Species and Varieties, Their Origin by Mutation Part 37

* On about 200 ears the variability ranges from 8-22 rows, and this leads approximately to one row more by each doubling of the numbers of instances. One ear with 22 rows in 200 would thus lead to the expectation of one ear with 32 rows in 100,000 ears.

[785] is exactly what is gained by repeated selections. To my mind this reduction of the size of the cultures is probably the sole effect of the repet.i.tion. But experience is lacking on this point, and exact comparisons should be made whenever possible, between the descendants of a unique but extreme choice, and a repeated but smaller selection. The effect of the repet.i.tion on the nourishment of the chosen representatives should be studied, for it is clear that a plant with 22 rows, the parents and grandparents of which had the same number, indicates a better condition of internal qualities than one with the same number of rows, produced accidentally from the common race. In this way it may perhaps be possible to explain, why in my experiment an ear with 22 rows gave an average offspring with 20, while the calculation, founded on the regression alone would require a parental ear with 32 rows.

However, as already stated, this discussion is only intended to convey some general idea as to the reduction of the cultures by means of repeated selections, as the material at hand is wholly inadequate for any closer calculation. This important point of the reduction may be ill.u.s.trated in still another manner.

The sowing of very large numbers is only required because it is impossible to tell from the [786] inspection of the seeds which of them will yield the desired individual. But what is impossible in the inspection of the seeds may be feasible, at least in important measure, in the inspection of the plants which bear the seeds. Whenever such an inspection demonstrates differences, in manifest connection with the quality under consideration, any one will readily grant that it would be useless to sow the seeds of the worst plants, and that even the whole average might be thrown over, if it were only possible to point out a number of the best. But it is clear that by this inspection of the parent plants the principle of repeated selection is introduced for two succeeding generations, and that its application to a larger series of generations is only a question of secondary importance.

Summing up our discussion of this first point we may a.s.sert that repeated selection is only selection on a small and practical scale, while a single choice would require numbers of individuals higher than are ordinarily available.

A second discussion in connection with our pedigree-culture of corn is the question whether the amelioration obtained was of a durable nature, or only temporary. In other words, whether the progeny of the race would remain constant, if cultivated after cessation of the selection. In order to ascertain this, [787] I continued the culture during several generations, choosing ears with less than the average number of rows.

The excellence of the race at once disappeared, and the ordinary average of the variety from which I had started seven years before, returned within two or three seasons. This shows that the attained improvement is neither fixed nor a.s.sured and is dependent on continued selection. This result only confirms the universal experience of breeders, which teaches the general dependency of improved races on continued selection. Here a striking contrast with elementary species or true varieties is obvious.

The strains which nature affords are true to their type; their average condition remains the same during all the succeeding generations, and even if it should be slightly altered by changes in the external conditions, it returns to the type, as soon as these changes come to an end. It is a real average, being the sum of the contribution of all the members of the strain. Improved races have only an apparent average, which is in fact biased by the exclusion of whole groups of individuals.

If left to themselves, their appearance changes, and the real average soon returns. This is the common experience of breeders.

A third point is to be discussed in connection [788] with the detailed pedigree-cultures. It is the question as to what might be expected from a continuation of improvement selection. Would it be possible to obtain any imaginable deviation from the original type, and to reach independency from further selection? This point has not until now attracted any practical interest, and from a practical point of view and within the limits of ordinary cultures, it seems impossible to obtain a positive answer. But in the theoretical discussion of the problems of descent it has become of the highest importance, and therefore requires a separate treatment, which will be reserved for the next lecture.

Here we come upon another equally difficult problem. It relates to the proportion of embryonic or individual fluctuation, to partial variation as involved in the process of selection. Probably all qualities which may be subjected to selection vary according to both principles, the embryonic decision giving only a more definite average, around which the parts of the individual are still allowed to oscillate. It is so with the corn, and whenever two or more ears are ripening or even only flowering on the same plant, differences of a partial nature may be seen in the number of their rows. These fluctuations are only small however, ordinarily not exceeding two and rarely four [789] rows. Choosing always the princ.i.p.al ear, the figures may be taken to indicate the degree of personal deviation from the average of the race. But whenever we make a mistake, and perchance sow from an ear, the deviation of which was largely due to partial variation, the regression should be expected to become considerably larger. Hence it must be conceded that exact calculations of the phenomena of inheritance are subject to much uncertainty, resulting from our very imperfect knowledge concerning the real proportion of the contributing factors, and the difficulty of ascertaining their influence in any given case. Here also we encounter more doubts than real facts, and much remains to be done before exact calculations may become of real scientific value.

