete loss. Quite on the contrary, it is very probable that real
losses are extremely rare, if not wholly lacking. Ordinarily the loss is
only apparent, the capacity becomes inactive only, but is not destroyed.
The character has become latent, as it is commonly stated, and therefore
may return to activity and to the full display of its peculiarity,
whenever occasion offers.
Such a return to activity was formerly called atavism. But as we have
seen, when dealing with the phenomena of latency at large, sundry cases
of latency are to be distinguished, in order to get a clear insight into
these difficult processes.
So it is with atavism, too. If any plant reverts to a known ancestor, we
have a positive and simple case. But ancestors with alternate specific
marks are as a rule neither historically nor experimentally manifest.
They are only reputed to be such, and the presumption rests [632] upon
the systematic affinity between the derivative species and its nearest
probable allies. Such reversions are now to be examined at some length
and may be adequately treated under the head of systematic atavism. To
this form of atavism pertain, on the basis of our definition, those
phenomena by which species assume one or more characters of allies, from
which they are understood to have descended by the loss of the character
under discussion. The phenomena themselves consist in the production of
anomalies and varieties, and as the genetic relation of the latter is
often hardly beyond doubt, the anomalies seem to afford the best
instances for the study of systematic atavism. This study has for its
chief aim the demonstration of the presence of the latent characters,
and to show that they return to activity suddenly and not by a slow and
gradual recovery of the former features. It supports the assertion that
the visible elementary characters are essentially an external display of
qualities carried by the bearers of heredity, and that these bearers are
separate entities, which may be mingled together, but are not fused into
a chaotic primitive life-substance. Systematic atavism by this means
leads us to a closer examination of the internal and concealed causes,
which rule the affinities and divergencies of [633] allied species. It
brings before us and emphasizes the importance of the conception of the
so-called unit-characters.
The primrose will serve as an example. In the second lecture we have
seen that the old species of Linnaeus, the _Primula veris_, was split up
by Jacquin into three smaller ones, which are called _P. officinalis_,
_P. elatior_ and _P. acaulis_. From this systematic treatment we can
infer that these three forms are assumed to be derived from a common
ancestor. Now two of them bear their flowers in bracted whorls,
condensed into umbels at the summits of a scape. The scapes themselves
are inserted in the axils of the basal leaves, and produce the flowers
above them. In the third species, _Primula acaulis_, this scape is
lacking and the flowers are inserted singly in the axils on long slender
stalks. For this reason the species is called acaulescent, indicating
that it has no other stem than the subterranean rootstock. But on closer
inspection we observe that the flower stalks are combined into little
groups, each group occupying the aril of one of the basal leaves. This
fact at once points to an analogy with the umbellate allies, and induces
us to examine the insertion of the flowers more critically. In doing so
we find that they are united at their base so as to constitute a sessile
umbel. [634] The scapes are not absolutely lacking, but only reduced to
almost invisible rudiments.
Relying upon this conclusion we infer that all of the three elementary
species have umbels, some pedunculate and the others not. On this point
they agree with the majority of the allied species in the genus and in
other genera, as for instance in _Androsace_. Hence the conclusion that
the common ancestors were perennial plants with a rootstock bearing
their flowers in umbels or whorls on scapes. Lacking in the _Primula
veris_, these scapes must obviously have been lost at the time of the
evolution of this form.
Proceeding on this line of speculation we at once see that a very
adequate opportunity for systematic atavism is offered here. According
to our general conception the apparent loss of a scape is no proof of a
corresponding internal loss, but might as well be caused simply by the
reduction of the scape-growing capacity to a latent or inactive state.
It might be awakened afterwards by some unknown agency, and return to
activity.
Now this is exactly what happens from time to time. In Holland the
acaulescent primrose is quite a common plant, filling the woods in the
spring with thousands of clusters of bright yellow flowers. It is a very
uniform type, but in [635] some years it is seen to return to atavistic
conditions in some rare individuals. More than once I have observed such
cases myself, and found that the variation is only a partial one,
producing one or rarely two umbels on the same plant, and liable to fail
of repetition when the varying specimens are transplanted into the
garden for further observation. But the fact remains that scapes occur.
