The chief object of all experimentation is to obtain explanations of
natural phenomena. Experiments are a repetition of things occurring in
nature with the conditions so guarded and so closely followed that it is
possible to make a clear analysis of facts and their causes, it being
rightfully assumed that the laws are the same in both cases.
Experiments on heredity and the experience of the breeder find their
analogy in the succession of generations in the wild state. The
stability of elementary species and of retrograde varieties is quite the
same under both conditions. Progression and retrogression are narrowly
linked everywhere, and the same laws govern the abundance of forms in
cultivated and in wild plants.
Elementary species and retrograde varieties are easily recognizable.
Ever-sporting varieties on the contrary are far less obvious, and in
many cases their hereditary relations have [431] had to be studied anew.
A clear analogy between them and corresponding types of wild plants has
yet to be pointed out. There can be no doubt that such analogy exists;
the conception that they should be limited to cultivated plants is not
probable. Striped flowers and variegated leaves, changes of stamens into
carpels or into petals may be extremely rare in the wild state, but the
"five-leaved" clover and a large number of monstrosities cannot be said
to be typical of the cultivated condition. These, however, are of rare
occurrence, and do not play any important part in the economy of nature.
In order to attain a better solution of the problem we must take a
broader view of the facts. The wide range of variability of
ever-sporting varieties is due to the presence of two antagonistic
characters which cannot be evolved at the same time and in the same
organ, because they exclude one another. Whenever one is active, the
other must be latent. But latency is not absolute inactivity and may
often only operate to encumber the evolution of the antagonistic
character, and to produce large numbers of lesser grades of its
development. The antagonism however, is not such in the exact meaning of
the word; it is rather a mutual exclusion, because one of the opponents
simply takes the place of the other when absent, or supplements [432] it
to the extent that it may be only imperfectly developed. This completion
ordinarily occurs in all possible degrees and thus causes the wide range
of the variability. Nevertheless it may be wanting, and in the case of
the double stocks only the two extremes are present.
It is rather difficult to get a clear conception of the substitution,
and it seems necessary to designate the peculiar relationship between
the two characters forming such a pair by a simple name. They might be
termed alternating, if only it were clearly understood that the
alternation may be complete, or incomplete in all degrees. Complete
alternation would result in the extremes, the incomplete condition in
the intermediate states. In some cases as with the stocks, the first
prevails, while in other cases, as with the poppies, the very extremes
are only rarely met with.
Taking such an alternation as a real character of the ever-sporting
varieties, a wide range of analogous cases is at once revealed among the
normal qualities of wild plants. Alternation is here almost universal.
It is the capacity of young organs to develop in two diverging
directions. The definitive choice must be made in extreme youth, or
often at a relatively late period of development. Once made, this [433]
choice is final, and a further change does not occur in the normal
course of things.
The most curious and most suggestive instance of such an alternation is
the case of the water-persicaria or _Polygonum amphibium_. It is known
to occur in two forms, one aquatic and the other terrestrial. These are
recorded in systematic works as varieties, and are described under the
names of _P. amphibium_ var. _natans_ Moench, and _P. amphibium_ var.
_terrestre_ Leers or _P. amphibium_ var. _terrestris_ Moench. Such
authorities as Koch in his German flora, and Grenier and Godron in their
French flora agree in the conception of the two forms as varieties.
Notwithstanding this, the two varieties may often be observed to sport
into one another. They are only branches of the same plant, grown under
different conditions. The aquatic form has floating or submerged stems
with oblong or elliptic leaves, which are glabrous and have long
petioles. The terrestrial plants are erect, nearly simple, more or less
hispid throughout, with lanceolate leaves and short petioles, often
nearly sessile. The aquatic form flowers regularly, producing its
peduncle at right angles from the floating stems, but the terrestrial
specimens are ordinarily seen without flower-spikes, which are but
rarely met with, at least as far as my own experience goes. Intermediate
[434] forms are very rare, perhaps wholly wanting, though in swamps the
terrestrial plants may often vary widely in the direction of the
floating type.