Returning to the question of the effects of selection in the long run, two essentially different cases are to be considered. Extremes may be selected from among the variants of ordinary fluctuating variability, or from ever-sporting varieties. These last we have shown to be double races. Their peculiar and wide range of variability is due to the subst.i.tution of two characters, which exclude one another, or if combined, are diminished in various degrees. Striped flowers and stocks, "five-leaved" clover, pistilloid opium-poppies and numerous other [790]

monstrosities have been dealt with as instances of such ever-sporting varieties.

Now the question may be put, what would be the effect of selection if in long series of years one of the two characters of such a double race were preferred continuously, to the complete exclusion of the other.

Would the race become changed thereby? Could it be affected to such a degree as to gradually lose the inactive quality, and cease to be a double race?

Here manifestly we have a means by which to determine what selection is able to accomplish. Physiologic experiments may be said to be too short to give any definite evidence. But cases may be cited where nature has selected during long centuries and with absolute constancy in her choice. Moreover unconscious selections by man have often worked in an a.n.a.logous manner, and many cultivated plants may be put to the test concerning the evidence they might give on this point. Stating beforehand the result of this inquiry, we may a.s.sert that long-continued selection has absolutely no appreciable effect. Of course I do not deny the splendid results of selection during the first few years, nor the necessity of continued selection to keep the improved races to the height of their ameliorated qualities. I only wish to state that the work [791] of selection here finds its limit and that centuries and perhaps geologic periods of continued effort in the same direction are not capable of adding anything more to the initial effect. Some ill.u.s.trative examples may suffice to prove the validity of this a.s.sertion. Every botanist who has studied the agricultural practice of plant-breeding, or the causes of the geographic distribution of plants, will easily recall to his mind numerous similar cases. Perhaps the most striking instance is afforded by cultivated biennial plants. The most important of them are forage-beets and sugar-beets. They are, of course, cultivated only as biennials, but some annual specimens may be seen each year and in nearly every field. They arise from the same seed as the normal individuals, and their number is obviously dependent on external conditions, and especially on the time of sowing. Ordinary cultures often show as much as 1% of these useless plants, but the exigencies of time and available labor often compel the cultivator to have a large part of his fields sown before spring. In central Europe, where the climate is unfavorable at this season, the beets respond by the production of far larger proportions of annual specimens, their number coming often up to 20% or more, thus const.i.tuting noticeable losses in the product [792] of the whole field. Rimpau, who has made a thorough study of this evil and has shown its dependency on various external conditions, has also tried to find methods of selection with the aim of overcoming it, or at least of reducing it to uninjurious proportions.

But in these efforts he has reached no practical result. The annuals are simply inexterminable.

Coming to the alternative side of the problem it is clear that annuals have always been excluded in the selection. Their seeds cannot be mixed with the good harvest, not even accidentally, since they have ripened in a previous year. In order to bear seeds in the second year beets must be taken from the field, and kept free from frost through the winter. The following spring they are planted out, and it is obvious that even the most careless farmer is not liable to mix them with annual specimens.

Hence we may conclude that a strict and unexcelled process of selection has been applied to the destruction of this tendency, not only for sugar-beets, since Vilmorin's time, when selection had become a well understood process, but also for forage-beets since the beginning of beet culture. Although unconscious, the selection of biennials must have been uninterrupted and strict throughout many centuries.

It has had no effect at all. Annuals are seen [793] to return every year. They are ineradicable. Every individual is in the possession of this latent quality and liable to convert it into activity as soon as the circ.u.mstances provoke its appearance, as proved by the increase of annuals in the early sowings. Hence the conclusion that selection in the long run is not adequate to deliver plants from injurious qualities.

Other proofs could be given by other biennials, and among them the stray annual plants of common carrots are perhaps the most notorious. In my own cultures of evening-primroses I have preferred the annuals and excluded the biennials, but without being able to produce a pure annual race. As soon as circ.u.mstances are favorable, the biennials return in large numbers. Cereals give a.n.a.logous proofs. Summer and winter varieties have been cultivated separately for centuries, but in trials it is often easy to convert the one into the other. No real and definite isolation has resulted from the effect of the long continued unconscious selection.

Striped flowers, striped fruits, and especially striped radishes afford further examples. It would be quite superfluous to dwell upon them.

Selection always tends to exclude the monochromatic specimens, but does not prevent their return in every generation. Numerous [794] rare monstrosities are in the same category, especially when they are of so rare occurrence as not to give any noticeable contribution to the seed-production, or even if they render their bearers incapable of reproduction. In such cases the selection of normal plants is very severe or even absolute, but the anomalies are by no means exterminated.

