Species and Varieties, Their Origin by Mutation Part 2

My second line of inquiry was an experimental repet.i.tion of the phenomena which were only partly discerned at the native locality. It was not my aim to intrude into the process, nor to try to bring out new features. My only object was to submit to the precepts just given concerning pure treatment, individual seed gathering, exclusion of crosses and accurate recording of all the facts. The result has been a pedigree which now permits of stating the relation between all the descendants of my original introduced plant. This pedigree at once exhibits the laws followed by the mutating species. The main fact is, that it does not change itself gradually, but remains unaffected during all succeeding generations. It only throws off new forms, which are sharply contrasted with the parent, and which are from the very beginning as perfect and as constant, as narrowly [29] defined and as pure of type as might be expected of any species.

These new species are not produced once or in single individuals, but yearly and in large numbers. The whole phenomenon conveys the idea of a close group of mutations, all belonging to one single condition of mutability. Of course this mutable state must have had a beginning, as it must sometime come to an end. It is to be considered as a period within the life-time of the species and probably it is only a small part of it.

The detailed description of this experiment, however, I must delay to a subsequent lecture, but I may be allowed to state, that the discovery of this period of mutability is of a definite theoretical importance. One of the greatest objections to the Darwinian theory of descent arose from the length of time it would require, if all evolution was to be explained on the theory of slow and nearly invisible changes. This difficulty is at once met and fully surmounted by the hypothesis of periodical but sudden and quite noticeable steps. This a.s.sumption requires only a limited number of mutative periods, which might well occur within the time allowed by physicists and geologists for the existence of animal and vegetable life on the earth.

[30] Summing up the main points of these introductory remarks, I propose to deal with the subjects mentioned above at some length, devoting to each of them, if possible at least an entire lecture. The decisive facts and discussions upon which the conclusions are based will be given in every case. Likewise I hope to point out the weak places and the lacunae in our present knowledge, and to show the way in which each of you may try to contribute his part towards the advancement of science in this subject. Lastly I shall try to prove that sudden mutation is the normal way in which nature produces new species and new varieties. These mutations are more readily accessible to observation and experiment than the slow and gradual changes surmised by Wallace and his followers, which are entirely beyond our present and future experience.

The theory of mutations is a starting-point for direct investigation, while the general belief in slow changes has held back science from such investigations during half a century.

Coming now to the subdivisions and headings under which my material is to be presented, I propose describing first the real nature of the elementary species and retrograde varieties, both in normal form and in hybridizations. A discussion of other types of varieties, including [31]

monstrosities will complete the general plan. The second subdivision will deal with the origin of species and varieties as taught by experiment and observation, treating separately the sudden variations which to my mind do produce new forms, and subsequently the fluctuations which I hold to be not adequate to this purpose.

[32]

B. ELEMENTARY SPECIES

LECTURE II

ELEMENTARY SPECIES IN NATURE

What are species? Species are considered as the true units of nature by the vast majority of biologists. They have gained this high rank in our estimation princ.i.p.ally through the influence of Linnaeus. They have supplanted the genera which were the accepted units before Linnaeus.

They are now to be replaced in their turn, by smaller types, for reasons which do not rest upon comparative studies but upon direct experimental evidence.

Biological studies and practical interests alike make new demands upon systematic botany. Species are not only the subject-material of herbaria and collections, but they are living ent.i.ties, and their life-history and life-conditions command a gradually increasing interest. One phase of the question is to determine the easiest manner to deal with the collected forms of a country, and another feature is the problem [33] as to what groups are real units and will remain constant and unchanged through all the years of our observations.

Before Linnaeus, the genera were the real units of the system. De Candolle pointed out that the old common names of plants, such as roses and clover, poplars and oaks, nearly all refer to genera. The type of the clovers is rich in color, and the shape of the flower-heads and the single flowers escape ordinary observation; but notwithstanding this, clovers are easily recognized, even if new types come to hand. White and red clovers and many other species are distinguished simply by adjectives, the generic name remaining the same for all.

Tournefort, who lived in the second half of the 17th century (1656-1708), is generally considered as the author of genera in systematic botany. He adopted, what was at that time the general conception and applied it throughout the vegetable kingdom. He grouped the new and the rare and the previously overlooked forms in the same manner in which the more conspicuous plants were already arranged by universal consent. Species were distinguished by minor marks and often indicated by short descriptions, but they were considered of secondary importance.

Based on the idea of a direct creation of all [34] living beings, the genera were then accepted as the created forms. They were therefore regarded as the real existing types, and it was generally surmised that species and varieties owed their origin to subsequent changes under the influence of external conditions. Even Linnaeus agreed with this view in his first treatises and in his "Philosophical Botany" he still kept to the idea that all genera had been created at once with the beginning of life.

