Experience must decide between the two main theories. It demonstrates the existence of polymorphous genera, such as _Draba_ and _Viola_ and hundreds of others. They clearly indicate a previous state of mutability. Their systematic relation is exactly what would be expected, if they were the result of such a period. Perhaps mutability has not wholly ceased in them, but, might be found to survive in some of their members. Such very rich genera however, are not the rule, but are exceptional cases, indicating the rarity of powerful mutative changes.

On the other hand, species may remain in a state of constancy during long, apparently during indefinite, ages.

[694] Many facts plead in favor of the constancy of species. This principle has always been recognized by systematists. Temporarily the current form of the theory of natural selection has a.s.sumed species to be inconstant, ever changing and continuously being improved and adapted to the requirements of the life-conditions. The followers of the theory of descent believed that this conclusion was unavoidable, and were induced to deny the manifest fact that species are constant ent.i.ties.

The mutation theory gives a clew to the final combination of the two contending ideas. Reducing the changeability of the species to distinct and probably short periods, it at once explains how the stability of species perfectly agrees with the principle of descent through modification.

On the other hand, the hypothesis of mutative periods is by no means irreconcilable with the observed facts of constancy. Such casual changes can be proved by observations such as those upon the evening-primrose, but it is obvious that a disproof can never be given. The principle grants the present constancy of the vast majority of living forms, and only claims the exceptional occurrence of definite changes.

Proofs of the constancy of species have been given in different ways.

The high degree of similarity of the individuals of most of our [695]

species has never been denied. It is observed throughout extended localities, and during long series of years. Other proofs are afforded by those plants which have been transported to distant localities some time since, but do not exhibit any change as a result of this migration.

Widely dispersed plants remain the same throughout their range, provided that they belong to a single elementary species. Many species have been introduced from America into Europe and have spread rapidly and widely.

The Canadian horsetail (_Erigeron canadensis_), the evening-primrose and many other instances could be given. They have not developed any special European features after their introduction. Though exposed to other environmental conditions and to compet.i.tion with other species, they have not succeeded in developing a new character. Such species as proved adequate to the new environment have succeeded, while those which did not have succ.u.mbed.

Much farther back is the separation of the species which now live both in arctic regions and on the summits of our highest mountaintops. If we compare the alpine flora with the arctic plants, a high degree of similarity at once strikes us. Some forms are quite identical; others are slightly different, manifestly representing elementary species of the same systematic [696] type. Still others are more distant or even belong to different genera. The latter, and even the diverging, though nearly allied, elementary species, do not yield adequate evidence in any direction.

They may as well have lived together in the long ages before the separation of the now widely distant floras, or have sprung from a common ancestor living at that time, and subsequently have changed their habits. After excluding these unreliable instances, a good number of species remain, which are quite the same in the arctic and alpine regions and on the summits of distant mountain ranges. As no transportation over such large distances can have brought them from one locality to the other, no other explanation is left than that they have been wholly constant and unchanged ever since the glacial period which separated them. Obviously they must have been subjected to widely changing conditions. The fact of their stability through all these outward changes is the best proof that the ordinary external conditions do not necessarily have an influence on specific evolution. They may have such a result in some instances, in others they obviously have not.

Many arctic forms bearing the specific name of _alpinus_ justify this conclusion. _Astragalus alpinus_, _Phleum alpinum_, _Hieracium alpinum_ and [697] others from the northern parts of Norway may be cited as examples.

Thus Primula imperialis has been found in the Himalayas, and many other plants of the high mountains of Java, Ceylon and northern India are identical forms. Some species from the Cameroons and from Abyssinia have been found on the mountains of Madagascar. Some peculiar Australian types are represented on the summit of Kini Balu in Borneo. None of these species, of course, are found in the intervening lowlands, and the only possible explanation of their ident.i.ty is the conception of a common post-glacial origin, coupled with complete stability. This stability is all the more remarkable as nearly allied but slightly divergent forms have also been reported from almost all of these localities. Other evidence is obtained by the comparison of ancient plants with their living representatives. The remains in tombs of ancient Egypt have always afforded strong support of the views of the adherents of the theory of stability, and to my mind they still do so.