Any favorable circ.u.mstances, or experimental selection in their behalf shows them to be still capable of full development. Numerous cases of such subordinate hereditary characters const.i.tute the greater part of the science of vegetable teratology.

If it should be objected that all these cases cover too short a time to be decisive, or at least fail in giving evidence relative to former times, alpine plants afford a proof which one can hardly expect to be surpa.s.sed. During the whole present geologic epoch they have been subjected to the never failing selection of their climate and other external conditions. They exhibit a full and striking adaptation to these conditions, but also possess the latent capacity for a.s.suming lowland characters as soon as they are transported into such environment. Obviously this capacity never becomes active on the mountains, and is always counteracted by selection. This agency is evidently without any effect, for as we have seen when dealing [795]

with the experiments of Nageli, Bonnier and others, each single individual may change its habits and its aspect in response to transplantation. The climate has an exceedingly great influence on each individual, but the continuance of this influence is without permanent result.

So much concerning ever-sporting varieties and double adaptations. We now come to the effects of a continuous selection of simple characters.

Here the sugar-beets stand preeminent. Since Vilmorin's time they have been selected according to the amount of sugar in their roots, and the result has been the most striking that has ever been attained, if considered from the standpoint of practice. But if critically examined, with no other aim than a scientific appreciation of the improvement in comparison with other processes of selection, the support of the evidence for the theory of acc.u.mulative influence proves to be very small.

The amount of sugar is expressed by percentage-figures. These however, are dependent on various causes, besides the real quant.i.ty of sugar produced. One of these causes is the quant.i.ty of watery fluid in the tissues, and this in its turn is dependent on the culture in dryer or moister soil, and on the amount of moisture in the air, and the same variety of sugar-beets [796] yields higher percentage-figures in a dry region than in a wet one. This is seen when comparing, for instance, the results of the a.n.a.lyses from the sandy provinces of Holland with those from the clay-meadows, and it is very well known that Californian beets average as high as 26% or more, while the best European beets remain at about 20%. As far as I have been able to ascertain, these figures however, are not indicative of any difference of race, but simply direct responses to the conditions of climate and of soil.

Apart from these considerations the improvement reached in half a century or in about twenty to thirty generations is not suggestive of anything absolute. Everything is fluctuating now, even as it was at the outset, and equally dependent on continual care. Vilmorin has given some figures for the beets of the first generations from which he started his race. He quotes 14% as a recommendable amount, and 7 and 21 as the extreme instances of his a.n.a.lyses. However incorrect these figures may be, they coincide to a striking degree with the present condition of the best European races. Of course minor values are excluded each year by the selection, and in consequence the average value has increased. For the year 1874 we find a standard of 10-14% considered as normal, [797]

bad years giving 10%, good years from 12% to 14% in the average. Extreme instances exceeded 17%. From that time the practice of the polarization of the juice for the estimate of the sugar has rapidly spread throughout Europe, and a definite increase of the average value soon resulted. This however, often does not exceed 14%, and beets selected in the field for the purpose of polarization come up to an average of 15 to 16%, varying downward to less than 10% and upward to 20 and 21%. In the main the figures are the same as those of Vilmorin, the range of variability has not been reduced, and higher extremes are not reached. An average increase of 1% is of great practical importance, and nothing can excel the industry and care displayed in the improvement of the beet-races.

Notwithstanding this a lasting influence has not been exercised; the methods of selection have been improved, and the number of polarized beets has been brought up to some hundreds of thousands in single factories, but the improvement is still as dependent upon continuous selection as it was half a century ago.

The process is practically very successful, but the support afforded by it to the selection theory vanishes on critical examination.

[798]

LECTURE XXVIII

ARTIFICIAL AND NATURAL SELECTION

The comparison of artificial and natural selection has furnished material support for the theory of descent, and in turn been the object of constant criticism since the time of Darwin. The criticisms, in greater part, have arisen chiefly from an imperfect knowledge of both processes. By the aid of distinctions recently made possible, the contrast between elementary species and improved races has become much more vivid, and promises to yield better results on which to base comparisons of artificial and natural selection.

Elementary species, as we have seen in earlier lectures, occur in wild and in cultivated plants. In older genera and systematic species they are often present in small numbers only, but many of the more recent wild types and also many of the cultivated forms are very rich in this respect. In agriculture the choice of the most adequate elementary forms for any special purpose is acknowledged [799] as the first step in the way of selection, and is designated by the name of variety-testing, applying the term variety to all the subdivisions of systematic species indiscriminately. In natural processes it bears the t.i.tle of survival of species. The fact that recent types show large numbers, and in some instances even hundreds of minor constant forms, while the older genera are considerably reduced in this respect, is commonly explained by the a.s.sumption of extinction of species on a correspondingly large scale.