Afterwards Linnaeus changed his opinion on this important point, and adopted species as the units of the system. He declared them to be the created forms, and by this decree, at once reduced the genera to the rank of artificial groups. Linnaeus was well aware that this conception was wholly arbitrary, and that even the species are not real indivisible ent.i.ties. But he simply forbade the study of lesser subdivisions. At his time he was quite justified in doing so, because the first task of the systematic botanists was the clearing up of the chaos of forms and the bringing of them into connection with their real allies.

Linnaeus himself designated the subdivisions of the species as varieties, but in doing so he followed two clearly distinct principles.

In some cases his species were real plants, and the varieties seemed to be derived from them by [35] some simple changes. They were subordinated to the parent-species. In other cases his species were groups of lesser forms of equal value, and it was not possible to discern which was the primary and which were the derivatives.

These two methods of subdivision seem in the main, and notwithstanding their relatively imperfect application in many single examples, to correspond with two really distinct cases. The derivative varieties are distinguished from the parent-species by some single, but striking mark, and often this attribute manifests itself as the loss of some apparent quality. The loss of spines and of hairs and the loss of blue and red flower-colors are the most notorious, but in rarer cases many single peculiarities may disappear, thereby const.i.tuting a variety. This relation of varieties to the parent-species is gradually increasing in importance in the estimation of botanists, sharply contrasting with those cases, in which such dependency is not to be met with.

If among the subdivisions of a species, no single one can be pointed out as playing a primary part, and the others can not be traced back to it, the relation between these lesser units is of course of another character. They are to be considered of equal importance. They are distinguished from each other by more than [36] one character, often by slight differences in nearly all their organs and qualities. Such forms have come to be designated as "elementary species." They are only varieties in a broad and vague systematic significance of the word, not in the sense accorded to this term in horticultural usage, nor in a sharper and more scientific conception.

Genera and species are, at the present time, for a large part artificial, or stated more correctly, conventional groups. Every systematist is free to delimit them in a wider or in a narrower sense, according to his judgment. The greater authorities have as a rule preferred larger genera, others of late have elevated innumerable subgenera to the rank of genera. This would work no real harm, if unfortunately, the names of the plants had not to be changed each time, according to current ideas concerning genera. Quite the same inconstancy is observed with species. In the Handbook of the British Flora, Bentham and Hooker describe the forms of brambles under 5 species, while Babington in his Manual of British Botany makes 45 species out of the same material. So also in other cases. For instance, the willows which have 13 species in one and 31 species in the other of these manuals, and the hawkweeds for which the figures are 7 and 32 [37] respectively.

Other authors have made still greater numbers of species in the same groups.

It is very difficult to estimate systematic differences on the ground of comparative studies alone. All sorts of variability occur, and no individual or small group of specimens can really be considered as a reliable representative of the supposed type. Many original diagnoses of new species have been founded on divergent specimens and of course, the type can afterwards neither be derived from this individual, nor from the diagnosis given.

This chaotic state of things has brought some botanists to the conviction that even in systematic studies only direct experimental evidence can be relied upon. This conception has induced them to test the constancy of species and varieties, and to admit as real units only such groups of individuals as prove to be uniform and constant throughout succeeding generations. The late Alexis Jordan, of Lyons in France, made extensive cultures in this direction. In doing so, he discovered that systematic species, as a rule, comprise some lesser forms, which often cannot easily be distinguished when grown in different regions, or by comparing dried material. This fact was, of course, most distasteful to the systematists of his time and even for a long period afterwards [38] they attempted to discredit it. Milde and many others have opposed these new ideas with some temporary success.

Only of late has the school of Jordan received due recognition, after Thuret, de Bary, Rosen and others tested its practices and openly p.r.o.nounced for them. Of late Wittrock of Sweden has joined them, making extensive experimental studies concerning the real units of some of the larger species of his country.

From the evidence given by these eminent authorities, we may conclude that systematic species, as they are accepted nowadays, are as a rule compound groups. Sometimes they consist of two or three, or a few elementary types, but in other cases they comprise twenty, or fifty, or even hundreds of constant and well differentiated forms.

The inner const.i.tution of these groups is however, not at all the same in all cases. This will be seen by the description of some of the more interesting of them. The European heartsease, from which our garden-pansies have been chiefly derived, will serve as an example. The garden-pansies are a hybrid race, won by crossing the _Viola tricolor_ with the large flowered and bright yellow _V. lutea_. They combine, as everyone knows, in their wide range of [39] varieties, the attributes of the latter with the peculiarities of the former species.

Besides the _lutea_, there are some other species, nearly allied to tricolor, as for instance, _cornuta_, _calcarata_, and _altaica_, which are combined with it under the head of _Melanium_ as a subgenus, and which together const.i.tute a systematic unity of undoubted value, but ranging between the common conceptions of genus and species. These forms are so nearly allied to the heartsease that they have of late been made use of in crosses, in order to widen the range of variability of garden-pansies.