That both types sport into each other has long been recognized in
field-observations, and has been the ground for the specific name of
_amphibium_, though in this respect herbarium material seems usually to
be scant. The matter has recently been subjected to critical and
experimental studies by the Belgian botanist Massart, who has shown that
by transplanting the forms into the alternate conditions, the change may
always be brought about artificially. If floating plants are established
on the shore they make ascending hairy stems, and if the terrestrial
shoots are submerged, their buds grow into long and slack, aquatic
stems. Even in such experiments, intermediates are rare, both types
agreeing completely with the corresponding models in the wild state.
Among all the previously described cases of horticultural plants and
monstrosities there is no clearer case of an ever-sporting variety than
this one of the water-persicaria. The var. _terrestris_ sports into the
var. _natans_, and as often as the changing life conditions may require
it. It is-true that ordinary sports occur without our discerning the
cause and without [435] any relation to adaptation. This however is
partly due to our lack of knowledge, and partly to the general rule that
in nature only such sports as are useful are spared by natural
selection, and what is useful we ordinarily term adaptive.
Another side of the question remains to be considered. The word variety,
as is now becoming generally recognized, has no special meaning
whatever. But here it is assumed in the clearly defined sense of a
systematic variety, which includes all subdivisions of species. Such
subdivisions may be, from a biological point of view, elementary species
and also be eversporting varieties. They may be retrograde varieties,
and the two alternating types may be described as separate varieties.
It is readily granted that many writers would not willingly accept this
conclusion. But it is simply impossible to avoid it. The two forms of
the water-persicaria must remain varieties, though they are only types
of the different branches of a single plant.
If not, hundreds and perhaps thousands of analogous cases are at once
exposed to doubt, and the whole conception of systematic varieties would
have to be thrown over. Biologists of course would have no objection to
this, but the student of the flora of any given country [436] or region
requires the systematic subdivisions and should always use his utmost
efforts to keep them as they are. There is no intrinsic difficulty in
the statement that different parts of the same plant should constitute
different varieties.
In some cases different branches of the same plant have been described
as species. So for instance with the climbing forms of figs. Under the
name of _Ficus repens_ a fine little plant is quite commonly cultivated
as a climber in flower baskets. It is never seen bearing figs. On the
other hand a shrub of our hothouses called _Ficus stipulata_, is
cultivated in pots and makes a small tree which produces quite large,
though non-edible figs. Now these two species are simply branches of the
same plant. If the _repens_ is allowed to climb up high along the walls
of the hothouses, it will at last produce stipulate branches with the
corresponding fruits. _Ficus radicans_ is another climbing form,
corresponding to the shrub _Ficus ulmifolia_ of our glasshouses. And
quite the same thing occurs with ivy, the climbing stems of which never
flower, but always first produce erect and free branches with rhombic
leaves. These branches have often been used as cuttings and yield little
erect and richly flowering shrubs, which are known in [437] horticulture
under the varietal name of _Hedera Helix arborea_.
Manifestly this classification is as nearly right as that of the two
varieties of the water-persicaria. Going one step further, we meet with
the very interesting case of alpine plants. The vegetation of the higher
regions of mountains is commonly called alpine, and the plants show a
large number of common features, differentiating them from the flora of
lower stations. The mountain plants have small and dense foliage, with
large and brightly-colored flowers. The corresponding forms of the
lowlands have longer and weaker stems, bearing their leaves at greater
distances, the leaves themselves being more numerous. The alpine forms,
if perennial, have thick, strongly developed and densely branched
rootstocks with heavy roots, in which a large amount of food material is
stored up during the short summer, and is available during the long
winter months of the year.
Some species are peculiar to such high altitudes, while many forms from
the lowlands have no corresponding type on the mountains. But a large
number of species are common to both regions, and here the difference of
course is most striking. _Lotus corniculatus_ and _Calamintha Acinos_,
_Calluna vulgaris_ and _Campanula_ [438] _rotundifolia_ may be quoted as
instances, and every botanist who has visited alpine regions may add
other examples. Even the edelweiss of the Swiss Alps, _Gnaphalium
Leontopodium_, loses its alpine characters, if cultivated in lowland
gardens. Between such lowland and alpine forms intermediates regularly
occur. They may be met with whenever the range of the species extends
from the plains upward to the limit of eternal snow.
In this case the systematists formerly enumerated the alpine plants as
_forma alpestris_, but whenever the intermediate is lacking the term
_Varietas alpestris_ was often made use of.