The scapes themselves are of varying length, often very short, and
seldom long, and their umbels display the involucre of bracts in a
manner quite analogous to that of the _Primula officinalis_ and _P.
elatior_. To my mind this curious anomaly strongly supports the view of
the latent condition of the scape in the acaulescent species, and that
such a dormant character must be due to a descent from ancestors with
active scapes, seems to be in no need of further reiteration. Returning
to activity the scapes at once show a full development, in no way
inferior to that of the allied forms, and only unstable in respect to
their length.
A second example is afforded by the bracts of the crucifers. This group
is easily distinguished by its cruciform petals and the grouping of the
flowers into long racemes. In other families each flower of such an
inflorescence would be subtended by a bract, according to the [636]
general rule that in the higher plants side branches are situated in the
arils of leaves. Bracts are reduced leaves, but the spikes of the
cruciferous plants are generally devoid of them. The flower-stalks, with
naked bases, seem to arise from the common axis at indefinite points.
Hence the inference that crucifers are an exception to a general rule,
and that they must have originated from other types which did comply
with this rule, and accordingly were in the possession of floral bracts.
Or, in other words, that the bracts must have been lost during the
original evolution of the whole family. This conclusion being accepted,
the accidental re-apparition of bracts within the family must be
considered as a case of systematic atavism, quite analogous to the
re-appearance of the scapes in the acaulescent primrose. The systematic
importance of this phenomenon, however, is far greater than in the first
case, in which we had only to deal with a specific character, while the
abolition of the bracts has become a feature of a whole family.
This reversion is observed to take place according to two widely
different principles. On one hand, bracts may be met with in a few stray
species, assuming the rank of a specific character. On the other hand
they may be seen [637] to occur as an anomaly, incompletely developed,
often very rare and with all the appearance of an accidental variation,
but sometimes so common as to seem nearly normal.
Coming now to particular instances, we may turn our attention in the
first place to the genus _Sisymbrium_. This is a group of about 50
species, of wide geographic distribution, among which the hedge mustard
(_S. officinalis_) is perhaps the most common of weeds. Two species are
reputed to have bracts, _Sisymbrium hirsutum_ and _S. supinum_. Each
flower-stalk of their long racemes is situated in the aril of such a
bract, and the peculiarity is quite a natural one, corresponding exactly
to what is seen in the inflorescence of other families. Besides the
_Sisymbrium some six other genera afford similar structures.
_Erucastrum pollichii_ has been already alluded to in a former lecture
when dealing with the same problem from another point of view. As
previously stated, it is one of the most manifest and most easily
accessible examples of a latent character becoming active through
systematic atavism. In fact, its bracts are found so often as to be
considered by some authors as of quite normal occurrence. Contrasted
with those of the above mentioned species of _Sisymbrium_, they are not
seen at the base of all the flower [638] stalks, but are limited to the
lowermost part of the raceme, adorning a few, often ten or twelve, and
rarely more flower-stalks. Moreover they exhibit a feature which is
indicative of the presence of an abnormality. They are not all of the
same size, but decrease in length from the base of the raceme upward,
and finally slowly disappear.
