Lecture Xxiii

TAXONOMIC ANOMALIES

The theory of descent is founded mainly on comparative studies, which have the advantage of affording a broad base and the convincing effect of concurrent evidence brought together from widely different sources. The theory of mutation on the other hand rests directly upon experimental investigations, and facts concerning the actual descent of one form from another are as yet exceedingly rare. It is always difficult to estimate the validity of conclusions drawn from isolated instances selected from the whole range of contingent phenomena, and this is especially true of the present case. Systematic and physiologic facts seem to indicate the existence of universal laws, and it is not probable that the process of production of new species would be different in the various parts of the animal and vegetable kingdoms. Moreover the principle of unit-characters, the preeminent significance of which has come to be more fully recognized of late, is in full harmony with the theory of sudden mutations. Together these two conceptions go to strengthen the probability of the sudden origin of all specific characters.

Experimental researches are limited in their extent, and the number of cases of direct observation of the process of mutation will probably never become large enough to cover the whole field of the theory of descent. Therefore it will always be necessary to show that the similarity between observed and other cases is such as to lift above all doubt the assertion of their resulting from the same causes.

Besides the direct comparison of the mutations described in our former lectures, with the analogous cases of the horticultural and natural production of species and varieties at large, another way is open to obtain the required proof. It is the study of the phenomena, designated by Casimir de Candolle by the name of taxonomic anomalies. It is the assertion that characters, which are specific in one case, may be observed to arise as anomalies or as varieties in other instances. If they can be shown to be identical or nearly so in both, it is obviously allowable to assume the same origin for the specific character and for the anomaly. In other terms, the specific marks may be considered as having originated according to the laws that govern the production of anomalies, and we may assume them to lie within reach of our experiments. The experimental treatment of the origin of species may also be looked upon as a method within our grasp.

The validity and the significance of these considerations will at once become clear, if we choose a definite example. The broadest and most convincing one appears to me to be afforded by the cohesion of the petals in gamopetalous flowers. According to the current views the families with the petals of their flowers united are regarded as one or two main branches of the whole pedigree of the vegetable kingdom. Eichler and others assume them to constitute one branch, and therefore one large subdivision of the system. Bessey, on the other hand, has shown the probability of a separate origin for those groups which have inferior ovaries. Apart from such divergencies the connation of the petals is universally recognized as one of the most important systematic characters.

How may this character have originated? The heath-family or the Ericaceae and their nearest allies are usually considered to be the lowest of the gamopetalous plants. In them the cohesion of the petals is still subject to reversionary exceptions. Such cases of atavism may be observed either as specific marks, or in the way of anomalies. Ledum, Monotropa and Pyrola, or the Labrador tea, the Indian pipe and wintergreen are instances of reversionary gamopetalism with free petals. In heaths (Erica Tetralix) and in rhododendrons the same deviation is observed to occur from time to time as an anomaly, and even the common Rhododendron ponticum of our gardens has a variety in which the corolla is more or less split. Sometimes it exhibits five free petals, while at other times only one or two are entirely free, the remaining four being incompletely loosened.

Such cases of atavism make it probable that the coherence of the petals has originally arisen by the same method, but by action in the opposite direction. The direct proof of this conclusion is afforded by a curious observation, made by Vilmorin upon the bright and large-flowered garden-poppy, Papaver bracteatum. Like all poppies it has four petals, which are free from one another. In the fields of Messrs. Vilmorin, where it is largely cultivated for its seeds, individuals occur from time to time which are anomalous in this respect. They exhibit a tendency to produce connate petals. Their flowers become monopetalous, and the whole strain is designated by the name of Papaver bracteatum monopetalum. Henry de Vilmorin had the kindness to send me some of these plants, and they have flowered in my garden during several years. The anomaly is highly variable. Some flowers are quite normal, exhibiting no sign of connation; others are wholly gamopetalous, the four petals being united from their base to the very margin of the cup formed. In consequence of the broadness of the petals however, this cup is so wide as to be very shallow.

