Species and Varieties, Their Origin by Mutation Part 27

On the other hand the mutants, that const.i.tute the first representatives of their race, exhibit all the attributes of the new type in full display at once. No series of generations, no selection, [559] no struggle for existence are needed to reach this end. In previous lectures I have mentioned that I have saved the seeds of the mutants whenever possible, and have always obtained repet.i.tions of the prototype only. Reversions are as absolutely lacking as is also a further development of the new type. Even in the case of the inconstant forms, where part of the progeny yearly return to the stature of _lamarckiana_, intermediates are not found. So it is also with _lata_, which is pistillate and can only be propagated by cross-fertilization. But though the current belief would expect intermediates at least in this case, they do not occur. I made a pedigree-culture of lata during eight successive generations, pollinating them in different ways, and always obtained cultures which were partly const.i.tuted of _lata_ and partly of _lamarckiana_ specimens. But the _lata_s remained _lata_ in all the various and most noticeable characters, never showing any tendency to gradually revert into the original form.

Intermediate forms, if not occurring in the direct line from one species to another, might be expected to appear perhaps on lateral branches. In this case the mutants of one type, appearing in the same year, would not be a pure type, but would exhibit different degrees of deviation from the parent. The best would then have to [560] be chosen in order to get the new type in its pure condition. Nothing of the kind, however, was observed. All the _oblonga_-mutants were pure _oblongas_. The pedigree shows hundreds of them in the succeeding years, but no difference was seen and no material for selection was afforded. All were as nearly equal as the individuals of old elementary species.

II. New forms spring laterally from the main stem.

The current conception concerning the origin of species a.s.sumes that species are slowly converted into others. The conversion is a.s.sumed to affect all the individuals in the same direction and in the same degree.

The whole group changes its character, acquiring new attributes. By inter-crossing they maintain a common line of progress, one individual never being able to proceed much ahead of the others.

The birth of the new species necessarily seemed to involve the death of the old one. This last conclusion, however, is hard to understand. It may be justifiable to a.s.sume that all the individuals of one locality are ordinarily intercrossed, and are moreover subjected to the same external conditions. They might be supposed to vary in the same direction if these conditions were changed slowly. But this could of course have no possible influence on the plants of the [561] same species growing in distant localities, and it would be improbable they should be affected in the same way. Hence we should conclude that when a species is converted into a new type in one locality this is only to be considered as one of numerous possible ones, and its alteration would not in the least change the aspect of the remainder of the species.

But even with this restriction the general belief is not supported by the evidence of the evening-primroses. There is neither a slow nor a sudden change of all the individuals. On the contrary, the vast majority remain unchanged; thousands are seen exactly repeating the original prototype yearly, both in the native field and in my garden. There is no danger that _lamarckiana_ might die out from the act of mutating, nor that the mutating strain itself would be exposed to ultimate destruction from this cause.

In older swarms, such as _Draba_ or _Helianthemum_, no such center, around which the various forms are grouped, is known. Are we to conclude therefore that the main strain has died out? Or is it perhaps concealed among the throng, being distinguished by no peculiar character? If our _gigas_ and _rubrinervis_ were growing in equal numbers with the _lamarckiana_ in the native field, would it be possible to decide [562]

which of them was the progenitor of the others? Of course this could be done by long and tedious crossing experiments, showing atavism in the progeny, and thereby indicating the common ancestor. But even this capacity seems to be doubtful and connected only with the state of mutability and to be lost afterwards. Therefore if this period of mutation were ended, probably there would be no way to decide concerning the mutual relations.h.i.+p of the single species.

Hence the lack of a recognizable main stem in swarms of elementary species makes it impossible to answer the question concerning their common origin.

Another phase of the opposition between the prevailing view and my own results seems far more important. According to the current belief the conversion of a group of plants growing in any locality and flowering simultaneously would be restricted to one type. In my own experiments several new species arose from the parental form at once, giving a wide range of new forms at the same time and under the same conditions.

III. New elementary species attain their full constancy at once.

Constancy is not the result of selection or of improvement. It is a quality of its own. It can neither be constrained by selection if it is absent [563] from the beginning, nor does it need any natural or artificial aid if it is present. Most of my new species have proved constant from the first. Whenever possible, the original mutants have been isolated during the flowering period and artificially self-fertilized. Such plants have always given a uniform progeny, all children exhibiting the type of the parent. No atavism was observed and therefore no selection was needed or even practicable.