The cereals and fruits and even the flowers and leaves in the funeral wreaths of Rameses and Amen-Hotep are the same that are still now cultivated in Egypt. Nearly a hundred or more species have been identified. Flowers of _Acacia_, leaves of _Mimusops_, [698] petals of _Nymphaea_ may be cited as instances, and they are as perfectly preserved as the best herbarium-specimens of the present time. The petals and stamens retain their original colors, displaying them as brightly as is consistent with their dry state.

Paleontologic evidence points to the same conclusion. Of course the remains are incomplete, and rarely adequate for a close comparison. The range of fluctuating variability should be examined first, but the test of elementary species given by their constancy from seed cannot, of course, be applied. Apart from these difficulties, paleontologists agree in recognizing the very great age of large numbers of species. It would require a too close survey of geologic facts to go into details on this point. Suffice it to say that in more recent Tertiary deposits many species have been identified with living forms. In the Miocene period especially, the similarity of the types of phanerogamic plants with their present offspring, becomes so striking that in a large number of cases specific distinctions rest in greater part on theoretical conceptions rather than on real facts. For a long time the idea prevailed that the same species could not have existed through more than one geologic period. Many distinctions founded on this belief have since had to be abandoned. [699] Species of algae belonging to the well-preserved group of the diatoms, are said to have remained unchanged from the Carboniferous period up to the present time.

Summing up the results of this very hasty survey, we may a.s.sert that species remain unchanged for indefinite periods, while at times they are in the alternative condition. Then at once they produce new forms often in large numbers, giving rise to swarms of subspecies. All facts point to the conclusion that these periods of stability and mutability alternate more or less regularly with one another. Of course a direct proof of this view cannot, as yet, be given, but this conclusion is forced upon us by a consideration of known facts bearing on the principle of constancy and evolution.

If we are right in this general conception, we may ask further, what is to be the exact place of our group of new evening-primroses in this theory? In order to give an adequate answer, we must consider the whole range of the observations from a broader point of view. First of all it is evident that the real mutating period must be a.s.sumed to be much longer than the time covered by my observations. Neither the beginning nor the end have been seen. It is quite obvious that _Oenothera lamarckiana_ was in a mutating condition when I first [700] saw it, seventeen years ago. How long had it been so? Had it commenced to mutate after its introduction into Europe, some time ago, or was it already previously in this state? It is as yet impossible to decide this point.

Perhaps the mutable state is very old, and dates from the time of the first importation of the species into Europe.

Apart from all such considerations the period of the direct observations, and the possible duration of the mutability through even more than a century, would const.i.tute only a moment, if compared with the whole geologic time. Starting from this conception the pedigree of our mutations must be considered as only one small group. Instead of figuring a fan of mutants for each year, we must condense all the succeeding swarms into one single fan, as might be done also for _Draba verna_ and other polymorphous species. In _Oenothera_ the main stem is prolonged upwards beyond the fan; in the others the main stem is lacking or at least undiscernable, but this feature manifestly is only of secondary importance. We might even prefer the image of a fan, adjusted laterally to a stem, which itself is not interrupted by this branch.

On this principle two further considerations are to be discussed. First the structure of the [701] fan itself, and secondly the combination of succeeding fans into a common genealogic tree.

The composition of the fan as a whole includes more than is directly indicated by the facts concerning the birth of new species. They arise in considerable quant.i.ties, and each of them in large numbers of individuals, either in the same or in succeeding years. This multiple origin must obviously have the effect of strengthening the new types, and of heightening their chances in the struggle for life. Arising in a single specimen they would have little chance of success, since in the field among thousands of seeds perhaps one only survives and attains complete development. Thousands or at least hundreds of mutated seeds are thus required to produce one mutated individual, and then, how small are its chances of surviving! The mutations proceed in all directions, as I have pointed out in a former lecture. Some are useful, others might become so if the circ.u.mstances were accidentally changed in definite directions, or if a migration from the original locality might take place. Many others are without any real worth, or even injurious.

Harmless or even slightly useless ones have been seen to maintain themselves in the field during the seventeen years of my research, as proved by _Oenothera laevifolia_ and _Oenothera_ [702] _brevistylis_.

Most of the others quickly disappear.

This failure of a large part of the productions of nature deserves to be considered at some length. It may be elevated to a principle, and may be made use of to explain many difficult points of the theory of descent.