Intermediate states occur, and not infrequently. Sometimes only two or three petals are united, or the connation does not extend the entire length of the petals. These cases are quite analogous to the imperfect splitting of the corolla of the rhododendron. Giving free rein to our imagination, we may for a moment assume the possibility of a new subdivision of the vegetable kingdom, arising from Vilmorin's poppy and having gamopetalous flowers for its chief character. If the character became fixed, so as to lose its present state of variability, such a group of supposititious gamopetalous plants might be quite analogous to the corresponding real gamopetalous families. Hence there can be no objection to the view, that the heaths have arisen in an analogous manner from their polypetalous ancestors. Other species of the same genus have also been recorded to produce gamopetalous flowers, as for instance, Papaver hybridum, by Hoffmann. Poppies are not the sole example of accidental gamopetaly. Linnaeus observed the same deviation long ago for Saponaria officinalis, and since, it has been seen in Clematis Vitalba by Jaeger, in Peltaria alliacea by Schimper, in Silene annulata by Boreau and in other instances. No doubt it is not at all of rare occurrence, and the origin of the present gamopetalous families is to be considered as nothing extraordinary. It is, as a matter of fact, remarkable that it has not taken place in more numerous instances, and the mallows show that such opportunities have been available at least more than once.

Other instances of taxonomic anomalies are afforded by leaves. Many genera, the species of which mainly bear pinnate or palmate leaves, have stray types with undivided leaves. Among the brambles, Rubus odoratus and R. flexuosus may be cited, among the aralias, Aralia crassifolia and A. papyrifera, and among the jasmines, the deliciously scented sambac (Jasminum Sambac). But the most curious instance is that of the telegraph-plant, or Desmodium gyrans, each complete leaf of which consists of a large terminal leaflet and two little lateral ones. These latter keep up, night and day, an irregular jerking movement, which has been compared to the movements of a semaphore. Desmodium is a papilionaceous plant and closely allied to the genus Hedysarum, which has pinnate leaves with numerous pairs of leaflets. Its place in the system leaves no doubt concerning its origin from pinnate-leaved ancestors. At the time of its origination its leaves must have become reduced as to the number of the blades, while the size of the terminal leaflet was correspondingly increased.

It might seem difficult to imagine this great change taking place suddenly. However, we are compelled to familiarize ourselves with such hypothetical assumptions. Strange as they may seem to those who are accustomed to the conception of continuous slow improvements, they are nevertheless in complete agreement with what really occurs. Fortunately the direct proof of this assertion can be given, and in a case which is narrowly related, and quite parallel to that of the Desmodium, since it affects a plant of the same family. It is the case of the monophyllous variety of the bastard-acacia or Robinia Pseud-Acacia. In a previous lecture we have seen that it originated suddenly in a French nursery in the year 1855. It can be propagated by seed, and exhibits a curious degree of variability of its leaves. In some instances these are one-bladed, the blade reaching a length of 15 cm., and hardly resembling those of the common bastard-acacia. Other leaves produce one or two small leaflets at the base of the large terminal one, and by this contrivance are seen to be very similar to those of the Desmodium, repeating its chief characters nearly exactly, and only differing somewhat in the relative size of the various parts. Lastly real intermediates are seen between the monophyllous and the pinnate types. As far as I have been able to ascertain, these are produced on weak twigs under unfavorable conditions; the size of the terminal leaflet decreases and the number of the lateral blades increases, showing thereby the presence of the original pinnate type in a latent condition.

The sudden origin of this "one-leaved" acacia in a nursery may be taken as a prototype of the ancient origin of Desmodium. Of course the comparison only relates to a single character, and the movements of the leaflets are not affected by it. But the monophylly, or rather the size of the terminal blade and the reduction of the lateral ones, may be held to be sufficiently illustrated by the bastard-acacia. It is worth while to state, that analogous varieties have also arisen in other genera. The "one-leaved" strawberry has already been referred to. It originated from the ordinary type in Norway and at Paris. The walnut likewise, has its monophyllous variety. It was mentioned for the first time as a cultivated tree about 1864, but its origin is unknown. A similar variety of the walnut, with "one-bladed" leaves but of varying shapes, was found wild in a forest near Dieppe in France some years ago, and appeared to be due to a sudden mutation.