_Viola tricolor_ is a common European weed. It is widely dispersed and very abundant, growing in many localities in large numbers. It is an annual and ripens its seeds freely, and if opportunity is afforded, it multiplies rapidly.

_Viola tricolor_ has three subspecies, which have been elevated to the rank of species by some authors, and which may here be called, for brevity's sake, by their binary names. One is the typical _V. tricolor_, with broad flowers, variously colored and veined with yellow, purple and white. It occurs in waste places on sandy soil. The second is called _V.

arvensis_ or the field-pansy; it has small inconspicuous flowers, with pale-yellowish petals which are shorter than the sepals. It pollinates itself without the [40] aid of insects, and is widely dispersed in cultivated fields. The third form, _V. alpestris_, grows in the Alps, but is of lesser importance for our present discussion.

Anywhere throughout the central part of Europe _V. tricolor_ and _V.

arvensis_ may be seen, each occupying its own locality. They may be considered as ranging among the most common native plants of the particular regions they inhabit. They vary in the color of the flowers, branching of the stems, in the foliage and other parts, but not to such an extent as to const.i.tute distinct strains. They have been brought into cultivation by Jordan, Wittrock and others, but throughout Europe each of them const.i.tutes a single type.

These types must be very old and constant, fluctuating always within the same distinct and narrow limits. No slow, gradual changes can have taken place. In different countries their various habitats are as old as the historical records, and probably many centuries older. They are quite independent of one another, the distance being in numerous cases far too great for the exchange of pollen or of seeds. If slow and gradual changes were the rule, the types could not have remained so uniform throughout the whole range of these two species. They would necessarily have split up into thousands [41] and thousands of minor races, which would show their peculiar characteristics if tested by cultures in adjacent beds. This however, is not what happens. As a matter of fact _V. tricolor_ and _V. arvensis_ are widely distributed but wholly constant types.

Besides these, there occur distinct types in numerous localities. Some of them evidently have had time and opportunity to spread more or less widely and now occupy larger regions or even whole countries. Others are narrowly limited, being restricted to a single locality. Wittrock collected seeds or plants from as many localities as possible in different parts of Sweden and neighboring states and sowed them in his garden near Stockholm. He secured seeds from his plants, and grew from them a second, and in many cases a third generation in order to estimate the amount of variability. As a rule the forms introduced into his garden proved constant, notwithstanding the new and abnormal conditions under which they were propagated.

First of all we may mention three perennial forms called by him _Viola tricolor ammotropha_, _V. tricolor coniophila_ and _V. stenochila_. The typical _V. tricolor_ is an annual plant; sowing itself in summer and germinating soon afterwards. The young plants thrive throughout [42] the latter part of the summer and during the fall, reaching an advanced stage of development of the branched stems before winter. Early in the spring the flowers begin to open, but after the ripening of the seeds the whole plant dies.

The three perennial species just mentioned develop in the same manner in the first year. During their flowering period, however, and afterwards, they produce new shoots from the lower parts of the stem. They prefer dry and sandy soils, often becoming covered with the sand that is blown on them by the winds. They are prepared for such seemingly adverse circ.u.mstances by the acc.u.mulation of food in the older stems and by the capacity of the new shoots to thrive on this food till they have become long enough to reach the light. _V. tricolor ammotropha_ is native near Ystad in Sweden, and the other two forms on Gotland. All three have narrowly limited habitats.

The typical tricolored heartsease has remained annual in all its other subspecies. It may be divided into two types in the first place, _V.

tricolor genuina_ and _V. tricolor versicolor_. Both of them have a wide distribution and seem to be the prototypes from which the rarer forms must have been derived. Among these latter Wittrock describes seven local types, which [43] proved to be constant in his pedigree-cultures.

Some of them have produced other forms, related to them in the way of varieties. They all have nearly the same general habit and do not exhibit any marked differences in their growth, in the structure and branching of the stems, or in the character of their foliage.

Differentiating points are to be found mainly in the colors and patterns of the flowers. The veins, which radiate from the centre of the corolla are branched in some and undivided in others; in one elementary species they are wholly lacking. The purple color may be absent, leaving the flowers of a pale or a deep yellow. Or the purple may be reddish or bluish. Of the petals all five may have the purple hue on their tips, or this attribute may be limited to the two upper ones. Contrasting with this wide variability is the stability of the yellow spot in the centre, which is always present and becomes inconspicuous only, when the whole petals are of the same hue. It is a general conception that colors and color-markings are liable to great variability and do not const.i.tute reliable standards. But the cultures of Wittrock have proved the contrary, at least in the case of the violets. No pattern, however quaint, appears changeable, if one elementary species only is considered. Hundreds of plants from seeds [44] from one locality may be grown, and all will exhibit exactly the same markings. Most of these forms are of very local occurrence. The most beautiful of all, the _ornatissima_, is found only in Jemtland, the _aurobadia_ only in Sodermanland, the anopetala_ in other localities in the same country, the _roseola_ near Stockholm, and the yellow _lutescens_ in Finmarken.