Briefly considering the different forms, we may state that the full experimental proof has been given for the origin of _gigas_ and _rubrinervis_, for _albida_ and _oblonga_, and even for _nanella_, which is to be considered as of a varietal nature; with _lata_ the decisive experiment is excluded by its unis.e.xuality. _laevifolia_ and _brevistylis_ were found originally in the field, and never appeared in my cultures. No observations were made as to their origin, and seeds have only been sown from later generations. But these have yielded uniform crops, thereby showing that there is no ground for the a.s.sumption that these two older varieties might behave otherwise than the more recent derivatives.

_Scintillans_ and _elliptica_ const.i.tute exceptions to the rule given.

They repeat their character, from pure seed, only in part of the offspring. I have tried to deliver the _scintillans_ from this [564]

incompleteness of heredity, but in vain. The succeeding generations, if produced from true representatives of the new type, and with pure fertilization, have repeated the splitting in the same numerical proportions. The instability seems to be here as permanent a quality as the stability in other instances. Even here no selection has been adequate to change the original form.

IV. Some of the new strains are evidently elementary species, while others are to be considered as retrograde varieties.

It is often difficult to decide whether a given form belongs to one or another of these two groups. I have tried to show that the best and strictest conception of varieties limits them to those forms that have probably originated by retrograde or degressive steps. Elementary species are a.s.sumed to have been produced in a progressive way, adding one new element to the store. Varieties differ from their species clearly in one point, and this is either a distinct loss, or the a.s.sumption of a character, which may be met with in other species and genera. _laevifolia_ is distinguished by the loss of the crinkling of the leaves, _brevistylis_ by the partial loss of the epigynous qualities of the flowers, and _nanella_ is a dwarf. These three new forms are therefore [565] considered to const.i.tute only retrograde steps, and no advance. This conclusion has been fully justified by some crossing experiments with _brevistylis_, which wholly complies with Mendel's law, and in one instance with _nanella_, which behaves in the same manner, if crossed with _rubrinervis_.

On the other hand, _gigas_ and _rubrinervis_, _oblonga_ and _albida_ obviously bear the characters of progressive elementary species. They are not differentiated from _lamarckiana_ by one or two main features.

They diverge from it in nearly all organs, and in all in a definite though small degree. They may be recognized as soon as they have developed their first leaves and remain discernible throughout life.

Their characters refer chiefly to the foliage, but no less to the stature, and even the seeds have peculiarities. There can be little doubt but that all the attributes of every new species are derived from one princ.i.p.al change. But why this should affect the foliage in one manner, the flowers in another and the fruits in a third direction, remains obscure. To gain ever so little an insight into the nature of these changes, we may best compare the differences of our evening-primroses with those between the two hundred elementary species of _Draba_ and other similar instances. In doing so we find the same main [566] feature, the minute differences in nearly all points.

V. The same new species are produced in a large number of individuals.

This is a very curious fact. It embraces two minor points, viz: the mult.i.tude of similar mutants in the same year, and the repet.i.tion thereof in succeeding generations. Obviously there must be some common cause. This cause must be a.s.sumed to lie dormant in the _Lamarckiana_s of my strain, and probably in all of them, as no single parent-plant proved ever to be wholly dest.i.tute of mutability. Furthermore the different causes for the sundry mutations must lie latent together in the same parent-plant. They obey the same general laws, become active under similar conditions, some of them being more easily awakened than others. The germs of the _oblonga_, _lata_ and _nanella_ are especially irritable, and are ready to spring into activity at the least summons, while those of _gigas_, _rubrinervis_ and _scintillans_ are far more difficult to arouse.

These germs must be a.s.sumed to lie dormant during many successive generations. This is especially evident in the case of _lata_ and _nanella__, which appeared in the first year of the pedigree culture and which since have been repeated yearly, and have been seen to arise by mutation [567] also during the last season (1903). Only _gigas_ appeared but once, but then there is every reason to a.s.sume that in larger sowings or by a prolongation of the experiments it might have made a second appearance.

Is the number of such germs to be supposed to be limited or unlimited?

My experiment has produced about a dozen new forms. Without doubt I could easily have succeeded in getting more, if I had had any definite reason to search for them. But such figures are far from favoring the a.s.sumption of indefinite mutability. The group of possible new forms is no doubt sharply circ.u.mscribed. Partly so by the morphologic peculiarities of _lamarckiana_, which seem to exclude red flowers, composite leaves, etc. No doubt there are more direct reasons for these limits, some changes having taken place initially and others later, while the present mutations are only repet.i.tions of previous ones, and do not contribute new lines of development to those already existing.

This leads us to the supposition of some common original cause, which produced a number of changes, but which itself is no longer at work, but has left the affected qualities, and only these, in the state of mutability.

