Lecture Xv

DOUBLE ADAPTATIONS

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 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 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 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 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 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 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 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 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 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. 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 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 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 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 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 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 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 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 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 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.

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 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 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 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.

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 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 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, 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 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.

E. MUTATIONS