Besides these rare cases there are quite a number of cruciferous species
on record, which have been observed to bear bracts. Penzig in his
valuable work on teratology gives a list of 33 such genera, many of them
repeating the anomaly in more than one species. Ordinary cabbages are
perhaps the best known instance, and any unusual abundance of
nourishment, or anomalous cause of growth seems to be liable to incite
the development of bracts. The hedge garlic or garlic mustard
(_Alliaria_), the shepherd's purse, the wormseed or _Erysimum
cheiranthoides_ and many others afford instances. In my cultures of
Heeger's shepherd's purse, the new species derived at Landau in Germany
from the common shepherd's purse, the anomaly was observed to occur more
than once, showing that the mutation, which changed the fruits, had not
in the least affected this subordinate anomalous peculiarity. In all
these cases the bracts behave as with the Erucastrum, [639] being
limited to the base of the spike, and decreasing in size from the lower
flowers upward. Connected with these atavistic bracts is a feature of
minor importance, which however, by its almost universal accompaniment
of the bracts, deserves our attention, as it is indicative of another
latent character. As a rule, the bracts are grown together with their
axillary flower-stalk. This cohesion is not complete, nor is it always
developed in the same degree. Sometimes it extends over a large part of
the two organs, leaving only their tips free, but on other occasions it
is limited to a small part of the base. But it is very interesting that
this same cohesion is to be seen in the shepherd's purse, in the
wormseed and in the cabbage, as well as in the case of the _Erucastrum_
and most of the other observed cases of atavistic bracts. This fact
suggests the idea of a common origin for these anomalies, and would lead
to the hypothesis that the original ancestors of the whole family,
before losing the bracts, exhibited this peculiar mode of cohesion.
Bracts and analogous organs afford similar cases of systematic atavism
in quite a number of other families. Aroids sometimes produce long
bracts from various places on their spadix, as may be seen in the
cultivated greenhouse species, _Anthurium scherzerianum_. [640] Poppies
have been recorded to bear bracts analogous to the little scales on the
flower-stalks of the pansies, on the middle of their flower stalks. A
similar case is shown by the yellow foxglove or _Digitalis parviflora_.
The foxgloves as a rule have naked flower-stalks, without the two little
opposite leafy organs seen in so many other instances. The yellow
species, however, has been seen to produce such scales from time to
time. The honeysuckle genus is, as a rule, devoid of the stipules at the
base of the petiole, but _Lonicera etrusca_ has been observed to develop
such organs, which were seen to be free in some, but in other specimens
were adnate to the base of the leaf, and even connate with those of the
opposite leaf.
Other instances could be given proving that bracts and stipules, when
systematically lacking, are liable to reappear as anomalies. In doing
so, they generally assume the peculiar characters that would be expected
of them by comparison with allied genera in which they are of normal
occurrence. There can be no doubt that their absence is due to an
apparent loss, resulting from the reduction of a formerly active quality
to inactivity. Resuming this effective state, the case attains the value
and significance accorded to systematic atavism.
A very curious instance of reduced bracts, developing [641] to unusual
size, is afforded by a variety of corn, which is called _Zea Mays
cryptosperma_, or _Zea Mays tunicata_. In ordinary corn the kernels are
surrounded by small and thin, inconspicuous and membranaceous scales.
Invisible on the integrate spikes, when ripe, they are easily detected
by pulling the kernels out. In _cryptosperma_ they are so strongly
developed as to completely hide the kernels. Obviously they constitute a
case of reversion to the characters of some unknown ancestor, since the
corn is the only member of the grass-family with naked kernels. The var.
_tunicata_, for this same reason, has been considered to be the original
wild form, from which the other varieties of corn have originated. But
as no historical evidence on this point is at hand, we must leave it as
it is, notwithstanding the high degree of attractiveness attached to the
suggestion.
The horsetail-family may be taken as a further support of our assertion.
Some species have stems of two kinds, the fertile being brownish and
appearing in early spring before the green or sterile ones. In others
the stems are all alike, green and crowned with a conelike spike of
sporangia-bearing scales. Manifestly the dimorphous cases are to be
considered as the younger ones, partly because they are obvious
exceptions to the common rule, and [642] partly because the division of
labor is indicative of a higher degree of evolution. But sometimes these
dimorphic species are seen to revert to the primary condition,
developing a fertile cone at the summit of the green summer-stem. I have
had the opportunity of collecting an instance of this anomaly on the
tall _Equisetum telmateja_ in Switzerland, and other cases are on record
in teratological literature. It is an obvious example of systematic
atavism, occurring suddenly and with the full development of all the
qualities needed for the normal production of sporangia and spores. All
of these must be concealed in a latent condition within the young
tissues of the green stems.