In nature, repeated mutations must be of far greater significance than isolated ones. How [568] great is the chance for a single individual to be destroyed in the struggle for life? Hundreds of thousands of seeds are produced by _lamarckiana_ annually in the field, and only some slow increase of the number of specimens can be observed. Many seeds do not find the proper circ.u.mstances for germination, or the young seedlings are destroyed by lack of water, of air, or of s.p.a.ce. Thousands of them are so crowded when becoming rosettes that only a few succeed in producing stems. Any weakness would have destroyed them. As a matter of fact they are much oftener produced in the seed than seen in the field with the usual unfavorable conditions; the careful sowing of collected seeds has given proof of this fact many times.

The experimental proof of this frequency in the origin of new types, seems to overcome many difficulties offered by the current theories on the probable origin of species at large.

VI. The relation between mutability and fluctuating variability has always been one of the chief difficulties of the followers of Darwin.

The majority a.s.sumed that species arise by the slow acc.u.mulation of slight fluctuating deviations, and the mutations were only to be considered as extreme fluctuations, obtained, in the main, by a continuous selection of small differences in a constant direction.

[569] My cultures show that quite the opposite is to be regarded as fact. All organs and all qualities of _lamarckiana_ fluctuate and vary in a more or less evident manner, and those which I had the opportunity of examining more closely were found to comply with the general laws of fluctuation. But such oscillating changes have nothing in common with the mutations. Their essential character is the heaping up of slight deviations around a mean, and the occurrence of continuous lines of increasing deviations, linking the extremes with this group. Nothing of the kind is observed in the case of mutations. There is no mean for them to be grouped around and the extreme only is to be seen, and it is wholly unconnected with the original type. It might be supposed that on closer inspection each mutation might be brought into connection with some feature of the fluctuating variability. But this is not the case.

The dwarfs are not at all the extreme variants of structure, as the fluctuation of the height of the _lamarckiana_ never decreases or even approaches that of the dwarfs. There is always a gap. The smallest specimens of the tall type are commonly the weakest, according to the general rule of the relations.h.i.+p between nourishment and variation, but the tallest dwarfs are of course the most robust specimens of their group. [570] Fluctuating variability, as a rule, is subject to reversion. The seeds of the extremes do not produce an offspring which fluctuates around their parents as a center, but around some point on the line which combines their attributes with the corresponding characteristic of their ancestors, as Vilmorin has put it. No reversion accompanies mutation, and this fact is perhaps the completest contrast in which these two great types of variability are opposed to each other.

The offspring of my mutants are, of course, subject to the general laws of fluctuating variability. They vary, however, around their own mean, and this mean is simply the type of the new elementary species.

VII. The mutations take place in nearly all directions.

Many authors a.s.sume that the origin of species is directed by unknown causes. These causes are a.s.sumed to work in each single case for the improvement of the animals and plants, changing them in a manner corresponding in a useful way to the changes that take place in their environment. It is not easy to imagine the nature of these influences nor how they would bring about the desired effect.

This difficulty was strongly felt by Darwin, and one of the chief purposes of his selection theory may be said to have been the attempt [571] to surmount it. Darwin tried to replace the unknown cause by natural agencies, which lie under our immediate observation. On this point Darwin was superior to his predecessors, and it is chiefly due to the clear conception of this point that his theory has gained its deserved general acceptance. According to Darwin, changes occur in all directions, quite independently of the prevailing circ.u.mstances. Some may be favorable, others detrimental, many of them without significance, neither useful nor injurious. Some of them will sooner or later be destroyed, while others will survive, but which of them will survive, is obviously dependent upon whether their particular changes agree with the existing environic conditions or not. This is what Darwin has called the struggle for life. It is a large sieve, and it only acts as such. Some fall through and are annihilated, others remain above and are selected, as the phrase goes. Many are selected, but more are destroyed; daily observation does not leave any doubt upon this point.

How the differences originate is quite another question. It has nothing to do with the theory of natural selection nor with the struggle for life. These have an active part only in the acc.u.mulation of useful qualities, and only in so [572] far as they protect the bearers of such characters against being crowded out by their more poorly const.i.tuted compet.i.tors.

