Chapter 31

Secondly the instability seems to be a constant quality, although the words themselves are at first sight, contradictory. I mean to convey the conception that the degree of instability remains unchanged during successive generations. This is a very curious fact, and strongly reminds us of the hereditary conditions of striped-flower varieties. But, on the contrary, the atavists, which are here the individuals with the stature and the characteristics of the _lamarckiana_, have become _lamarckiana_s in their hereditary qualities, too. If their seed is saved and sown, their progeny does not contain any _scintillans_, or at least no more than might arise by ordinary mutations.

One other inconstant new species is to be noted, but as it was very rare both in the field and in my cultures, and as it was difficult of cultivation, little can as yet be said about it. It is the _Oenothera elliptica_, with narrow elliptical leaves and also with elliptical petals. It repeats [546] its type only in a very small proportion of its seed.

All in all we thus have a group of a dozen new types, springing from an original form in one restricted locality, and seen to grow there, or arising in the garden from seeds collected from the original locality. Without any doubt the germs of the new types are fully developed within the seed, ready to be evolved at the time of germination. More favorable conditions in the field would no doubt allow all of the described new species to unfold their attributes there, and to come into competition with each other and with the common parents. But obviously this is only of secondary importance, and has no influence on the fact that a number of new types, analogous to the older swarms of _Draba_, _Viola_ and of many other polymorphous species, have been seen to arise directly in the wild state.

[547]

LECTURE XIX

EXPERIMENTAL PEDIGREE-CULTURES

The observation of the production of mutants in the field at Hilversum, and the subsequent cultivation of the new types in the garden at Amsterdam, gives ample proof of the mutability of plants. Furthermore it furnishes an analogy with the hypothetical origin of the swarms of species of _Draba_ and _Viola_. Last but not least important it affords material for a complete systematic and morphologic study of the newly arisen group of forms.

The physiologic laws, however, which govern this process are only very imperfectly revealed by such a study. The instances are too few. Moreover the seeds from which the mutants spring, escape observation. It is simply impossible to tell from which individual plants they have been derived. The laevifolia and the brevistylis have been found almost every year, the first always recurring on the same spot, the second on various parts of the original field. It is therefore allowable to assume a common [548] origin for all the observed individuals of either strain. But whether, besides this, similar strains are produced anew by the old _lamarckiana_ group, it is impossible to decide on the sole ground of these field-observations.

The same holds good with the other novelties. Even if one of them should germinate repeatedly, without ever opening its flowers, the possibility could not be excluded that the seeds might have come originally from the same capsule but lain dormant in the earth during periods of unequal length.

Other objections might be cited that can only be met by direct and fully controlled experiments. Next to the native locality comes the experimental garden. Here the rule prevails that every plant must be fertilized with pollen of its own, or with pollen of other individuals of known and recorded origin. The visits of insects must be guarded against, and no seeds should be saved from flowers which have been allowed to open without this precaution. Then the seeds of each individual must be saved and sown separately, so as to admit of an appreciation, and if necessary, a numerical determination of the nature of its progeny. And last but not least the experiments should be conducted in a similar manner during a series of successive years.

[549] I have made four such experiments, each comprising the handling of many thousands of individual plants, and lasting through five to nine generations. At the beginning the plants were biennial, as in the native locality, but later I learned to cultivate them in annual generations. They have been started from different plants and seeds, introduced from the original field into my garden at Amsterdam.

It seems sufficient to describe here one of these pedigree-cultures, as the results of all four were similar. In the fall of 1886 I took nine large rosettes from the field, planted them together on an isolated spot in the garden, and harvested their seeds the next year. These nine original plants are therefore to be considered as constituting the first generation of my race. The second generation was sown in 1888 and flowered in 1889. It at once yielded the expected result. 15,000 seedlings were tested and examined, and among them 10 showed diverging characters. They were properly protected, and proved to belong to two new types. 5 of them were _lata_ and 5 _nanella_. They flowered next year and displayed all the characters as described in our preceding lecture. Intermediates between them and the general type were not found, and no indication of their appearance was noted in their parents. [550] They came into existence at once, fully equipped, without preparation or intermediate steps. No series of generations, no selection, no struggle for existence was needed. It was a sudden leap into another type, a sport in the best acceptation of the word. It fulfilled my hopes, and at once gave proof of the possibility of the direct observation of the origin of species, and of the experimental control thereof.