More than once I have had occasion to deal with the phenomenon of
torsions, as exhibited by the teasels and some other plants. This
anomaly has been shown to be analogous to the cases described as double
adaptations. The capacity of evolving antagonistic characters is
prominent in both. The antagonists are assumed to lie quietly together
while inactive. But as soon as evolution calls them into activity they
become mutually exclusive, because only one of them can come to full
display in the same organ. External influences decide which of the two
becomes dominant and which remains dormant. This decision must take
place separately [643] for each stem and each branch, but as a rule, the
stronger ages are more liable to furnish anomalies than the weaker.
Exactly the same thing is true of double adaptations. Every bud of the
water-persicaria may develop either into an erect or into a floating
stem, according as it is surrounded by water or by relatively dry soil.
In other cases utility is often less manifest, but some use may either
be proved, or shown to be very probable. At all events the term
adaptation includes the idea of utility, and obviously useless
contrivances could hardly be brought under the same head.
We have also dealt with the question of heredity. It is obvious that
from the flowers of the floating and erect stems of the water-persicaria
seeds will result, each capable of yielding both forms. Quite the same
thing was the case with the teasels. Some 40% of the progeny produce
beautifully twisted stems, but whether the seed was saved from the most
completely twisted specimens or from the straight plants of the race was
of no importance.
This phenomenon of twisting may now be considered from quite another
point of view. It is a case of systematic atavism, or of the
reacquirement of some ancient and long-lost quality. This quality is the
alternate position of [644] the leaves, which has been replaced in the
teasel family by a grouping in pairs. In order to prove the validity of
this assertion, it will be necessary to discuss two points separately,
viz.: relative positions of the leaves, and the manner in which the
alternate position causes the stems to become twisted.
Leaves are affixed to their stems and branches in various ways. Among
them one is of wide occurrence throughout the whole realm of the higher
plants, while all the others are more rare. Moreover these subordinate
arrangements are, as a rule, confined to definite systematic groups.
Such groups may be large, as for instance, the monocotyledons, that have
their leaves arranged in two opposite rows in many families, or small,
as genera or subdivisions of genera. Apart from these special cases the
main stem and the greater part of the branches of the pedigree of the
higher plants exhibit a spiral condition or a screw arrangement, all
leaves being inserted at different points and on different sides of the
stem. This condition is assumed to be the original one, from which the
more specialized types have been derived. As is usual with characters in
general, it is seen to vary around an average, the spiral becoming
narrower and looser. A narrow spiral condenses the leaves, while a [645]
loose one disperses them. According to such fluctuating deviations the
number of leaves, inserted upon a given number of spiral circuits, is
different in different species. In a vast majority of cases 13 leaves
are found on 5 circuits, and as we have only to deal with this
proportion in the teasels we will not consider others.
In the teasels this screw-arrangement has disappeared, and has been
replaced by a decussate grouping. The leaves are combined into pairs,
each pair occupying the opposite sides of one node. The succeeding pairs
alternate with one another, so as to place their leaves at right angles.
The leaves are thus arranged on the whole stem in four equidistant rows.
On the normal stem of a teasel the two members of a pair are tied to one
another in a comparatively complicated way. The leaves are broadly
sessile and their bases are united so as to constitute a sort of cup.
The margins of these cups are bent upward, thereby enabling them to hold
water, and after a rainfall they may be seen filled to the brim. It is
believed that these little reservoirs are useful to the plant during the
flowering period, because they keep the ants away from the honey.
Considering the internal structure of the stem at the base of these cups
we find that the vascular bundles of the two opposite leaves are
strongly connected [646] with one another, constituting a ring which
narrowly surrounds the stem, and which would impede an increase in
thickness, if such were in the nature of the plant. But since the stems
end their existence during the summer of their development, this
structure is of no real harm.
The grouping of the leaves in alternate pairs may be seen within the bud
as well as on the adult stems. In order to do this, it is necessary to
make transverse sections through the heart of the rosette of the leaves
of the first year. If cut through the base, the pair exhibit connate
wings, corresponding to the water-cups; if cut above these, the leaves
seem to be free from one another.