The researches of Wittrock included only a small number of elementary species, but every one who has observed the violets in the central parts of Europe must be convinced that many dozens of constant forms of the typical _Viola tricolor_ might easily be found and isolated.

We now come to the field pansy, the _Viola arvensis_, a very common weed in the grain-fields of central Europe. I have already mentioned its small corolla, surpa.s.sed by the lobes of the calyx and its capacity of self-fertilization. It has still other curious differentiating characters; the pollen grains, which are square in _V. tricolor_, are five-sided in _V. arvensis_. Some transgressive fluctuating variability may occur in both cases through the admixture of pollen-grains. Even three-angled pollen grains are seen sometimes. Other marks are observed in the form of the anthers and the spur.

There seem to be very many local subspecies [45] of the field-pansy.

Jordan has described some from the vicinity of Lyons, and Wittrock others from the northern parts of Europe. They diverge from their common prototype in nearly all attributes, the flowers not showing the essential differentiating characters as in the _V. tricolor_. Some have their flower-stalks erect, and in others the flowers are held nearly at right angles to the stem. _V. pallescens_ is a small, almost unbranched species with small pale flowers. _V. segetalis_ is a stouter species with two dark blue spots on the tips of the upper petals. _V. agrestis_ is a tall and branched, hairy form. _V. nemausensis_ attains a height of only 10 cm., has rounded leaves and long flower-stalks. Even the seeds afford characters which may be made use of in isolating the various species.

The above-mentioned elementary forms belong to the flora of southern France, and Wittrock has isolated and cultivated a number of others from the fields of Sweden. A species from Stockholm is called _Viola patens_; _V. arvensis curtisepala_ occurs in Gotland, and _V. arvensis striolata_ is a distinct form, which has appeared in his cultures without its true origin being ascertained.

The alpine violets comprise a more widespread type with some local elementary species [46] derived exactly in the same way as the tricolored field pansies.

Summarizing the general result of this description we see that the original species _Viola tricolor_ may be split up into larger and lesser groups of separate forms. These last prove to be constant in pedigree-cultures, and therefore are to be considered as really existent units. They are very numerous, comprising many dozens in each of the two larger subdivisions.

All systematic grouping of these forms, and their combination into subspecies and species rests on the comparative study of their characters. The result of such studies must necessarily depend on principles which underlie them. According to the choice of these principles, the construction of the groups will be found to be different. Wittrock trusts in the first place to morphologic characters, and considers the development as pa.s.sing from the more simple to the more complex types. On the other hand the geographic distribution may be considered as an indication of the direction of evolution, the wide-spread forms being regarded as the common parents of the minor local species.

However, such considerations are only of secondary importance. It must be borne in mind that an ordinary systematic species may include [47]

many dozens of elementary forms, each of which remains constant and unchanged in successive generations, even if cultivated in the same garden and under similar external conditions.

Leaving the violets, we may take the vernal whitlow-gra.s.s or _Draba verna_ for a second ill.u.s.tration. This little annual cruciferous plant is common in the fields of many parts of the United States, though originally introduced from Europe. It has small basal rosettes which develop during summer and winter, and produce numerous leafless flowering stems early in the spring. It is a native of central Europe and western Asia, and may be considered as one of the most common plants, occurring anywhere in immense numbers on sandy soils. Jordan was the first to point out that it is not the same throughout its entire range. Although a hasty survey does not reveal differences, they show themselves on closer inspection. De Bary, Thuret, Rosen and many others confirmed this result, and repeated the pedigree-cultures of Jordan.

Every type is constant and remains unchanged in successive generations.

The anthers open in the flower-buds and pollinate the stigmas before the expansion of the flowers, thus a.s.suring self-fertilization. Moreover, these inconspicuous little flowers are only sparingly visited by insects. Dozens of subspecies [48] may be cultivated in the same garden without any real danger of their intercrossing. They remain as pure as under perfect isolation.

It is very interesting to observe the aspect of such types, when growing near each other. Hundreds of rosettes exhibit one type, and are undoubtedly similar. The alternative group is distinguishable at first sight, though the differentiating marks are often so slight as to be traceable with difficulty. Two elementary species occur in Holland, one with narrow leaves in the western provinces and one with broader foliage in the northern parts. I have cultivated them side by side, and was as much struck with the uniformity within each group, as with the contrast between the two sets.