Part 27 (1/2)
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.