In order to compare the position of leaves of the twisted plants with
this normal arrangement, the best way is to make a corresponding section
through the heart of the rosette of the first year. It is not necessary
to make a microscopic preparation. In the fall the changed disposition
may at once be seen to affect the central leaves of the group. All the
rosettes of the whole race commence with opposite leaves; those that are
to produce straight stems remain in this condition, but the preparation
for twisting begins at the end of the first year as shown by a special
arrangement of the leaves. This [647] disposition may then be seen to
extend to the very center of the rosette, by use of microscopical
sections. Examining sections made in the spring, the original
arrangement of the leaves of the stem is observed to continue until the
beginning of the growth of the shoot. It is easy to estimate the number
of leaves corresponding to a given number of spiral circuits in these
sections and the proportion is found to indicate 13 leaves on 5 turns.
These figures are the same as those given above for the ordinary
arrangement of alternate leaves in the main lines of the pedigree of the
vegetable kingdom.
Leaving aside for the moment the subsequent changes of this spiral
arrangement, it becomes at once clear that here we have a case of
systematic atavism. The twisted teasels lose their decussation, but in
doing so the leaves are not left in a disorderly dispersion, but a
distinct new arrangement takes its place, which is to be assumed as the
normal one for the ancestors of the teasel family. The case is to be
considered as one of atavism. Obviously no other explanation is
possible, than the supposition that the 5-13 spiral is still latent,
though not displayed by the teasels. But in the very moment when the
faculty of decussation disappears, it resumes its place, and becomes
[648] as prominent as it must once have been in the ancestors, and is
still in that part of their offspring, which has not become changed in
this respect. Thus the proof of our assertion of systematic atavism is,
in this case, not obtained by the inspection of the adult, but by the
investigation of the conditions in an early stage. It remains to be
explained how the twisting may finally be caused by this incipient
grouping of the leaves. Before doing so, it may be as well to state that
the case of the teasel is not an isolated one, and that the same
conclusions are supported by the valerian, and a large number of other
examples. In early spring some rosettes show a special condition of the
leaves, indicating thereby at once their atavism and their tendency to
become twisted as soon as they begin to expand. The Sweet William or
_Dianthus barbatus_ affords another instance; it is very interesting
because a twisted race is available, which may produce thousands of
instances developed in all imaginable degrees, in a single lot of
plants. _Viscaria oculata_ is another instance belonging to the same
family.
The bedstraw (_Galium_) also includes many species which from time to
time produce twisted stems. I have found them myself in Holland on
_Galium verum_ and _G. Aparine_. Both seem [649] to be of rare
occurrence, as I have not succeeded in getting any repetition by
prolonged culture.
Species, which generally bear their leaves in whorls, are also subjected
to casual atavisms of this kind, as for instance the tall European
horsetail, _Equisetum Telmateja_, which occasionally bears cones on its
green summer stems. Its whorls are changed on the twisted parts into
clearly visible spirals. The ironwood or _Casuarina quadrivalvis_ is
sometimes observed to produce the same anomaly on its smaller lateral
branches.
Coming now to the discussion of the way in which the twisting is the
result of the spiral disposition of the leaves, we may consider this
arrangement on stems in the adult state. These at once show the spiral
line and it is easy to follow this line from the base up to the apex. In
the most marked cases it continues without interruption, not rarely
however, ending in a whorl of three leaves and a subsequent straight
internode, of which there may even be two or three. The spiral exhibits
the basal parts of the leaves, with the axillary lateral branches. The
direction of the screw is opposed to that of the twisting, and the
spiral ribs are seen to cross the line of insertion of the leaves at
nearly right angles. On this line the leaves are nearer [650] to one
another than would correspond to the original proportion of 5 turns for
13 leaves. In fact, 10 or even 13 leaves may not rarely be counted on a
single turn. Or the twist may become so strong locally as to change the
spiral into a longitudinal line. On this line all inserted leaves extend
themselves in the same direction, resembling an extended flag.