Something more is known concerning the "one-bladed" ashes, varieties of which are often seen in our parks and gardens. The common form has broad and deeply serrate leaves, which are far more rounded than the leaflets of the ordinary ash. The majority of the leaves are simple, but some produce one or two smaller leaflets at their base, closely corresponding in this respect to the variations of the "one-bladed" bastard-acacia, and evidently indicating the same latent and atavistic character. In some instances this analogy goes still further, and incompletely pinnate leaves are produced with two or more pairs of leaflets. Besides this variable type another has been described by Willdenow. It has single leaves exclusively, never producing smaller lateral leaflets, and it is said to be absolutely constant from seed, while the more variable types seem to be also more inconstant when propagated sexually. The difference is so striking and affords such a reliable feature that Koch proposed to make two distinct varieties of them, calling the pure type Fraxinus excelsior monophylla, and the varying trees F. excelsior exheterophylla. Some writers, and among them Willdenow, have preferred to separate the "one-leaved" forms from the species, and to call them Fraxinus simplicifolia.

According to Smith and to Loudon, the "one-leaved" ashes are found wild in different districts in-England. Intermediate forms have not been recorded from these localities. This mode of origin is that already detailed for the laciniate varieties of alders and so many other trees. Hence it may be assumed that the "one-leaved" ashes have sprung suddenly but frequently from the original pinnate species. The pure type of Willdenow should, in this case, be considered as due to a slightly different mutation, perhaps as a pure retrograde variety, while the varying strains may only be eversporting forms. This would likewise explain part of their observed inconstancy.

In this respect the historic dates, as collected by Korshinsky, are not very convincing. Vicinism has of course, almost never been excluded, and part of the multiformity of the offspring must obviously be due to this most universal agency. Indirect vicinism also plays some part, and probably affords the explanation of some reputed mutative productions of the variety. So, for instance, in the case of Sinning, who after sowing the seeds of the common ash, got as large a proportion as 2% of monophyllous trees in a culture of some thousand plants. It is probable that his seeds were taken partly from normal plants, and partly from hybrids between the normal and the "one-bladed" type, assuming that these hybrids have pinnate leaves like their specific parent, and bear the characters of the other parent only in a latent condition.

Our third example relates to peltate leaves. They have the stalk inserted in the middle of the blade, a contrivance produced by the connation of the two basal lobes. The water-lilies are a well known instance, exhibiting sagittate leaves in the juvenile stage and changing in many species, into nearly circular peltate forms, of which Victoria regia is a very good example, although its younger stages do not always excite all the interest they deserve. The Indian cress (Tropaeolum), the marsh pennywort or Hydrocotyle, and many other instances could be quoted. Sometimes the peltate leaves are not at all orbicular, but are elongated, oblong or elliptic, and with only the lobes at the base united. The lemon-scented Eucalyptus citriodora is one of the most widely known cases. In other instances the peltate leaves become more or less hollow, constituting broad ascidia as in the case of the crassulaceous genus Umbilicus.

