Species and Varieties, Their Origin by Mutation Part 32

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_, [672] _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. [673] 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 a.s.sertion 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 princ.i.p.al 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 a.n.a.logous origin for both. Those of the _Croton_, according to our knowledge regarding similar cases, must [674] 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 const.i.tute 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 const.i.tuted of two opposite leaves, accidentally connated at their base or along some part of their margins. If the leaves are sessile, the a.n.a.logy with the teasels is complete, as shown, for instance, in a case of _Cotyledon_, a cra.s.sulaceous plant which is [675] 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, const.i.tute 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 a.s.sumes 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 const.i.tute 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 [676] has broad elliptical undivided leaves. On the middle of the midvein these leaves are seen to bear small cl.u.s.ters of flowers; indeed this is the only place where flowers are produced. Each cl.u.s.ter 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 advent.i.tious flowers, are the inner scales of the spikelets, and not on green leaves as in the [677] 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 dest.i.tute 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 a.n.a.logy 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 [678] to the a.s.sumption 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 h.o.m.ologous to those parts of other gra.s.ses which have the same name. It is const.i.tuted 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 [679] 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. [680] 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 [681] 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 advent.i.tious flower, two little swollen bodies are evolved. In the normal flowers of barley and other grains and gra.s.ses their function is to open the flowers by swelling, and afterwards collapse and allow them to close.

In the advent.i.tious 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 a.n.a.logous 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 advent.i.tious flower are more or less subject to arrest of development, [682] 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 [683] 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 a.s.sume 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 a.s.sume the same in the case of the _Helwingia_ and other normal instances. In this way we gain a further support for our a.s.sertion, 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. [684] 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.

[685] 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 circ.u.mstances 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 a.n.a.logous 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.

[686]

LECTURE XXIV

THE HYPOTHESIS OF PERIODIC MUTATIONS

The prevailing belief that slow and gradual, nearly invisible changes const.i.tute the process of evolution in the animal and vegetable kingdom, did not offer a strong stimulus for experimental research. No appreciable response to any external agency was of course to be expected. Responses were supposed to be produced, but the corresponding outward changes would be too small to betray themselves to the investigator.

The direct observation of the mutations of the evening-primrose has changed the whole aspect of the problem at once. It is no longer a matter dealing with purely hypothetical conditions. Instead of the vague notions, uncertain hopes, and a priori conceptions, that have hitherto confused the investigator, methods of observation have been formulated, suitable for the attainment of definite results, the general nature of which is already known.

To my mind the real value of the discovery [687] of the mutability of the evening-primrose lies in its usefulness as a guide for further work.

The view that it might be an isolated case, lying outside of the usual procedure of nature, can hardly be sustained. On such a supposition it would be far too rare to be disclosed by the investigation of a small number of plants from a limited area. Its appearance within the limited field of inquiry of a single man would have been almost a miracle.

The a.s.sumption seems justified that a.n.a.logous cases will be met with, perhaps even in larger numbers, when similar methods of observation are used in the investigation of plants of other regions. The mutable condition may not be predicated of the evening-primroses alone. It must be a universal phenomenon, although affecting a small proportion of the inhabitants of any region at one time: perhaps not more than one in a hundred species, or perhaps not more than one in a thousand, or even fewer may be expected to exhibit it. The exact proportion is immaterial, because the number of mutable instances among the many thousands of species in existence must be far too large for all of them to be submitted to close scrutiny.

It is evident from the above discussion that next in importance to the discovery of the prototype of mutation is the formulation of methods [688] for bringing additional instances to light. These methods may direct effort toward two different modes of investigation. We may search for mutable plants in nature, or we may hope to induce species to become mutable by artificial methods. The first promises to yield results most quickly, but the scope of the second is much greater and it may yield results of far more importance. Indeed, if it should once become possible to bring plants to mutate at our will and perhaps even in arbitrarily chosen directions, there is no limit to the power we may finally hope to gain over nature.

What is to guide us in this new line of work? Is it the minute inspection of the features of the process in the case of the evening-primroses? Or are we to base our hopes and our methods on broader conceptions of nature's laws? Is it the systematic study of species and varieties, and the biologic inquiry into their real hereditary units? Or is the theory of descent to be our starting-point?

Are we to rest our conceptions on the experience of the breeder, or is perhaps the geologic pedigree of all organic life to open to us better prospects of success?