It is simply impossible to decide concerning the real relation between
the alpine and lowland types without experiments. About the middle of
the last century it was quite a common thing to collect plants not only
for herbarium-material, but also for the purpose of planting them in
gardens and thus to observe their behavior under new conditions. This
was done with the acknowledged purpose of investigating the systematic
significance of observed divergencies. Whenever these held good in the
garden they were considered to be reliable, but if they disappeared they
were regarded as the results of climatic conditions, or of the influence
of soil or nourishment. Between [439] these two alternatives, many
writers have tried to decide, by transplanting their specimens after
some time in the garden, into arid or sandy soil, in order to see
whether they would resume their alpine character.
Among the systematists who tested plants in this way, Nageli especially,
directed his attention to the hawkweeds or _Hieracium_. On the Swiss
Alps they are very small and exhibit all the characters of the pure
alpine type. Thousands of single plants were cultivated by him in the
botanical garden of Munich, partly from seed and partly from introduced
rootstocks. Here they at once assumed the tall stature of lowland forms.
The identical individual, which formerly bore small rosettes of basal
leaves, with short and unbranched flower-stalks, became richly leaved
and often produced quite a profusion of flower-heads on branched stems.
If then they were transplanted to arid sand, though remaining in the
same garden and also under the same climatic conditions they resumed
their alpine characters. This proved nutrition to be the cause of the
change and not the climate.
The latest and most exact researches on this subject are due to Bonnier,
who has gone into all the details of the morphologic as well as of the
physiologic side of the problem. [440] His purpose was the study of
partial variability under the influence of climate and soil. In every
experiment he started from a single individual, divided it into two
parts and planted one half on a mountain and the other half on the
plain. The garden cultures were made chiefly at Paris and Fontainebleau,
the alpine cultures partly in the Alps, partly in the Pyrenees. From
time to time the halved plants were compared with each other, and the
cultures lasted, as a rule, during the lifetime of the individual, often
covering many years.
The common European frostweed or _Helianthemum vulgare_ will serve to
illustrate his results. A large plant growing in the Pyrenees in an
altitude of 2,400 meters was divided. One half was replanted on the same
spot, and the other near Cadeac, at the base of the mountain range (740
M.). In order to exclude the effect of a change of soil, a quantity of
the earth from the original locality was brought into the garden and the
plant put therein. Further control experiments were made at Paris. As
soon as the two halved individuals commenced to grow and produced new
shoots, the influence of the different climates made itself felt. On the
mountain, the underground portions remained strong and dense, the leaves
and internodes small and hairy, the flowering stems nearly [441]
procumbent, the flowers being large and of a deep yellow. At Cadeac and
at Paris the whole plant changed at once, the shoots becoming elongated
and loose, with broad and flattened, rather smooth leaves and numerous
pale-hued flowers. The anatomical structure exhibited corresponding
differences, the intercellular spaces being small in the alpine plant
and large in the one grown in the lowlands, the wood-tissues strong in
the first and weak in the second case.
The milfoil (_Achillea Millefolium_) served as a second example, and the
experiments were carried on in the same localities. The long and thick
rootstocks of the alpine plant bearing short stems only with a few dense
corymbs contrasted markedly with the slender stems, loose foliage and
rich groups of flowerheads of the lowland plant. The same differences,
in inner and outer structures were observed in numerous instances,
showing that the alpine type in these cases is dependent on the climate,
and that the capacity for assuming the antagonistic characters is
present in every individual of the species. The external conditions
decide which of them becomes active and which remains inactive, and the
case seems to be exactly parallel to that of the water-persicaria.
In the experiments of Bonnier the influence of the soil was, as a rule,
excluded by transplanting [442] part of the original earth with the
transplanted half of the plant. From this he concluded that the observed
changes were due to the inequality of the climate. This involved three
main factors, light, moisture and temperature. On the mountains the
light is more intense, the air drier and cooler. Control-experiments
were made on the mountains, depriving the plants of part of the light.