However, the differentiating characteristics of elementary species are only very small. How widely distant they are from the beautiful adaptative organizations of orchids, of insectivorous plants and of so many others! Here the difference lies in the acc.u.mulation of numerous elementary characters, which all contribute to the same end. Chance must have produced them, and this would seem absolutely improbable, even impossible, were it not for Darwin's ingenious theory. Chance there is, but no more than anywhere else. It is not by mere chance that the variations move in the required direction. They do go, according to Darwin's view, in all directions, or at least in many. If these include the useful ones, and if this is repeated a number of times, c.u.mulation is possible; if not, there is simply no progression, and the type remains stable through the ages. Natural selection is continually acting as a sieve, throwing out the useless changes and retaining the real improvements. Hence the acc.u.mulation in apparently predisposed directions, and hence the increasing adaptations to the more specialized conditions of life. It must be obvious to any one who can free himself from the current ideas, [573] that this theory of natural selection leaves the question as to how the changes themselves are brought about, quite undecided. There are two possibilities, and both have been propounded by Darwin. One is the acc.u.mulation of the slight deviations of fluctuating variability, the other consists of successive sports or leaps taking place in the same direction.

In further lectures a critical comparison of the two views will be given. Today I have only to show that the mutations of the evening-primroses, though sudden, comply with the demands made by Darwin as to the form of variability which is to be accepted as the cause of evolution and as the origin of species.

Some of my new types are stouter and others weaker than their parents, as shown by _gigas_ and _albida_. Some have broader leaves and some narrower, _lata_ and _oblonga_. Some have larger flowers (_gigas_) or deeper yellow ones (_rubrinervis_), or smaller blossoms (_scintillans_), or of a paler hue (_albida_). In some the capsules are longer (_rubrinervis_), or thicker (_gigas_), or more rounded (_lata_), or small (_oblonga_), and nearly dest.i.tute of seeds (_brevistylis_). The unevenness of the surface of the leaves may increase as in _lata_, or decrease as in _laevifolia_. The tendency to become annual prevails in _rubrinervis_, but _gigas_ tends to become [574] biennial. Some are rich in pollen, while _scintillans_ is poor. Some have large seeds, others small. _Lata_ has become pistillate, while _brevistylis_ has nearly lost the faculty to produce seeds. Some undescribed forms were quite sterile, and some I observed which produced no flowers at all. From this statement it may be seen that nearly all qualities vary in opposite directions and that our group of mutants affords wide material for the sifting process of natural selection. On the original field the _laevifolia_ and _brevistylis_ have held their own during sixteen years and probably more, without, however, being able to increase their numbers to any noticeable extent. Others perish as soon as they make their appearance, or a few individuals are allowed to bloom, but probably leave no progeny.

But perhaps the circ.u.mstances may change, or the whole strain may be dispersed and spread to new localities with different conditions. Some of the latter might be found to be favorable to the robust _gigas_, or to _rubrinervis_, which requires a drier air, with rainfall in the springtime and suns.h.i.+ne during the summer. It would be worth while to see whether the climate of California, where neither _O. lamarckiana_ nor _O. biennis_ are found wild, would not exactly [575] suit the requirements of the new species _rubrinervis_ and _gigas_.

NOTE. _Oenothera_s are native to America and all of the species growing in Europe have escaped from gardens directly, or may have arisen by mutation, or by hybridization of introduced species. A fixed hybrid between _O. cruciata_ and _O. biennis_ const.i.tuting a species has been in cultivation for many years. The form known as _O. biennis_ in Europe, and used by de Vries in all of the experiments described in these lectures, has not yet been found growing wild in America and is not identical with the species bearing that name among American botanists.

Concerning this matter Professor de Vries writes under date of Sept. 12, 1905: "The '_biennis_' which I collected in America has proved to be a motley collection of forms, which at that time I had no means of distinguis.h.i.+ng. No one of them, so far as they are now growing in my garden is identical with our _biennis_ of the sand dunes." The same appears to be the case with _O. muricata_. Plants from the Northeastern American seaboard, identifiable with the species do not entirely agree with those raised from seed received from Holland.

_O. lamarckiana_ has not been found growing wild in America in recent years although the evidence at hand seems to favor the conclusion that it was seen and collected in the southern states in the last century.

(See MacDougal, Vail, Shull, and Small: Mutants and Hybrids of the _Oenotheras_. Publication 24. Carnegie Inst.i.tution. Was.h.i.+ngton, D.C., 1905.) EDITOR.

[576]

LECTURE XX

THE ORIGIN OF WILD SPECIES AND VARIETIES

New species and varieties occur from time to time in the wild state.

Setting aside all theoretical conceptions as to the common origin of species at large, the undoubted fact remains that new forms are sometimes met with. In the case of the peloric toad-flax the mutations are so numerous that they seem to be quite regular. The production of new species of evening-primroses was observed on the field and afterwards duplicated in the garden. There is no reason to think that these cases are isolated instances. Quite on the contrary they seem to be the prototypes of repeated occurrences in nature.

If this conception is granted, the question at once arises, how are we to deal with a.n.a.logous cases, when fortune offers them, and what can we expect to learn from them?