This extinction is considered to affect the unfit in a higher measure than the fit. Consequently the former vanish, often without leaving any trace of their existence, and only those that prove to be sufficiently adapted to the surrounding external conditions, resist and survive.

This selection exhibits far-reaching a.n.a.logies between the artificial and the natural processes, and is in both cases of the very highest importance. In nature the dying out of unfit mutations is the result of the great struggle for life. In a previous lecture we have compared its agency with that of a sieve. All elements which are too small or too weak fall through, and only those are preserved which resist the sifting process. Reduced in number they thrive and multiply and are thus enabled to [800] strike out new mutative changes. These are again submitted to the sifting tests, and the frequent repet.i.tion of this process is considered to give a good explanation of the manifold, highly complicated, and admirable structures which strike the beginner as the only real adaptations in nature.

Exactly in the same way artificial selection isolates and preserves some elementary species, while it destroys others. Of course the time is not sufficient to secure new mutations, or at least these are only rare at present, and their occurrence is doubtful in historic periods. Apart from this unavoidable difference the a.n.a.logy between natural and artificial selection appears to me to be very striking.

This form of selection may be termed selection between species. Opposed to it stands the selection within the elementary species or variety. It has of late, alone come to be known as selection, though in reality it does not deserve this distinction. I have already detailed the historical evidence which gives preference to selection between species.

The process can best be designated by the name of intraspecific selection, if it is understood that the term intraspecific is meant to apply to the conception of small or elementary species.

I do not wish to propose new terms, but [801] I think that the princ.i.p.al differences might better become understood by the introduction of the word election into the discussion of questions of heredity. Election meant formerly the preferential choice of single individuals, while the derivation of the word selection points to a segregation of a.s.semblies into their larger parts. Or to state it in a shorter way, individual selection is exactly what is usually termed election. Choosing one man from among thousands is to elect him, but a select party is a group of chosen persons. There would be no great difficulty in the introduction of the word election, as breeders are already in the habit of calling their choice individuals "elite," at least in the case of beets and of cereals.

This intraspecific selection affords a second point for the comparison between natural and artificial processes. This case is readily granted to be more difficult than the first, but there can be no doubt that the similarity is due to strictly comparable causes. In practice this process is scarcely second in importance to the selection between species, and in numerous cases it rests upon it, and crowns it, bringing the isolated forms up to their highest possible degree of usefulness. In nature it does quite the same, adapting strains of individuals to the local conditions of their environment. Improved [802] races do not generally last very long in practice; sooner or later they are surpa.s.sed by new selections. Exactly so we may imagine the agency of natural intraspecific selection. It produces the local races, the marks of which disappear as soon as the special external conditions cease to act. It is responsible only for the smallest lateral branches of the pedigree, but has nothing in common with the evolution on the main stems. It is of very subordinate importance.

These a.s.sertions of course, are directly opposed to the current run of scientific belief, but they are supported by facts. A considerable part of the evidence has already been dealt with and for our closing discussion only an exact comparison remains to be made between the two detailed types of intraspecific selection. In coming to this I will first dwell upon some intermediate types and conclude with a critical discussion of the features of artificial selection, which to my mind prove the invalidity of the conclusions drawn from it in behalf of an explanation of the processes of nature.

Natural selection occurs not only in the wild state, but is also active in cultivated fields. Here it regulates the struggle of the selected varieties and improved races with the older types, and even with the wild species. In a previous [803] lecture I have detailed the rapid increase of the wild oats in certain years, and described the experiments of Risler and Rimpau in the running out of select varieties.

The agency is always the same. The preferred forms, which give a larger harvest, are generally more sensitive to injurious influences, more dependent on rich manure and on adequate treatment. The native varieties have therefore the advantage, when climatic or cultural conditions are unfavorable for the fields at large. They suffer in a minor degree, and are thereby enabled to propagate themselves afterwards more rapidly and to defeat the finer types. This struggle for life is a constant one, and can easily be followed, whenever the composition of a strain is noted in successive years. It is well appreciated by breeders and farmers, because it is always liable to counteract their endeavors and to claim their utmost efforts to keep their races pure. There can be no doubt that exactly the same struggle exempt from man's intrusion is fought out in the wild state.