In various ways they were more or less shaded, and as a rule responded
to this treatment in the same way as to transplantation to the plain
below. Bonnier concluded that, though more than one factor takes part in
inciting the morphologic changes, light is to be considered as the chief
agency. The response is to be considered as a useful one, as the whole
structure of the alpine varieties is fitted to produce a large amount of
organic material in a short time, which enables the plants to thrive
during the short summers and long winters of their elevated stations.
In connection with these studies on the influences of alpine climates,
Bonnier has investigated the internal structure of arctic plants, and
made a series of experiments on growth in continuous electric light. The
arctic climate is cold, but wet, and the structure of the leaves is
correspondingly loose, though the plants become [443] as small as on the
Alps. Continuous electric light had very curious effects; the plants
became etiolated, as if growing in darkness, with the exception that
they assumed a deep green tinge. They showed more analogy with the
arctic than with the alpine type.
The influence of the soil often produces changes similar to that of
climate. This was shown by the above cited experiments of Nageli with
the hawkweeds, and may easily be controlled in other cases. The
ground-honeysuckle or _Lotus corniculatus_ grows in Holland partly on
the dry and sandy soil of the dunes, and occasionally in meadows. It is
small and dense in the first case, with orange and often very darkly
colored petals, while it is loose and green in the meadows, with
yellower flowers. Numerous analogous cases might be given. On mountain
slopes in South Africa, and especially in Natal, a species of composite
is found, which has been introduced into culture and is used as a
hanging plant. It is called _Othonna crassifolia_ and has fleshy, nearly
cylindrical leaves, and exactly mimics some of the crassulaceous
species. On dry soil the leaves become shorter and thicker and assume a
reddish tinge, the stems remain short and woody and bear their leaves in
dense rosettes. On moist and rich garden-soil this aspect becomes [444]
changed at once, the stems grow longer and of a deeper green.
Intermediates occur, but notwithstanding this the two extremes
constitute clearly antagonistic types.
The flora of the deserts is known to exhibit a similar divergent type.
Or rather two types, one adapted to paucity of water, and the other to a
storage of fluid at one season in order to make use of it at other
times, as is the case with the cactuses. Limiting ourselves to the
alternate group, we observe a rich and dense branching, small and
compact leaves and extraordinarily long roots. Here the analogy with the
alpine varieties is manifest, and the dryness of the soil evidently
affects the plants in a similar way, as do the conditions of life in
alpine regions. The question at once comes up as to whether here too we
have only instances of partial variability, and whether many of the
typical desert-species would lose their peculiar character by
cultivation under ordinary conditions. The varieties of _Monardella
macrantha_, described by Hall, from the San Jacinto Mountain, Cal., are
suggestive of such an intimate analogy with the cases studied by
Bonnier, that it seems probable that they might yield similar results,
if tested by the same method.
Leaving now the description of these special [445] cases, we may resume
our theoretical discussion of the subject, and try to get a clearer
insight into the analogy of ever-sporting varieties and the wild species
quoted. All of them may be characterized by the general term of
dimorphism. Two types are always present, though not in the same
individual or in the same organ. They exclude one another, and during
their juvenile stage a decision is taken in one direction or in the
other. Now, according to the theory of natural selection, wild species
can only retain useful or at least innocuous qualities, since all
mutations in a wrong direction must perish sooner or later. Cultivated
species on the other hand are known to be largely endowed with
qualities, which would be detrimental in the wild condition.
Monstrosities are equally injurious and could not hold their own if left
to themselves.
These same principles may be applied to ever-sporting or antagonistic
pairs of characters. According to the theory of mutations such pairs may
be either useful or useless. But only the useful will stand further
test, and if they find suitable conditions will become specific or
varietal characters. On this conclusion it becomes at once clear, why
natural dimorphism is, as a rule, a very useful quality, while the
cultivated dimorphous varieties [446] strike us as something unnatural.
The relation between cause and effect, is in truth other than it might
seem to be at first view, but nevertheless it exists, and is of the
highest importance.
From this same conclusion we may further deduce some explanation of the
hereditary races characterized by monstrosities. It is quite evident
that the twisted teasels are inadequate for the struggle with their tall
congeners, or with the surrounding plants. Hence the conclusion that a
pure and exclusively twisted race would soon die out. The fact that such
races are not in existence finds its explanation in this circumstance,
and therefore it does not prove the impossibility or even the
improbability that some time a pure twisted race might arise. If chance
should put such an accidental race in the hands of an experimenter, it
could be protected and preserved, and having no straight atavistic
branches, but being twisted in all its organs, might yield the most
curious conceivable monstrosity, surpassing even the celebrated dwarf
twisted shrubs of Japanese horticulturists.