This connation of the basal lobes is universally considered as a good and normal specific character. Nevertheless it has its manifest analogy in the realm of the anomalies. This is the pitcher or ascidium. On some trees it is of quite common occurrence, as on the lime-tree (Tilia parvifolia) and the magnolia (Magnolia obovata and its hybrids). It is probable that both these forms have varieties with, and others without, ascidia. Of the lime-tree, instances are known of single trees which produce hundreds of such anomalous leaves yearly, and one such a tree is growing in the neighborhood of Amsterdam at Lage Vuursche. I have alluded to these cases more than once, but on this occasion a closer inspection of the structure of the ascidium is required. For this purpose we may take the lime-tree as an example. Take the shape of the normal leaves in the first place. These are cordate at their base and mainly inequilateral, but the general shape varies to a considerable extent. This variation is closely related to the position of the leaves on the twigs, and shows distinct indications of complying with the general law of periodicity. The first leaves are smaller, with more rounded lobes, the subsequent leaves attain a larger size, and their lobes slightly change their forms. In the first leaves the lobes are so broad as to touch one another along a large part of their margins, but in organs formed later this contact gradually diminishes and the typical leaves have the lobes widely separated. Now it is easily understood that the contact or the separation of the lobes must play a part in the construction of the ascidia, as soon as the margins grow together. Leaves which touch one-another, may be affected by the connation without any further malformation. They remain flat, become peltate and exhibit a shape which in some way holds a middle position between the pennyworts and the lemon-scented eucalyptus. Here we have the repetition of the specific characters of these plants by the anomaly of another. Whenever the margins are not in contact, and become connate, notwithstanding their separation, the blade must be folded together in some slight degree, in order to produce the required contact. This is the origin of the ascidium. It is quite superfluous to insist upon the fact that their width or narrowness must depend upon the corresponding normal form. The more distant the lobes, the deeper the ascidium will become. It should be added that this explanation of the different shapes of ascidia is of general validity.

Ascidia of the snake-plantain or Plantago lanceolata are narrow tubes, because the leaves are oblong or lanceolate, while those of the broad leaved species of arrowhead, as for instance, the Sagittaria japonica, are of a conical shape.

From the evidence of the lime-tree we may conclude that normal peltate leaves may have originated in the same way. And from the fact that pitchers are one of the most frequent anomalies, we may conclude that the chance of producing peltate leaves must have been a very great one, and wholly sufficient to account for all observed cases. In every instance the previously existing shape of the leaf must have decided whether peltate or pitcher-like leaves would be formed. As far as we can judge peltate anomalies are quite uninjurious, while ascidia are forms which must impede the effect of the light on the leaf, as they conceal quite an important part of the upper surface. In this way it is easily conceivable that peltate leaves are a frequent specific character, while ascidia are not, as they only appear in the special cases of limited adaptation, as in the instances of the so called pitcher-plants. The genera Nepenthes, Sarracenia and some others are very well known and perhaps even the bladderworts or Utricularia might be included here.

The reproduction of specific characters by anomalous ascidia is not at all limited to the general case as described above. More minute details may be seen to be duplicated in the same way. Proofs are afforded on one side by incomplete ascidia, and on the other by the double cups.

Incomplete ascidia are those of the Nepenthes. The leaf is divided into three parts, a blade, a tendril and the pitcher. Or in other words, the limb produces a tendril at its summit, by means of which the plant is enabled to fasten itself to surrounding shrubs and to climb between their branches. But the end of this tendril bears a well-formed urn, which however, is produced only after the revolving and grasping movements of the tendril have been made. Some species have more rounded and some more elongated ascidia and often the shape is seen to change with the development of the stem. The mouth of the urn is strengthened by a thick rim and covered with a lid. Numerous curious contrivances in these structures to catch ants and other insects have been described, but as they have no relation to our present discussion, we shall abstain from dealing with them. Likewise we must refrain from a consideration of the physiologic qualities of the tendril, and confine our attention to the combination of a limb, a naked midvein and an ascidium. This combination is to be the basis of our discussion. It is liable to be produced all of a sudden. This assertion is proved by its occurrence as a varietal mark in one of our most ordinary cultivated plants. It is the group known as Croton, belonging to the genus Codiaeum. A variety is called interruptum and another appendiculatum, and these names both relate to the interruption of the leaves by a naked midvein. The leaves are seen to be built up of three parts. The lower half retains the aspect of a limb; it is crowned by a vein without lateral nerves or blade-like expansions, and this stalk in its turn bears a short limb on its summit. The base of this apical limb exhibits two connate lobes, forming together a wide cup or ascidium. It should be stated that these interruptum varieties are highly variable, especially in the relative size of the three principal parts of the leaf. Though it is of course conceded that the ascidium of Nepenthes has many secondary devices which are lacking in Croton, it seems hardly allowable to deny the possibility of an analogous origin for both. Those of the Croton, according to our knowledge regarding similar cases, must have arisen at once, and hence the conclusion that the ascidia of Nepenthes are also originally due to a sudden mutation. Interrupted leaves, with an ascidium on a naked prolongation of the midvein, are by no means limited to the Croton varieties. As stray anomalies they have often been observed, and I myself had the opportunity of collecting them on magnolia, on clover and on some other species. They are additional evidence in support of the explanation given above.