If, in order to secure one good novelty, nature must produce ten or twenty or perhaps more bad ones at the same time, the possibility of improvements coming by pure chance must be granted at once. All hypotheses concerning the direct causes of adaptation at once become superfluous, and the great principle enunciated by Darwin once more reigns supreme.

In this way too, the mutation-period of the evening-primrose is to be considered as a prototype. a.s.suming it as such provisionally, it may aid us in arranging the facts of descent so as to allow of a deeper insight and a closer scrutiny. All swarms of elementary species are the remains of far larger initial groups. All species containing only a few subspecies may be supposed to have thrown off at the outset far more numerous lateral branches, out of which however, the greater part have been lost, being unfit for the surrounding conditions. It is the principle of the struggle for life between elementary species, followed by the survival of the [703] fittest, the law of the selection of species, which we have already laid stress upon more than once.

Our second consideration is also based upon the frequent repet.i.tion of the several mutations. Obviously a common cause must prevail. The faculty of producing _nanella_ or _lata_ remains the same through all the years. This faculty must be one and the same for all the hundreds of mutative productions of the same form. When and how did it originate? At the outset it must have been produced in a latent condition, and even yet it must be a.s.sumed to be continuously present in this state, and only to become active at distant intervals. But it is manifest that the original production of the characters of _Oenothera gigas_ was a phenomenon of far greater importance than the subsequent accidental transition of this quality into the active state. Hence the conclusion that at the beginning of each series of a.n.a.logous mutations there must have been one greater and more intrinsic mutation, which opened the possibility to all its successors. This was the origination of the new character itself, and it is easily seen that this incipient change is to be considered as the real one. All others are only its visible expressions.

Considering the mutative period of our evening-primrose [704] as one unit-stride section in the great genealogic tree, this period includes two nearly related, but not identical changes. One is the production of new specific characters in the latent condition, and the other is the bringing of them to light and putting them into active existence. These two main factors are consequently to be a.s.sumed in all hypothetic conceptions of previous mutative periods.

Are all mutations to be considered as limited to such periods? Of course not. Stray mutations may occur as well. Our knowledge concerning this point is inadequate for any definite statement. Swarms of variable species are easily recognized, if the remnants are not too few. But if only one or two new species have survived, how can we tell whether they have originated-alone or together with others. This difficulty is still more p.r.o.nounced in regard to paleontologic facts, as the remains of geologic swarms are often found, but the absence of numerous mutations can hardly be proved in any case.

I have more than once found occasion to lay stress on the importance of a distinction between progressive and retrograde mutations in previous lectures. All improvement is, of course, by the first of these modes of evolution, but apparent losses of organs or qualities are [705] perhaps of still more universal occurrence. Progression and regression are seen to go hand in hand everywhere. No large group and probably even no genus or large species has been evolved without the joint agency of these two great principles. In the mutation-period of the evening-primroses the observed facts give direct support to this conclusion, since some of the new species proved, on closer inspection, to be retrograde varieties, while others manifestly owe their origin to progressive steps. Such steps may be small and in a wrong direction; notwithstanding this they may be due to the acquisition of a wholly new character and therefore belong to the process of progression at large.

Between them however, there is a definite contrast, which possibly is in intimate connection with the question of periodic and stray mutations.

Obviously each progressive change is dependent upon the production of a new character, for whenever this is lacking, no such mutation is possible. Retrograde changes, on the other hand, do not require such elaborate preliminary work. Each character may be converted into the latent condition, and for all we know, a special preparation for this purpose is not at all necessary. It is readily granted that such special preparation may occur, because the [706] great numbers in which our dwarf variety of the _Oenothera_ are yearly produced are suggestive of such a condition. On the other hand, the _laevifolia_ and _brevistylis_ mutations have not been repeated, at least not in a visible way.

From this discussion we may infer that it is quite possible that a large part of the progressive changes, and a smaller part of the retrograde mutations, are combined into groups, owing their origin to common external agencies. The periods in which such groups occur would const.i.tute the mutative periods. Besides them the majority of the retrograde changes and some progressive steps might occur separately, each being due to some special cause. Degressive mutations, or those which arise by the return of latent qualities to activity, would of course belong with the latter group.