Such varieties however, do not exist at present. The ordinary twisted
races on the other hand, are found in the wild state and have only to be
isolated and cultivated to yield large numbers [447] of twisted
individuals. In nature they are able to maintain themselves during long
centuries, quite as well as normal species and varieties. But they owe
this quality entirely to their dimorphous character. A twisted race of
teasels might consist of successive generations of tall atavistic
individuals, and produce yearly some twisted specimens, which might be
destroyed every time before ripening their seeds. Reasoning from the
evidence available, and from analogous cases, the variety would, even
under such extreme circumstances, be able to last as long as any other
good variety or elementary species. And it seems to me that this
explanation makes clear how it is possible that varieties, which are
potentially rich in their peculiar monstrosity, are discovered from time
to time among plants when tested by experimental methods.
Granting these conclusions, monstrosities on the one side, and
dimorphous wild species on the other, constitute the most striking
examples of the inheritance of latent characters.
The bearing of the phenomena of dimorphism upon the principles of
evolution formulated by Lamarck, and modified by his followers to
constitute Neo-Lamarckianism, remains to be considered. Lamarck assumed
that the external conditions directly affected the organisms in [448]
such a way as to make them better adapted to life, under prevailing
circumstances. Nageli gave to this conception the name "Theory of direct
causation" (Theorie der directen Bewirkung), and it has received the
approval of Von Wettstein, Strasburger and other German investigators.
According to this conception a plant, when migrating from lowlands into
the mountains would slowly be changed and gradually assume alpine
habits. Once acquired this habit would become fixed and attain the rank
of specific characters. In testing this theory by field-observations and
culture-experiments, the defenders of the Nagelian principle could
easily produce evidence upon the first point. The change of
lowland-plants into alpine varieties can be brought about in numerous
cases, and corresponding changes under the influence of soil, or
climate, or life-conditions are on record for the most various
characters and qualities.
The second point, however, is as difficult to prove as the first is of
easy treatment. If after hundreds and thousands of years of exposure to
alpine or other extreme conditions a fixed change is proved to have
taken place, the question remains unanswered, whether the change has
been a gradual or a sudden one. Darwin pointed out that long periods of
life afford a [449] chance for a sudden change in the desired direction,
as well as for the slow accumulation of slight deviations. Any mutations
in a wrong direction would at once be destroyed, but an accidental
change in a useful way would be preserved, and multiply itself. If in
the course of centuries this occurred, they would be nearly sure to
become established, however rare at the outset. Hence the positive
assertion is scarcely capable of direct proof.
On the other hand the negative assertion must be granted full
significance. If the alpine climate has done no more than produce a
transitory change, it is clear that thousands of years do not,
necessarily, cause constant and specific alterations. This requirement
is one of the indispensable supports of the Lamarckian theory. The
matter is capable of disproof however, and such disproof seems to be
afforded by the direct evidence of the present condition of the alpine
varieties at large, and by many other similar cases.
Among these the observations of Holtermann on some desert-plants of
Ceylon are of the highest value. Moreover they touch questions which are
of wide importance for the study of the biology of American deserts. For
this reason I may be allowed to introduce them here at some length.
[450] The desert of Kaits, in Northern Ceylon, nourishes on its dry and
torrid sands some species, represented by a large number of individuals,
together with some rarer plants. The commonest forms are _Erigeron
Asteroides_, _Vernonia cinerea_, _Laurea pinnatifida_, _Vicoa
auriculata_, _Heylandia latebrosa_ and _Chrysopogon montanus_. In direct
contrast with the ordinary desert-types they have a thin epidermis, with
exposed stomata, features that ordinarily were characteristic of species
of moister regions. They are annuals, growing rapidly, blooming and
ripening their seeds before the height of the dry season. Evidently they
are to be considered as the remainder of the flora of a previous period,
when the soil had not yet become arid. They might be called relics. Of
course they are small and dwarf-like, when compared with allied forms.