In the same way double ascidia may be made use of to explain the foliar cups of the teasels and some other plants, as for instance, some European snakeroots (Eryngium maritimum and E. campestre), or the floral leaves of the honeysuckle. The leaves on the stems of the teasels are disposed in pairs, and the bases of the two leaves of each pair are connate so as to constitute large cups. We have already mentioned these cups, and recall them in the present connection to use them as a prototype of the double ascidia. These are constituted of two opposite leaves, accidentally connated at their base or along some part of their margins. If the leaves are sessile, the analogy with the teasels is complete, as shown, for instance, in a case of Cotyledon, a crassulaceous plant which is known to produce such cups from time to time. They are narrower than those of the teasel, but this depends, as we have seen for the "one-leaved" ascidium, on the shape of the original leaf. In other respects they exactly imitate the teasel cups showing thereby how these cups may probably have originated.

In numerous cases of anomalies some accidental structures are parallel to specific characters, while others are not, being obviously injurious to their bearers. So it is also with the double ascidia. In the case of stalked leaves the two opposite stalks must, of course, constitute a long and very narrow tube, when growing together. This tube must bear at its summit the conical ascidium produced by the two connate limbs. At its base however, it includes the terminal bud of the stem, and frequently the tube is so narrow as to impede its further development. By this contrivance the double ascidium assumes a terminal position. Instances have been observed on magnolia, in Boehmeria and in other cases.

Flowers on leaves are of rare occurrence. Notwithstanding this, they constitute specific characters in some instances, accidental anomalies in others. Helwingia rusciflora flora is the most curious and best known instance. It is a little shrub, belonging to the Cornaceae, and has broad elliptical undivided leaves. On the middle of the midvein these leaves are seen to bear small clusters of flowers; indeed this is the only place where flowers are produced. Each cluster has from 13-15 flowers, of which some are staminate and borne on stalks, while others are pistillate and nearly sessile. These flowers are small and of a pale greenish color and yield small stone-fruits, with a thin coating of pulpy tissue. As the name indicates, this mode of flowering is closely similar to that of Ruscus, which however, does not bear its flowers and berries on real leaves, but on leaflike expansions of the twigs. Phyllonoma ruscifolia, a saxifragaceous plant, bears the same specific name, indicating a similar origin of the flowers. Other instances have been collected by Casimir de Candolle, but their number is very small.

As a varietal mark, flowers on leaves likewise rarely occur. One instance however, is very remarkable, and we have already dealt with it, when treating of constant varieties, and of the lack of vicinism in the case of species with exclusive self-fertilization.

It is the "Nepaul-barley" or Hordeum trifurcatum. The leaves, which in this case bear the adventitious flowers, are the inner scales of the spikelets, and not on green leaves as in the cases already alluded to. But this of course makes no real difference. The character is variable to a high degree, and this fact indicates its varietal nature, though it should be recalled that at least with the Helwingia, the majority of the leaves are destitute of flowers, and that in this way some degree of variability is present in this normal case too.

All in all there are three sorts of "Nepaul-barley." They have the same varietal mark, but belong to different species of barley. These are differentiated according to the number of the rows in which the grains are seen on the spikes. These numbers may be two, four or six, giving rise to the specific names of Hordeum distichum, tetrastichum and hexastichum. Whether these three varieties are of independent, but parallel origin, or are to be considered as due to a single mutation and subsequent crosses is not known, all of them being of ancient origin. Historic evidence concerning their birth is wholly wanting. From analogy it would seem probable that the character had arisen by a mutation in one of the three named species, and had been transferred to others by means of accidental crosses, even as it has been artificially transmitted of late to quite a number of other sorts. But however admissible this conception may seem, there is of course no real objection to the assumption of independent and parallel mutations.