The third generation was in the main a repetition of the second. I tried some 10,000 seedlings and found three _lata_ and three _nanella_, or nearly the same proportion as in the first instance. But besides these a _rubrinervis_ made its appearance and flowered the following year. This fact at once revealed the possibility that the instability of _lamarckiana_ might not be restricted to the three new types now under observation. Hence the question arose how it would be possible to obtain other types or to find them if they were present. It was necessary to have better methods of cultivation and examination of the young plants. Accordingly I devoted the three succeeding years to working on this problem.

I found that it was not at all necessary to sow any larger quantities of seed, but that the young plants must have room enough to develop into full and free rosettes. Moreover I observed [551] that the attributes of _lata_ and _nanella_, which I now studied in the offspring of my first mutants, were clearly discernible in extreme youth, while those of _rubrinervis_ remained concealed some weeks longer. Hence I concluded that the young plants should be examined from time to time until they proved clearly to be only normal _lamarckiana_. Individuals exhibiting any deviation from the type, or even giving only a slight indication of it, were forthwith taken out of the beds and planted separately, under circumstances as favorable as possible. They were established in pots with well-manured soil and kept under glass, but fully exposed to sunshine. As a rule they grew very fast, and could be planted out early in June. Some of them, of course, proved to have been erroneously taken for mutants, but many exhibited new characters.

All in all I had 334 young plants which did not agree with the parental type. As I examined some 14,000 seedlings altogether, the result was estimated at about 2.5%. This proportion is much larger than in the yields of the two first generations and illustrates the value of improved methods. No doubt many good mutations had been overlooked in the earlier observations.

As was to be expected, _lata_ and _nanella_ [552] were repeated in this third generation (1895). I was sure to get nearly all of them, without any important exceptions, as I now knew how to detect them at almost any age. In fact, I found many of them; as many as 60 _nanella_ and 73 _lata_, or nearly 5% of each. _Rubrinervis_ also recurred, and was seen in 8 specimens. It was much more rare than the two first-named types.

But the most curious fact in that year was the appearance of _oblonga_. No doubt I had often seen it in former years, but had not attached any value to the very slight differences from the type, as they then seemed to me. I knew now that any divergence was to be esteemed as important, and should be isolated for further observation. This showed that among the selected specimens not less than 176, or more than 1% belonged to the _oblonga_ type. This type was at that time quite new to me, and it had to be kept through the winter, to obtain stems and flowers. It proved to be as uniform as its three predecessors, and especially as sharply contrasted with _lamarckiana_. The opportunity for the discovery of any intermediates was as favorable as could be, because the distinguishing marks were hardly beyond doubt at the time of the selection and removal of the young plants. But no connecting links were found.

[553] The same holds good for _albida_, which appeared in 15 specimens, or in 0.1%, of the whole culture. By careful cultivation these plants proved not to be sickly, but to belong to a new, though weak type. It was evident that I had already seen them in former years, but having failed to recognize them had allowed them to be destroyed at an early age, not knowing how to protect them against adverse circumstances. Even this time I did not succeed in getting them strong enough to keep through the winter.

Besides these, two new types were observed, completing the range of all that have since been recorded to regularly occur in this family. They were _scintillans_ and _gigas_. The first was obtained in the way just described. The other hardly escaped being destroyed, not having showed itself early enough, and being left in the bed after the end of the selection. But as it was necessary to keep some rosettes through the winter in order to have biennial flowering plants to furnish seeds, I selected in August about 30 of the most vigorous plants, planted them on another bed and gave them sufficient room for their stems and branches in the following summer. Most of them sent up robust shoots, but no difference was noted till the first flowers opened. One plant had a much larger crown of bright blossoms than any of the others. [554] As soon as these flowers faded away, and the young fruits grew out, it became clear that a new type was showing itself. On that indication I removed all the already fertilized flowers and young fruits, and protected the buds from the visits of insects. Thus the isolated flowers were fertilized with their own pollen only, and I could rely upon the purity of the seed saved. This lot of seeds was sown in the spring of 1897 and yielded a uniform crop of nearly 300 young _gigas_ plants.

Having found how much depends upon the treatment, I could gradually decrease the size of my cultures. Evidently the chance of discovering new types would be lessened thereby, but the question as to the repeated production of the same new forms could more easily and more clearly be answered in this way. In the following year (1896) I sowed half as many seeds as formerly, and the result proved quite the same. With the exception of _gigas_ all the described forms sprang anew from the purely fertilized ancestry of normal _lamarckiana_s. It was now the fifth generation of my pedigree, and thus I was absolutely sure that the descendants of the mutants of this year had been pure and without deviation for at least four successive generations.