These curious little desert-plants disprove the Nagelian views in two
important points. First, they show that extreme conditions do not
necessarily change the organisms subjected to them, in a desirable
direction. During the many centuries that these plants must have existed
in the desert in annual generations, no single feature in the anatomical
structure has become changed. Hence the conclusion that small leaves,
abundant rootstocks and short [451] stems, a dense foliage, a strongly
cuticularized epidermis, few and narrow air-cavities in the tissues and
all the long range of characteristics of typical desert-plants are not a
simple result of the influence of climate and soil. There is no direct
influence in this sense.
The second point, in which Nageli's idea is broken down by Holtermann's
observations, results from the behavior of the plants of the Kaits
desert when grown or sown on garden soil. When treated in this way they
at once lose the only peculiarity which might be considered as a
consequence of the desert-life of their ancestors, their dwarf stature.
They behave exactly like the alpine plants in Bonnier's experiments, and
with even more striking differences. In the desert they attain a height
of a few centimeters, but in the garden they attain half a meter and
more in height. Nothing in the way of stability has resulted from the
action of the dry soil, not even in such a minor point as the height of
the stems.
From the facts and discussions we may conclude that double adaptation is
not induced by external influences, at least not in any way in which it
might be of use to the plant. It may arise by some unknown cause, or may
not be incited at all. In the first case the plant becomes capable of
living under the alternating [452] circumstances, and if growing near
the limits of such regions it will overlap and get into the new area.
All other species, which did not acquire the double habit, are of course
excluded, with such curious exceptions as those of Kaits. The typical
vegetation under such extreme conditions however, finds explanation
quite as well by the one as by the other view.
Leaving these obvious cases of double adaptation, there still remains
one point to be considered. It is the dwarf stature of so many desert
and alpine plants. Are these dwarfs only the extremes of the normal
fluctuating variability, or is their stature to be regarded as the
expression of some peculiar adaptive but latent quality? It is as yet
difficult to decide this question, because statistical studies of this
form of variability are still wanting. The capacity of ripening the seed
on individuals of dwarf stature however, is not at all a universal
accompaniment of a variable height. Hence it cannot be considered as a
necessary consequence of it. On the other hand the dwarf varieties of
numerous garden-plants, as for instance: of larkspurs, snapdragon,
opium-poppies and others are quite stable and thence are obviously due
to peculiar characteristics. Such characteristics, if combined with tall
stature into a pair of antagonists, would yield a double [453]
adaptation, and on such a base a hypothetical explanation could no doubt
be rested. Instead of discussing this problem from the theoretical side,
I prefer to compare those species which are capable of assuming a dwarf
stature under less uncommon conditions than those of alpine and
desert-plants. Many weeds of our gardens and many wild species have this
capacity. They become very tall, with large leaves, richly branched
stems and numerous flowers in moist and rich soil. On bad soil, or if
germinating too late, when the season is drier, they remain very small,
producing only a few leaves and often limiting themselves to one
flower-head. This is often seen with thorn-apples and amaranths, and
even with oats and rye, and is notoriously the case with buckwheat.
Gauchery has observed that the extremes differ often as much from one
another as 1:10. In the case of the Canadian horseweed or _Erigeron
canadensis_, which is widely naturalized in Europe, the tallest
specimens are often twenty-five times as tall as the smallest, the
difference increasing to greater extremes, if besides the main stem, the
length of the numerous branches of the tall plants are taken into
consideration. Other instances studied by the French investigator are
_Erythraea pulchella_ and _Calamintha Acinos_.
[454] Dimorphism is of universal occurrence in the whole vegetable
kingdom. In some cases it is typical, and may easily be discerned from
extreme fluctuating variability. In others the contrast is not at all
obvious, and a closer investigation is needed to decide between the two
possibilities. Sometimes the adaptive quality is evident, in other cases
it is not. A large number of plants bear two kinds of leaves linked with
one another by intermediate forms. Often the first leaves of a shoot, or
those of accidentally strong shoots, exhibit deviating shapes, and the
usefulness of such occurrences seems to be quite doubtful. The
elongation of stems and linear leaves, and the reduction of lateral
organs in darkness, is manifestly an adaptation. Many plants have
stolons with double adaptations which enable them to retain their
character of underground stems with bracts or to exchange it for the
characteristics of erect stems with green leaves according to the outer
circumstances. In some shrubs and trees the capacity of a number of buds
to produce either flowers or shoots with leaves seems to be in the same
condition. The capacity of producing spines is also a double adaptation,
active on dry and arid soil and latent in a moist climate or under
cultivation, as with the wild and cultivated apple, and in the
experiments of Lothelier [455] with _Berberis_, _Lycium_ and other
species, which lose their spines in damp air.