For the purpose of a comparison with the Helwingia type we are however, not at all concerned with the species to which the trifurcatum variety belongs, but only with the varietal mark itself. The spikelets may be one-, two- or three-flowered, according to the species. If we choose for further consideration the hexastichum type, each spikelet produces three normal flowers and afterwards three normal grains. Morphologically however, the spikelet is not homologous to those parts of other grasses which have the same name. It is constituted of three real spikelets, and thus deserves the name of a triple construction. Each of these three little organs has its normal pair of outer scales or glumae. These are linear and short, ending in a long and narrow spine. Those of the middle-most spikelets stand on its outer side, while those of the lateral part are placed transversely. In this way they form a kind of involucre around the central parts. The latter consist of the inner and outer palets or scales, each two of which include one of the flowers. The outer palet is to be considered as the metamorphosed leaf, in the aril of which the flower is produced. In the common sorts of barley it bears a long awn, giving thereby its typical aspect to the whole spike. The axillary flower is protected on the opposite side by a two-keeled inner palet. Each flower exhibits three stamens and an ovary. In the six-rowed barley all the three flowers of a triple spikelet are fertile, and each of them has a long awn on the top of the outer palet. But in the two-rowed species only the middle-most flower is normal and has an awn, the two remaining being sterile and more or less rudimentary and with only very short awns. From this description it is easily seen that the species of barley may be distinguished from one another, even at a casual glance, by the number of the rows of the awns, and therefore by the shape of the entire spikes. This striking feature, however, does not exist in the "Nepaul-barley." The awns are replaced by curiously shaped appendices, which are three-lobed. The central lobe is oblong and hollow, and forms a kind of hood, which covers a small supernumerary floret. The two lateral lobes are narrower, often linear and extended into a smaller or longer awn. These awns are mostly turned away from the center of the spike. The central lobe may sometimes bear two small florets, but ordinarily only one is to be found, and this is often incomplete, having only one or two stamens, or is different in some other way. These narrow lateral lobes heighten the abnormal aspect of the whole spike.

They are only produced at a somewhat advanced stage of the development of the palet, are united to one another and to the central part by strong veins, which form transversal anastomoses at their insertion. The length of these awns is very variable, and this quality is perhaps the most striking of the whole variety. Often they reach only 1-2 mm., or the majority may become longer and attain even 1 cm., while here and there, between them, longer ones are inserted, extending in some instances even as far as 3 cm. from the spike. Their transverse position in such cases is strikingly contrasted with the ordinary erect type of the awns.

These lateral lobes are to be regarded, from the morphologic point of view, as differentiated parts of the blade of the leaf. Before they are formed, or coincidently with the beginning of their development, the summit of the central lobe becomes hollow, and the development of the supernumerary flower commences. In different varieties, and especially in the most recent crosses of them, this development is excessively variable.

The accidental flower arises at some distance beneath the summit of the scale, on its middle vein. The development begins with the protrusion of a little scale, and the flower itself is situated beneath this scale, and is to be protected by it and by the primary scale, but is turned upside down at the same time. Opposite to this organ, which represents the outer palet of the adventitious flower, two little swollen bodies are evolved. In the normal flowers of barley and other grains and grasses their function is to open the flowers by swelling, and afterwards collapse and allow them to close.

In the adventitious flowers of the "Nepaul-barley," however, this function is quite superfluous. The stamens occur in varying numbers; typically there are three, but not rarely less, or more, are seen. In some instances the complete double whorl of six, corresponding to the ancestral monocotyledonous type, has been found. This is a very curious case of systematic atavism, quite analogous to the Iris pallida abavia, previously alluded to, which likewise has six stamens, and to the cases given in a previous lecture. But for our present discussion it is of no further interest. The ovary is situated in the middle of the flower, and in some instances two have been observed. This is also to be considered as a case of atavism.