The spiral on the stem is simply the continuation of the spiral line
from within the rosettes of the first year. Accordingly it is seen to
become gradually less steep at the base. For this reason it must be one
and the same with this line, and in extreme youth it must have produced
its leaves at the same mutual distances as this line. Transverse
sections of the growing summits of the stems support this conclusion.
From these several facts we may infer that the steepness of the spiral
line increases on the stem, as it is gradually changed into a screw.
Originally 5 turns were needed for 13 leaves, but this number diminishes
and 4 or 3 or even 2 turns may take the same number of foliar organs,
until the screw itself is changed into a straight line.
This change consists in an unwinding of the whole spiral, and in order
to effect this the stem must become wound up in the opposite direction.
The winding of the foliar screw must [651] curve the longitudinal ribs.
The straighter and steeper the screw becomes, the more the ribs will
become twisted. That this happens in the opposite direction is obvious,
without further discussion. The twisting is the inevitable consequence
of the reversal of the screw.
Two points remain to be dealt with. One is the direct proof of the
reversal of the screw, the other the discussion of its cause. The first
may be observed by a simple experiment. Of course it proceeds only
slowly, but all that is necessary is to mark the position of one of the
younger leaves of a growing stem of a twisting individual and to observe
the change in its position in a few hours. It will be seen to have
turned some way around the stem, and finally may be seen to make a
complete revolution in the direction opposite to the screw, and thereby
demonstrating the fact of its uncurling.
The cause of this phenomenon is to be sought in the intimate connection
of the basal parts of the leaves, which we have detailed above. The
fibrovascular strands constitute a strong rope, which is twisted around
the stem along the line on which the leaves are inserted. The
strengthening of the internodes may stretch this rope to some extent,
but it is too strong to be rent asunder. Hence it opposes the normal
growth, and the only manner in which the internodes [652] may adjust
themselves to the forces which tend to cause their expansion is by
straightening the rope. In doing so they may find the required space, by
growing out in an unusual direction, bending their axes and twisting the
ribs.
To prove the validity of this explanation, a simple experiment may be
given. If the fibrovascular rope is the mechanical impediment which
hinders the normal growth, we may try the effect of cutting through this
rope. By this means the hindrance may at least locally be removed. Now,
of course, the operation must be made in an early stage before, or at
the beginning of the period of growth, in every case before the
uncurling of the rope begins. Wounds made at this time are apt to give
rise to malformations, but notwithstanding this difficulty I have
succeeded in giving the necessary proof. Stems operated upon become
straight where the rope is cut through, though above and under the
wounded part they go on twisting in the usual way.
Sometimes the plants themselves succeed in tearing the rope asunder, and
long straight internodes divide the twisted stems in two or more parts
in a very striking manner. A line of torn leaf-bases connects the two
parts of the screw and gives testimony of what has passed within [653]
the tissues. At other times the straightening may have taken place
directly internal to a leaf, and it is torn and may be seen to be
attached to the stem by two distinct bases.
Summing up this description of the hereditary qualities of our twisted
teasels and of their mechanical consequences, we may say that the loss
of the normal decussation is the cause of all the observed changes. This
special adaptation, which places the leaves in alternating pairs,
replaced and concealed the old and universal arrangement on a screw
line. In disappearing, it leaves the latter free, and according to the
rule of systematic atavism, this now becomes active and takes its place.
If the fibrovascular connection of the leaf-bases were lost at the same
time the stems would grow and become straight and tall. This change
however, does not occur, and the bases of the leaves now constitute a
continuous rope instead of separate rings, and thereby impede the
stretching of the internodes. These in their turn avoid the difficulty
by twisting themselves in a direction opposite to that of the spiral of
the leaves.