In some conifers the evolution of horizontal branches may be modified by
simply turning the buds upside down. Or the lateral branches can be
induced to become erect stems by cutting off the normal summit of a
tree. Numerous organs and functions lie dormant until aroused by
external agencies, and many other cases could be cited, showing the wide
occurrence of double adaptation.
There are, however, two points, which should not be passed over without
some mention. One of them is the influence of sun and shade on leaves,
and the other the atavistic forms, often exhibited during the juvenile
period.
The leaves of many plants, and especially those of some shrubs and
trees, have the capacity of adapting themselves either to intense or to
diffuse light. On the circumference of the crown of a tree the light is
stronger and the leaves a small and thick, with a dense tissue. In the
inner parts of the crown the light is weak and the leaves are broader in
order to get as much of it as possible. They become larger but thinner,
consisting often of a small number of cell layers. The definitive
formation is made in extreme youth, often even during the previous
summer, at the time of the [456] very first evolution of the young
organs within the buds. _Iris_, and _Lactuca Scariola_ or the prickly
lettuce, and many other plants afford similar instances. As the
definitive decision must be made in these cases long before the direct
influence of the conditions which would make the change useful is felt,
it is hardly conceivable how they could be ascribed to this cause.
It is universally known that many plants show deviating features when
very young, and that these often remind us of the characters of their
probable ancestors. Many plants that must have been derived from their
nearest systematic relatives, chiefly by reductions, are constantly
betraying this relation by a repetition of the ancestral marks during
their youth.
There can be hardly a doubt that the general law of natural selection
prevails in such cases as it does in others. Or stated otherwise, it is
very probable, that in most cases the atavistic characters have been
retained during youth because of their temporary usefulness.
Unfortunately, our knowledge of utility of qualities is as yet, very
incomplete. Here we must assume that what is ordinarily spared by
natural selection is to be considered as useful, [457] until direct
experimental investigations have been made.
So it is for instance with the submerged leaves of water-plants. As a
rule they are linear, or if compound, are reduced to densely branching
filiform threads. Hence we may conclude that this structure is of some
use to them. Now two European and some corresponding American species of
water-parsnip, the _Sium latifolium_ and _Berula angustifolia_ with
their allies, are umbellifers, which bear pinnate instead of bi- or
tri-pinnate leaves. But the young plants and even the young shoots when
developing from the rootstocks under water comply with the above rule,
producing very compound, finely and pectinately dissected leaves. From a
systematic point of view these leaves indicate the origin of the
water-parsnips from ordinary umbellifers, which generally have bi- and
tripinnate leaves.
Similar cases of double adaptation, dependent on external conditions at
different periods of the evolution of the plant are very numerous. They
are most marked among leguminous plants, as shown by the trifoliolate
leaves of the thorn-broom and allies, which in the adult state have
green twigs destitute of leaves.
As an additional instance of dimorphism and probable double adaptation
to unrecognized external [458] conditions I might point to the genus
_Acacia_. As we have seen in a previous lecture some of the numerous
species of this genus bear bi-pinnate leaves, while others have only
flattened leaf-stalks. According to the prevailing systematic
conceptions, the last must have been derived from the first by the loss
of the blades and the corresponding increase of size and superficial
extension of the stalk. In proof of this view they exhibit, as we have
described, the ancestral characters in the young plantlets, and this
production of bi-pinnate leaves has probably been retained at the period
of the corresponding negative mutations, because of some distinct,
though still unknown use.
Summarizing the results of this discussion, we may state that useful
dimorphism, or double adaptation, is a substitution of characters quite
analogous to the useless dimorphism of cultivated ever-sporting
varieties and the stray occurrence of hereditary monstrosities. The same
laws and conditions prevail in both cases.
[459]