This a.s.sumption of a stray and isolated production of varieties is to a large degree supported by experience in horticulture. Here there are no real swarms of mutations. Sudden leaps in variability are not rare, but then they are due to hybridization. Apart from this mixture of characters, varieties as a rule appear separately, often with intervals of dozens of years, and without the least suggestion of a common cause.

It is quite superfluous to go into details, as we have dealt with the horticultural [707] mutations at sufficient length on a previous occasion. Only the instance of the peloric toadflax might be recalled here, because the historic and geographic evidence, combined with the results of our pedigree-experiment, plainly show that peloric mutations are quite independent of any periodic condition. They may occur anywhere in the wide range of the toad-flax, and the capacity of repeatedly producing them has lasted some centuries at least, and is perhaps even as old as the species itself.

Leaving aside such stray mutations, we may now consider the probable const.i.tution of the great lines of the genealogic tree of the evening primroses, and of the whole vegetable and animal kingdom at large. The idea of drawing up a pedigree for the chief groups of living organisms is originally due to Haeckel, who used this graphic method to support the Darwinian theory of descent. Of course, Haeckel's genealogic trees are of a purely hypothetic nature, and have no other purpose than to convey a clear conception of the notion of descent, and of the great lines of evolution at large. Obviously all details are subject to doubt, and many have accordingly been changed by his successors. These changes may be considered as partial improvements, and the somewhat picturesque form of Haeckel's pedigree might well be replaced by [708] more simple plans. But the changes have by no means removed the doubts, nor have they been able to supplant the general impression of distinct groups, united by broad lines. This feature is very essential, and it is easily seen to correspond with the conception of swarms, as we have deduced it from the study of the lesser groups.

Genealogic trees are the result of comparative studies; they are far removed from the results of experimental inquiry concerning the origin of species. What are the links which bind them together? Obviously they must be sought in the mutative periods, which have immediately preceded the present one. In the case of the evening-primrose the systematic arrangement of the allied species readily guides us in the delimitations of such periods. For manifestly the species of the large genus of _Oenothera_ are grouped in swarms, the youngest or most recent of which we have under observation. Its immediate predecessor must have been the subgenus _Onagra_, which is considered by some authors as consisting of a single systematic species, _Oenothera biennis_. Its multifarious forms point to a common origin, not only morphologically but also historically. Following this line backward or downward we reach another apparent mutation-period, which includes the origin of [709] the group called _Oenothera_, with a large number of species of the same general type as the _Onagra-forms, Still farther downward comes the old genus _Oenothera_ itself, with numerous subgenera diverging in sundry characters and directions.

Proceeding still farther we might easily construct a main stem with numerous succeeding fans of lateral branches, and thus reach, from our new empirical point of view, the theoretical conclusion already formulated.

Paleontologic facts readily agree with this conception. The swarms of species and varieties are found to succeed one another like so many stories. The same images are repeated, and the single stories seem to be connected by the main stems, which in each tier produce the whole number of allied forms. Only a few prevailing lines are prolonged through numerous geologic periods; the vast majority of the lateral branches are limited each to its own storey. It is simply the extension of the pedigree of the evening-primroses backward through ages, with the same construction and the same leading features. There can be no doubt that we are quite justified in a.s.suming that evolution has followed the same general laws through the whole duration of life on earth. Only a moment of their lifetime is disclosed to us, but it [710] is quite sufficient to enable us to discern the laws and to conjecture the outlines of the whole scheme of evolution.

A grave objection which has, often, and from the very outset, been urged against Darwin's conception of very slow and nearly imperceptible changes, is the enormously long time required. If evolution does not proceed any faster than what we can see at present, and if the process must be a.s.sumed to have gone on in the same slow manner always, thousands of millions of years would have been needed to develop the higher types of animals and plants from their earliest ancestors.

Now it is not at all probable that the duration of life on earth includes such an incredibly long time. Quite on the contrary the lifetime of the earth seems to be limited to a few millions of years.

The researches of Lord Kelvin and other eminent physicists seem to leave no doubt on this point. Of course all estimates of this kind are only vague and approximate, but for our present purposes they may be considered as sufficiently exact.

