Species and Varieties, Their Origin by Mutation Part 22

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] circ.u.mstances, 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 a.s.suming 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 pulch.e.l.la_ 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 circ.u.mstances. 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 pa.s.sed 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 circ.u.mference 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 p.r.i.c.kly 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 repet.i.tion 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 a.s.sume 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 dest.i.tute 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 subst.i.tution of characters quite a.n.a.logous 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]

E. MUTATIONS

LECTURE XVI

THE ORIGIN OF THE PELORIC TOAD-FLAX

I have tried to show previously that species, in the ordinary sense of the word, consist of distinct groups of units. In systematic works these groups are all designated by the name of varieties, but it is usually granted that the units of the system are not always of the same value.

Hence we have distinguished between elementary species and varieties proper. The first are combined into species whose common original type is now lost or unknown, and from their characters is derived an hypothetical image of what the common ancestor is supposed to have been.

The varieties proper are derived in most cases from still existing types, and therefore are subjoined to them. A closer investigation has shown that this derivation is ordinarily produced by the loss of some definite attribute, or by the re-acquisition of an apparently [460] lost character. The elementary species, on the other hand, must have arisen by the production of new qualities, each new acquisition const.i.tuting the origin of a new elementary form.

Moreover we have seen, that such improvements and such losses const.i.tute sharp limits between the single unit-forms. Every type, of course, varies around an average, and the extremes of one form may sometimes reach or even overlap those of the nearest allies, but the offspring of the extremes always return to the type. The transgression is only temporary and a real transition of one form to another does not come within ordinary features of fluctuating variability. Even in the cases of eversporting varieties, where two opposite types are united within one race, and where the succeeding individuals are continually swinging from one extreme to the other, pa.s.sing through a wide range of intermediate steps, the limits of the variety are as sharply defined and as free from real transgression as in any other form.

In a complete systematic enumeration of the real units of nature, the elementary species and varieties are thus observed to be discontinuous and separated by definite gaps. Every unit may have its youth, may lead a long life in the adult state and may finally die. But through [461]

the whole period of its existence it remains the same, at the end as sharply defined from its nearest allies as in the beginning. Should some of the units die out, the gaps between the neighboring ones will become wider, as must often have been the case. Such segregations, however important and useful for systematic distinctions, are evidently only of secondary value, when considering the real nature of the units themselves.

We may now take up the other side of the problem. The question arises as to how species and varieties have originated. According to the Darwinian theory they have been produced from one another, the more highly differentiated ones from the simpler, in a graduated series from the most simple forms to the most complicated and most highly organized existing types. This evolution of course must have been regular and continuous, diverging from time to time into new directions, and linking all organisms together into one common pedigree. All lacunae in our present system are explained by Darwin as due to the extinction of the forms, which previously filled them.

Since Lamarck first propounded the conception of a common origin for all living beings, much has been done to clear up our ideas as to the real nature of this process. The broader [462] aspect of the subject, including the general pedigree of the animal and vegetable kingdom, may be said to have been outlined by Darwin and his followers, but this phase of the subject lies beyond the limits of our present discussion.

The other phase of the problem is concerned with the manner in which the single elementary species and varieties have sprung from one another.

There is no reason to suppose that the world is reaching the end of its development, and so we are to infer that the production of new species and varieties is still going on. In reality, new forms are observed to originate from time to time, both wild and in cultivation, and such facts do not leave any doubt as to their origin from other allied types, and according to natural and general laws.

In the wild state however, and even with cultivated plants of the field and garden, the conditions, though allowing of the immediate observation of the origination of new forms, are by no means favorable for a closer inquiry into the real nature of the process. Therefore I shall postpone the discussion of the facts till another lecture, as their bearing will be more easily understood after having dealt with more complete cases.

These can only be obtained by direct experimentation. Comparative studies, of course, [463] are valuable for the elucidation of general problems and broad features of the whole pedigree, but the narrower and more practical question as to the genetic relation of the single forms to one another must be studied in another way, by direct experiment. The exact methods of the laboratory must be used, and in this case the garden is the laboratory. The cultures must be guarded with the strictest care and every precaution taken to exclude opportunities for error. The parents and grandparents and their offspring must be kept pure and under control, and all facts bearing upon the birth or origin of the new types should be carefully recorded.