Owing partly to improved methods of selection, [555] partly no doubt to chance, even more mutants were found this year than in the former. Out of some 8,000 seedlings I counted 377 deviating ones, or nearly 5%, which is a high proportion. Most of them were _oblonga_ and _lata_, the same types that had constituted the majority in the former year.

_Albida_, _nanella_ and _rubrinervis_ appeared in large numbers, and even _scintillans_, of which I had but a single plant in the previous generation, was repeated sixfold.

New forms did not arise, and the capacity of my strain seemed exhausted. This conclusion was strengthened by the results of the next three generations, which were made on a much smaller scale and yielded the same, or at least the mutants most commonly seen in previous years.

Instead of giving the figures for these last two years separately, I will now summarize my whole experiment in the form of a pedigree. In this the normal _lamarckiana_ was the main line, and seeds were only sown from plants after sufficient isolation either of the plants themselves, or in the latter years by means of paper bags enclosing the inflorescences. I have given the number of seedlings of _lamarckiana_ which were examined each year in the table below. Of course by far the largest number of them were [556] thrown away as soon as they showed their differentiating characters in order to make room for the remaining ones. At last only a few plants were left to blossom in order to perpetuate the race. I have indicated for each generation the number of mutants of each of the observed forms, placing them in vertical columns underneath their respective heads. The three first generations were biennial, but the five last annual.

PEDIGREE OF A MUTATING FAMILY OF _OENOTHERA LAMARCKIANA_ IN THE EXPERIMENTAL GARDEN AT AMSTERDAM

Gener: O.gig. albida obl. rubrin. Lam. nanella lata. scint. VIII. 5 1 0 1700 21 1 VII. 9 0 3000 11 VI. 11 29 3 1800 9 5 1 V. 25 135 20 8000 49 142 6 IV. 1 15 176 8 14000 60 73 1 III. 1 10000 3 3 II. 15000 5 5 I. 9

It is most striking that the various mutations of the evening-primrose display a great degree of regularity. There is no chaos of forms, no indefinite varying in all degrees and in all directions. Quite on the contrary, it is at once evident that very simple rules govern the whole phenomenon.

I shall now attempt to deduce these laws from [557] my experiment. Obviously they apply not only to our evening-primroses, but may be expected to be of general validity. This is at once manifest, if we compare the group of new mutants with the swarms of elementary forms which compose some of the youngest systematic species, and which, as we have seen before, are to be considered as the results of previous mutations. The difference lies in the fact that the evening-primroses have been seen to spring from their ancestors and that the _drabas_ have not. Hence the conclusion that in comparing the two we must leave out the pedigree of the evening-primroses and consider only the group of forms as they finally show themselves. If in doing so we find sufficient similarity, we are justified in the conclusion that the _drabas_ and others have probably originated in the same way as the evening-primroses. Minor points of course will differ, but the main lines cannot have complied with wholly different laws. All so-called swarms of elementary species obviously pertain to a single type, and this type includes our evening-primroses as the only controlled case.

Formulating the laws of mutability for the evening-primroses we therefore assume that they hold good for numerous other corresponding cases.

[558] I. The first law is, that new elementary species appear suddenly, without intermediate steps.

This is a striking point, and the one that is in the most immediate contradiction to current scientific belief. The ordinary conception assumes very slow changes, in fact so slow that centuries are supposed to be required to make the differences appreciable. If this were true, all chance of ever seeing a new species arise would be hopelessly small. Fortunately the evening-primroses exhibit contrary tendencies. One of the great points of pedigree-culture is the fact that the ancestors of every mutant have been controlled and recorded. Those of the last year have seven generations of known _lamarckiana_ parents preceding them. If there had been any visible preparation towards the coming mutation, it could not have escaped observation. Moreover, if visible preparation were the rule, it could hardly go on at the same time and in the same individuals in five or six diverging directions, producing from one parent, _gigas_ and _nanella_, _lata_ and _rubrinervis_, _oblonga_ and _albida_ and even _scintillans_.

On the other hand the mutants, that constitute 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 repetitions 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 constituted 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 assumes that species are slowly converted into others. The conversion is assumed 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 assume 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 relationship 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 unisexuality. _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 assumption that these two older varieties might behave otherwise than the more recent derivatives.