In a paper published in 1862 Sir William Thomson (now Lord Kelvin) first endeavored to show that great limitation had to be put upon the enormous demand for time made by Lyell, Darwin and other biologists. From a consideration [711] of the secular cooling of the earth, as deduced from the increasing temperature in deep mines, he concluded that the entire age of the earth must have been more than twenty and less than forty millions of years, and probably much nearer twenty than forty. His views have been much criticised by other physicists, but in the main they have gained an ever-increasing support in the way of evidence. New mines of greater depth have been bored, and their temperatures have proved that the figures of Lord Kelvin are strikingly near the truth. George Darwin has calculated that the separation of the moon from the earth must have taken place some fifty-six millions of years ago. Geikie has estimated the existence of the solid crust of the earth at the most as a hundred million years. The first appearance of the crust must soon have been succeeded by the formation of the seas, and a long time does not seem to have been required to cool the seas to such a degree that life became possible. It is very probable that life originally commenced in the great seas, and that the forms which are now usually included in the plankton or floating-life included the very first living beings.

According to Brooks, life must have existed in this floating condition during long primeval epochs, and evolved nearly all the main branches of the animal and vegetable kingdom [712] before sinking to the bottom of the sea, and later producing the vast number of diverse forms which now adorn the sea and land.

All these evolutions, however, must have been very rapid, especially at the beginning, and together cannot have taken more time than the figures given above.

The agency of the larger streams, and the deposits which they bring into the seas, afford further evidence. The amount of dissolved salts, especially of sodium chloride, has been made the subject of a calculation by Joly, and the amount of lime has been estimated by Eugene Dubois. Joly found fifty-five and Dubois thirty-six millions of years as the probable duration of the age of the rivers, and both figures correspond to the above dates as closely as might be expected from the discussion of evidence so very incomplete and limited.

All in all it seems evident that the duration of life does not comply with the demands of the conception of very slow and continuous evolution. Now it is easily seen, that the idea of successive mutations is quite independent of this difficulty. Even a.s.suming that some thousands of characters must have been acquired in order to produce the higher animals and plants of the present time, no valid objection is raised. The demands of the biologists and the results of [713] the physicists are harmonized on the ground of the theory of mutation.

The steps may be surmised to have never been essentially larger than in the mutations now going on under our eyes, and some thousands of them may be estimated as sufficient to account for the entire organization of the higher forms. Granting between twenty and forty millions of years since the beginning of life, the intervals between two successive mutations may have been centuries and even thousands of years. As yet there has been no objection cited against this a.s.sumption, and hence we see that the lack of harmony between the demands of biologists and the results of the physicists disappears in the light of the theory of mutation.

Summing up the results of this discussion, we may justifiably a.s.sert that the conclusions derived from the observations and experiments made with evening-primroses and other plants in the main agree satisfactorily with the inferences drawn from paleontologic, geologic and systematic evidence. Obviously these experiments are wonderfully supported by the whole of our knowledge concerning evolution. For this reason the laws discovered in the experimental garden may be considered of great importance, and they may guide us in our further inquiries. Without doubt many minor [714] points are in need of correction and elaboration, but such improvements of our knowledge will gradually increase our means of discovering new instances and, new proofs.

The conception of mutation periods producing swarms of species from time to time, among which only a few have a chance of survival, promises to become the basis for speculative pedigree-diagrams, as well as for experimental investigations.

[715]

LECTURE XXV

GENERAL LAWS OF FLUCTUATION

The principle of unit-characters and of elementary species leads at once to the recognition of two kinds of variability. The changes of wider amplitude consist of the acquisition of new units, or the loss of already existing ones. The lesser variations are due to the degree of activity of the units themselves.

Facts ill.u.s.trative of these distinctions were almost wholly lacking at the time of the first publication of Darwin's theories. It was a bold conception to point out the necessity for such distinction on purely theoretical grounds. Of course some sports were well known and fluctuations were evident, but no exact a.n.a.lysis of the details was possible, a fact that was of great importance in the demonstration of the theory of descent. The lack of more definite knowledge upon this matter was keenly felt by Darwin, [716] and exercised much influence upon his views at various times.

Quetelet's famous discovery of the law of fluctuating variability changed the entire situation and cleared up many difficulties. While a clear conception of fluctuations was thus gained, mutations were excluded from consideration, being considered as very rare, or non-existent. They seemed wholly superfluous for the theory of descent, and very little importance was attached to their study. Current scientific belief in the matter has changed only in recent years.