Two great difficulties have of late stood in the way of such experimental investigation. One of them is of a theoretical, the, other of a practical nature. One is the general belief in the supposed slowness of the process, the other is the choice of adequate material for experimental purposes. Darwin's hypothesis of natural selection as the means by which new types arise, is now being generally interpreted as stating the slow transformation of ordinary fluctuating divergencies from the average type into specific differences. But in doing so it is overlooked that Quetelet's law of fluctuating variability was not yet discovered at the time, when Darwin propounded his theory. So there [464] is no real and intimate connection between these two great conceptions. Darwin frequently pointed out that a long period of time might be needed for slow improvements, and was also a condition for the occurrence of rare sports. In any case those writers have been in error, according to my opinion, who have refrained from experimental work on the origin of species, on account of this narrow interpretation of Darwin's views. The choice of the material is quite another question, and obviously all depends upon this choice. Promising instances must be sought for, but as a rule the best way is to test as many plants as possible. Many of them may show nothing of interest, but some might lead to the desired end.

For to-day's lecture I have chosen an instance, in which the grounds upon which the choice was based are very evident. It is the origin of the peloric toad-flax (_Linaria vulgaris peloria_).

The ground for this choice lies simply in the fact that the peloric toad-flax is known to have originated from the ordinary type at different times and in different countries, under more or less divergent conditions. It had arisen from time to time, and hence I presumed that there was a chance to see it arise again. If this should happen under experimental circ.u.mstances [465] the desired evidence might easily be gathered. Or, to put it in other words, we must try to arrange things so as to be present at the time when nature produces another of these rare changes.

There was still another reason for choosing this plant for observational work. The step from the ordinary toad-flax to the peloric form is short, and it appears as if it might be produced by slow conversion. The ordinary species produces from time to time stray peloric flowers. These occur at the base of the raceme, or rarely in the midst of it. In other species they are often seen at the summit. Terminal pelories are usually regular, having five equal spurs. Lateral pelories are generally of zygomorphic structure, though of course in a less degree than the normal bil.a.b.i.ate flowers, but they have unequal spurs, the middle one being of the ordinary length, the two neighboring being shorter, and those standing next to the opposite side of the flower being the shortest of all. This curious remainder of the original, symmetrical structure of the flower seems to have been overlooked hitherto by the investigators of peloric toad-flaxes.

The peloric variety of this plant is characterized by its producing only peloric flowers. No single bil.a.b.i.ate or one-spurred flower remains.

[466] I once had a lot of nearly a hundred specimens of this fine variety, and it was a most curious and beautiful sight to observe the many thousands of nearly regular flowers blooming at the same time. Some degree of variability was of course present, even in a large measure.

The number of the spurs varied between four and six, transgressing these limits in some instances, but never so far as to produce really one-spurred flowers. Comparing this variety with the ordinary type, two ways of pa.s.sing over from the one to the other might be imagined. One would entail a slow increase of the number of the peloric flowers on each plant, combined with a decrease of the number of the normal ones, the other a sudden leap from one extreme to the other without any intermediate steps. The latter might easily be overlooked in field observations and their failure may not have the value of direct proof.

They could never be overlooked, on the other hand, in experimental culture.

The first record of the peloric toad-flax is that of Zioberg, a student of Linnaeus, who found it in the neighborhood of Upsala. This curious discovery was described by Rudberg in his dissertation in the year 1744.

Soon afterwards other localities were discovered by Link near Gottingen in Germany about 1791 and afterwards [467] in the vicinity of Berlin, as stated by Ratzeburg, 1825. Many other localities have since been indicated for it in Europe, and in my own country some have been noted of late, as for instance near Zandvoort in 1874 and near Oldenzaal in 1896. In both these last named cases the peloric form arose spontaneously in places which had often been visited by botanists before the recorded appearance, and therefore, without any doubt, they must have been produced directly and independently by the ordinary species which grows in the locality. The same holds good for other occurrences of it. In many instances the variety has been recorded to disappear after a certain lapse of time, the original specimens dying out and no new ones being produced. _Linaria_ is a perennial herb, multiplying itself easily by buds growing on the roots, but even with this means of propagation its duration seems to have definite limits.

There is one other important point arguing strongly for the independent appearance of the peloric form in its several localities. It is the difficulty of fertilization and the high degree of sterility, even if artificially pollinated. Bees and b.u.mble-bees are unable to crawl into the narrow tubular flowers, and to bring the fertilizing pollen to the stigma. Ripe capsules with seeds [468] have never been seen in the wild state. The only writer who succeeded in sowing seeds of the peloric variety was Wildenow and he got only very few seedlings. But even in artificial pollination the result is the same, the anthers seeming to be seriously affected by the change. I tried both self-fertilization and cross-pollination, and only with utmost care did I succeed in saving barely a hundred seeds. In order to obtain them I was compelled to operate on more than a thousand flowers on about a dozen peloric plants.