All these parts of the adventitious flower are more or less subject to arrest of development, in a later stage. They may even sometimes become abnormal. Stamens may unite into pairs, or carpels bear four stigmas. The pollen-sacs are as a rule barren, the mother-cells undergoing atrophy, while normal grains are seen but rarely. Likewise the ovaries are rudimentary, but Wittmack has observed the occasional production of ripe grains from these abnormal florets.

The scale is seldom seen to extend any farther upwards than the supernumerary flower. But in the rare instances where it does prolong its growth, it may repeat the abnormality and bear a second floret above the first. This of course is generally much weaker, and more rudimentary.

Raciborsky, who has lately given a full and very accurate description of this anomaly, lays great stress upon the fact that it is quite useless. It is perhaps the most obviously useless structure in the whole vegetable kingdom. Notwithstanding this, it has come to be as completely hereditary as any of the most beautiful adaptations in nature. Therefore it is one of the most serious objections to the hypothesis of slow and gradual improvements on the sole ground of their usefulness. The struggle for life and natural selection are manifestly inadequate to give even the slightest indication of an explanation of this case. It is simply impossible to imagine the causes that might have produced such a character. The only way out of this difficulty is to assume that it has arisen at once, in its present apparently differentiated and very variable condition, and that, being quite uninjurious and since it does not decrease the fertility of the race, it has never been subjected to natural selection, and so has saved itself from destruction.

But if we once grant the probability of the origin of the "Nepaul-barley" by a sudden mutation, we obviously must assume the same in the case of the Helwingia and other normal instances. In this way we gain a further support for our assertion, that even the strangest specific characters may have arisen suddenly.

After having detailed at some length those proofs which seem to be the most striking, and which had not been previously described with sufficient detail, we may now take a hasty survey of other contingent cases. In the first place the cruciate flowers of some onagraceous plants should be remembered. Small linear petals occur as a specific character in Oenothera cruciata of the Adirondacks, but have been seen to arise as sudden mutations in the common evening-primrose (O. biennis) in Holland, and in the willow-herb (Epilobium hirsutum) in England. Leaves placed in whorls of three are very rare. The oleander, juniper and some few other plants have ternate whorls as a specific character. As an anomaly, ternate whorls are far more common, and perhaps any plant with opposite leaves may from time to time produce them. Races rich in this abnormality are found in the wild state in the yellow loosestrife or Lysimachia vulgaris, in which it is a very variable specific character, the whorls varying from two to four leaves. In the cultivated state it is met with in the myrtle or Myrtus communis, where it has come to be of some importance in Israelitic ritual. Crisped leaves are known in a mallow, Malva crispa, and as a variety in cabbages, parsley, lettuce and others. The orbicular fruits of Heeger's shepherd's purse (Capsella heegeri) recall similar fruits of other cruciferous genera, as for instance, Camelina. Screw-like stems with wide spirals are specific in the flower-stalks of Cyclamen and Vallisneria, varietal in Juncus effusus spiralis and accidental in Scirpus lacustris. Dormant buds or small bulbs in inflorescences are normal for wild onions, Polygonum viviparum and others, varietal in Poa alpina vivipara and perhaps in Agave vivipara, and accidental in plantains (Plantago lanceolata), Saxifraga umbrosa and others. Cleft leaves, one of the most general anomalies, are typical in Boehmeria biloba. The adnation of the peduncles of the inflorescences to the stem is typical in Solanum and accidental in many other cases.

It seems quite superfluous to add further proof. It is a very general phenomenon that specific characters occur in other genera as anomalies, and under such circumstances that the idea of a slow evolution on the ground of utility is absolutely excluded. No other explanation remains than that of a sudden mutation, and once granted for the abnormal cases, this explanation must obviously likewise be granted for the analogous specific characters.

Our whole discussion shows that mutations, once observed in definite instances, afford the most probable basis for the explanation of specific characters at large.