Local races of wild plants have not been the object for field observations recently. Some facts however, are known concerning them. On the East Friesian Islands in the North Sea the flowers are strikingly larger and brighter colored than those of the same species on the [804]

neighboring continent. This local difference is ascribed by Behrens to a more severe selection by the pollinating insects in consequence of their lesser frequency on these very windy isles. Seeds of the pines from the Himalayas yield cold-resisting young plants if gathered from trees in a high alt.i.tude, while the seeds of the same species from lower regions yield more sensitive seedlings. Similar instances are afforded by _Rhododendron_ and other mountain species. According to Cieslar corresponding differences are shown by seeds of firs and larches from alpine and lowland provinces.

Such changes are directly dependent on external influences. This is especially manifest in experiments extending the cultures in higher or in more northern regions. The shorter summer is a natural agent of selection; it excludes all individuals which cannot ripen their seeds during so short a period. Only the short lived ones survive. Schubeler made very striking experiments with corn and other different cereals, and has succeeded in making their culture possible in regions of Norway where it formerly failed. In the district of Christiania, corn had within some few years reduced its lifetime from 123 to 90 days, yielding smaller stems and fewer kernels, but still sufficient to make its culture profitable under the existing conditions. [805] This change was not permanent, but was observed to diminish rapidly and to disappear entirely, whenever the Norwegian strain was cultivated in the southern part of Germany. It was a typical improved race, dependent on continual selection by the short summers which had produced it. Similar results have been reached by Von Wettstein in the comparison of kinds of flax from different countries. The a.n.a.logy between such cultivated local races and the local races of nature is quite striking. The practice of seed exchange rests for a large part on the experience that the characters, acquired under the definite climatic and cultural conditions of some select regions, hold good for one or two, and sometimes even more generations, before they decrease to practical uselessness. The Probstei, the Hanna and other districts owe their wealth to this temporary superiority of their wheat and other cereals.

Leaving these intermediate forms of selection, we now come to our princ.i.p.al point. It has already been discussed at some length in the previous lecture, but needs further consideration. It is the question whether intraspecific selection may be regarded as a cause of lasting and ever-increasing improvement. This is a.s.sumed by biologists who consider fluctuating variability as the main source of progression [806]

in the organic world. But the experience of the breeders does not support this view, since the results of practice prove that selection according to a constant standard soon reaches a limit which it is not capable of transgressing. In order to attain further improvements the method of selection itself must be improved. A better and sharper method a.s.sures the choice of more valuable representatives of the race, even if these must be sought for in far larger numbers of individuals, as is indicated by the law of Quetelet.

Continuous or even prolonged improvement of a cultivated race is not the result of frequently repeated selection, but of the improvement of the standard of appreciation. Nature, as far as we know, changes her standard from time to time only in consequence of the migrations of the species, or of local changes of climate. Afterwards the new standard remains unchanged for centuries.

Selection, according to a constant standard, reaches its results in few generations. The experience of Van Mons and other breeders of apples shows that the limit of size and lusciousness may be soon attained.

Vilmorin's experiments with wild carrots and those of Carriere with radishes lead to the same conclusion as regards roots. Improvements of flowers in [807] size and color are usually easy and rapid in the beginning, but an impa.s.sable limit is soon reached. Numerous other instances could be given.

Contrasted with these simple cases is the method of selecting sugar beets. More than once I have alluded to this splendid example of the influence of man upon domestic races, and tried to point out how little support it affords to the current scientific opinion concerning the power of natural selection. For this reason it is interesting to see how a gradual development of the methods of selection has been, from the very outset, one of the chief aims of the breeders. None of them doubts that an improvement of the method alone is adequate to obtain results.

This result, in the main, is the securing of a few percent more of sugar, a change hardly comparable with that progress in evolution, which our theories are destined to explain.

Vilmorin's original method was a very simple one. Polarization was still undiscovered in his time. He determined the specific weight of his beets, either by weighing them as a whole, or by using a piece cut from the base of the roots and deprived of its bark, in order to test only the sugar tissues. The pieces were floated in solutions of salt, which were diluted until the pieces [808] began to sink. Their specific weight at that moment was determined and considered to be a measure of the corresponding value of the beet. This principle was afterwards improved in two ways. The first was a selection after the salt solution method, but performed on a large scale. After some few determinations, a solution was made of such strength as to allow the greater number of the beets to float, and only the best to sink down. In large vessels thousands of beets could be tested in this way, to select a few of the very heaviest. The other improvement was the determination of the specific weight of the sap, pressed out from the tissue. It was more tedious and more expensive, but more direct, as the influence of the air cavities of the tissue was excluded. It prepared the way for polarization.