The variety being wholly barren in nature, the a.s.sumption that the plants in the different recorded localities might have a common origin is at once excluded. There must have been at least nearly as many mutations as localities. This strengthens the hope of seeing such a mutation happen in one's own garden. It should also be remembered that peloric flowers are known to have originated in quite a number of different species of _Linaria_, and also with many of the allied species within the range of the l.a.b.i.atiflorae.

I will now give the description of my own experiment. Of course this did not give the expected result in the first year. On the contrary, it was only after eight years' work that I had the good fortune of observing the mutation. [469] But as the whole life-history of the preceding generations had been carefully observed and recorded, the exact interpretation of the fact was readily made.

My culture commenced in the year 1886. I chose some plants of the normal type with one or two peloric flowers besides the bil.a.b.i.ate majority which I found on a locality in the neighborhood of Hilversum in Holland.

I planted the roots in my garden and from them had the first flowering generation in the following summer. From their seeds I grew the second generation in three following years. They flowered profusely and produced in 1889 only one, and in 1890 only two peloric structures. I saved the seeds in 1889 and had in 1890-1891 the third generation. These plants likewise flowered only in the second year, and gave among some thousands of symmetrical blossoms, only one five-spurred flower. I pollinated this flower myself, and it produced abundant fruit with enough seeds for the entire culture in 1892, and they only were sown.

Until this year my generations required two years each, owing to the perennial habit of the plants. In this way the prospects of the culture began to decrease, and I proposed to try to heighten my chances by having a new generation yearly. With this intention I sowed the [470]

selected seeds in a pan in the gla.s.shouse of my laboratory and planted them out as soon as the young stems had reached a length of some few centimeters. Each seedling was put in a separate pot, in heavily manured soil. The pots were kept under gla.s.s until the beginning of June, and the young plants produced during this period a number of secondary stems from the curious hypocotylous buds which are so characteristic of the species. These stems grew rapidly and as soon as they were strong enough, the plants were put into the beds. They all, at least nearly all, some twenty specimens, flowered in the following month.

I observed only one peloric flower among the large number present. I took the plant bearing this flower and one more for the culture of the following year, and destroyed all others. These two plants grew on the same spot, and were allowed to fertilize each other by the agency of the bees, but were kept isolated from any other congener. They flowered abundantly, but produced only one-spurred bil.a.b.i.ate flowers during the whole summer. They matured more than 10 cu. cm. of seeds.

It is from this pair of plants that my peloric race has sprung. And as they are the ancestors of the first closely observed case of peloric mutation, [471] it seems worth while to give some details regarding their fertilization.

Isolated plants of _Linaria vulgaris_ do not produce seed, even if freely pollinated by bees. Pollen from other plants is required. This requirement is not at all restricted to the genus _Linaria_, as many instances are known to occur in different families. It is generally a.s.sumed that the pollen of any other individual of the same species is capable of producing fertilization, although it is to be said that a critical examination has been made in but few instances.

This, however, is not the case, at least not in the present instance. I have pollinated a number of plants, grown from seed of the same strain and combined them in pairs, and excluded the visits of insects, and pollen other than that of the plant itself and that of the specimen with which it was paired. The result was that some pairs were fertile and others barren. Counting these two groups of pairs, I found them nearly equal in number, indicating thereby that for any given individual the pollen of half of the others is potent, but that of the other half impotent. From these facts we may conclude the presence of a curious case of dimorphy, a.n.a.logous to that proposed for the primroses, but without visible differentiating marks in the flowers. At least such opposite characters [472] have as yet not been ascertained in the case of our toad-flax.

In order to save seed from isolated plants it is necessary, for this reason, to have at least two individuals, and these must belong to the two physiologically different types. Now in the year 1892, as in other years, my plants, though separated at the outset by distances of about 20 cm. from each other, threw out roots of far greater length, growing in such a way as to abolish the strict isolation of the individuals. Any plot may produce several stems from such roots, and it is manifestly impossible to decide whether they all belong to one original plant or to the mixed roots of several individuals. No other strains were grown on the same bed with my plants however, and so I considered all the stems of the little group as belonging to one plant. But their perfect fertility showed, according to the experience described, that there must have been at least two specimens mingled together.