The answer to all such questions is a very simple one. All possibilities must be considered, and no line of investigation ignored. For myself I have based my field-researches and my [689] testing of native plants on the hypothesis of unit-characters as deduced from Darwin's _Pangenesis_.

This conception led to the expectation of two different kinds of variability, one slow and one sudden. The sudden ones known at the time were considered as sports, and seemed limited to retrograde changes, or to cases of minor importance. The idea that sudden steps might be taken as the princ.i.p.al method of evolution could be derived from the hypothesis of unit characters, but the evidence might be too remote for a starting point for experimental investigation.

The success of my test has given proof to the contrary. Hence the a.s.sertion that no evidence is to be considered as inadequate for the purpose under discussion. Sometime a method of discovering, or of producing, mutable plants may be found, but until this is done, all facts of whatever nature or direction must be made use of. A very slight indication may change forever the whole aspect of the problem.

The probabilities are now greatly in favor of our finding out the causes of evolution by a close scrutiny of what really happens in nature. A persistent study of the physiologic factors of this evolution is the chief condition of success. To this study field-observations may contribute as well as direct experiments, [690] microscopical investigations as well as extended pedigree-cultures. The cooperation of many workers is required to cover the field. Somewhere no doubt the desired principle lies hidden, but until it is discovered, all methods must be tried.

With this conception as the best starting point for further investigation, we may now make a brief survey of the other phase of the problem. We shall try to connect our observations on the evening-primroses with the theory of descent at large.

We start with two main facts. One is the mutability of Lamarck's primrose, and the second is the immutable condition of quite a number of other species. Among them are some of its near allies, the common and the small flowered evening-primrose, or _Oenothera biennis_ and _O.

muricata_.

From these facts, a very important question arises in connection with the theory of descent. Is the mutability of our evening-primroses temporary, or is it a permanent condition? A discussion of this problem will give us the means of reaching a definite idea as to the scope of our inquiries.

Let us consider the present first. If mutability is a permanent condition, it has of course no beginning, and moreover is not due to the [691] agency of external circ.u.mstances. Should this be granted for the evening-primrose, it would have to be predicated for other species found in a mutable state. Then, of course, it would be useless to investigate the causes of mutability at large, and we should have to limit ourselves to the testing of large numbers of plants in order to ascertain which are mutable and which not.

If, on the other hand, mutability is not a permanent feature, it must once have had a beginning, and this beginning itself must have had an external cause. The amount of mutability and its possible directions may be a.s.sumed to be due to internal causes. The determination of the moment at which they will become active can never be the result of internal causes. It must be a.s.signed to some external factor, and as soon as this is discovered the way for experimental investigation is open.

In the second place we must consider the past. On the supposition of permanency all the ancestors of the evening-primrose must have been mutable. By the alternative view mutability must have been a periodic phenomenon, producing at times new qualities, and at other times leaving the plants unchanged during long successions of generations. The present mutable state must then have been preceded by an immutable [692]

condition, but of course thousands of mutations must have been required to produce the evening-primroses from their most remote ancestors.

If we take the species into consideration that are not mutable at present, we may ask how we are to harmonize them with each of the two theories proposed. If mutability is permanent, it is manifest that the whole pedigree of the animal and vegetable kingdom is to be considered as built up of main mutable lines, and that the thousands of constant species can only be taken to represent lateral branches of the genealogic tree.

These lateral branches would have lost the capacity of mutating, possessed by all their ancestors. And as the principle of the hypothesis under discussion does not allow a resumption of this habit, they would be doomed to eternal constancy until they finally die out. Loss of mutability, under this conception, means loss of the capacity for all further development. Only those lines of the main pedigree which have retained this capacity would have a future; all others would die out without any chance of progression.

If, on the other hand, mutability is not permanent, but a periodic condition, all lines of the genealogic tree must be a.s.sumed to show alternatively [693] mutating and constant species. Some lines may be mutating at the present moment; others may momentarily be constant. The mutating lines will probably sooner or later revert to the inactive state, while the powers of development now dormant may then become awakened on other branches.

The view of permanency represents life as being surrounded with unavoidable death, the principle of periodicity, on the contrary, follows the idea of resurrection, granting the possibility of future progression for all living beings. At the same time it yields a more hopeful prospect for experimental inquiry.