As a last example of systematic atavism I will refer to the reversionary
changes, afforded by the tomatoes. Though the culture of this plant is a
recent one, it seems to be at present in a state of mutability,
producing new strains, or [654] assuming the features of their
presumable ancestors. In his work "The Survival of the Unlike," Bailey
has given a detailed description of these various types. Moreover, he
has closely studied the causes of the changes, and shown the great
tendency of the tomatoes to vicinism. By far the larger part of the
observed cases of running out of varieties are caused by accidental
crosses through the agency of insects. Even improvements are not rarely
due to this cause. Besides these common and often unavoidable changes,
others of greater importance occur from time to time. Two of them
deserve to be mentioned. They are called the "Upright" and the "Mikado"
types, and differ as much or even more from their parents than the
latter do from any one of their wild congeners. Their characters come
true from seed. The "Mikado" race or the _Lycopersicum grandifolium_
(_L. latifolium_) has larger and fewer leaflets than the slender and
somewhat flimsy foliage of the common form. Flat or plane blades with
decurrent margins constitute another character. This variety, however,
does not concern our present discussion. The upright type has stiff and
self-sustaining stems and branches, resembling rather a potato-plant
than a tomato. Hence the name _Lycopersicum solanopsis_ or _L. validum_,
under which it is usually described. [655] The foliage of the plant is
so distinct as to yield botanical characters of sufficient importance to
justify this specific designation. The leaflets are reduced in numbers
and greatly modified, and the flowers in the inflorescence are reduced
to two or three. This curious race came in suddenly, without any
premonition, and the locality and date of its mutation are still on
record. Until some years ago it had not made its appearance for a second
time. Obviously it is to be considered as a reversionary form. The limp
stems of the common tomatoes are in all respects indicative of the
cultivated condition. They cannot hold themselves erect, but must be
tied up to supports. The color of the leaves is a paler green than
should be expected from a wild plant. Considering other species of the
genus _Solanum_, of which the _Lycopersicum_ is a subdivision, the stems
are as a rule erect and self-supporting, with some few exceptions.
These, however, are special adaptations as shown by the winding stems of
the bitter-sweet.
From this discussion we seem justified in concluding that the original
appearance of the upright type was of the nature of systematic atavism.
It differs however, from the already detailed cases in that it is not a
monstrosity, nor an ever-sporting race, but is as constant a form [656]
as the best variety or species. Even on this ground it must be
considered as a representative of a separate group of instances of the
universal rule of systematic reversions.
Of late the same mutation has occurred in the garden of C.A. White at
Washington. The parent form in this case was the "Acme," of the ordinary
weak and spreading habit of growth. It is known as one of the best and
most stable of the varieties and was grown by Mr. White for many years,
and had not given any sign of a tendency towards change. Seeds from some
of the best plants in 1899 were sown the following spring, and the young
seedlings unexpectedly exhibited a marked difference from their parents.
From the very outset they were more strong and erect, more compact and
of a darker green than the "Acme." When they reached the fruiting stage
they had developed into typical representatives of the _Lycopersicum
solanopsis_ or upright division. The whole lot of plants comprised only
some 30 specimens, and this number, of course, is too small to base
far-reaching conclusions upon. But all of the lot showed this type, no
true "Acme" being seen among them. The fruit differed in flavor,
consistency and color from that of the parent, and it also ripened
earlier than the latter. No seed was saved from [657] these plants, but
the following year the "Acme" was sown again and found true to its type.
Seeds saved from this generation in 1900 have, however, repeated the
mutation, giving rise to exactly the same new upright form in 1901. This
was called by its originator "The Washington." Seeds from this second
mutation were kindly sent to me by Mr. White, and proved true to their
type when sown in my garden.
Obviously it is to be assumed in the case of the tomatoes as well as in
instances from other genera cited, that characters of ancestors, which
are not displayed in their progeny, have not been entirely lost, but are
still present, though in a latent condition. They may resume their
activity unexpectedly, and at once develop all the features which they
formerly had borne.
Latency, from this point of view, must be one of the most common things
in nature. All organisms are to be considered as internally formed of a
host of units, partly active and partly inactive. Extremely minute and
almost inconceivably numerous, these units must have their material
representatives within the most intimate parts of the cells.
[658]
LECTURE XXIII