Lobster Claw
Fig. 12.Right claw of lobster bearing a pair of extra dactylopodites (after van Beneden). The fine toothing on R suggests that this is part of a cutting claw, though the limb bearing it is a crusher.
If these indications are reliable[9]and are established by further observation we shall be led to the conclusion that the bud which becomes an extra pair of limbs does not merely contain the parts proper to the side on which it grows, but is comparable with the original zygotic cell, and consists not simply of two halves, but of two halves differentiated as a right and a left like the two halves of the normal body.
Phenomena of this kind, evoked by mutilation or injury, together with the cognate observations on regeneration throwvery curious lights on the nature of living things. To an understanding of the nature of the mechanics of living matter and its relation to matter at large they offer the most hopeful line of approach. I allude especially to the examples in which it has been established that the part which is produced after mutilation is a structure different from that which was removed. The term "regeneration" was introduced before such phenomena were discovered, and though every one recognizes its inapplicability to these remarkable cases, the word still misleads us by presenting a wrong picture to the mind. The expression "heteromorphosis" (Loeb) has been appropriately applied to various phenomena of this kind, and Morgan has given the name "morphallaxis" to another group of cases in which the renewal occurs by the transformation of a previously existing part.[10]But we must continually remember that all these occurrences which we know only as abnormalities and curiosities must in reality be exemplifications of the normal mechanics of division and growth. The conditions needed to call them forth are abnormal, but the responses which the system makes are evidences of its normal constitution. When therefore, for example, the posterior end of a worm produces a reversed tail from its cut end we have a proof that there must be in the normal body forces ready to cause this outgrowth. The new structure is not an ill-shaped head-end, for, as Morgan shows, the nephridial ducts have their funnels perforating the segments in a reversed direction. The "tension" of growth is actually reversed.[11]So also when in a Planarian amputation of the body immediately behind the head leads to the formation of a new reversed head at the back of the normal head, while amputation further back leads to the regeneration of a new tail, these responses give indications of forces normally present in the body of the Planarian. Such facts open up a great field of speculation and research. Especially important it would be to determine where the critical region may be at which the one response is replaced by theother. I suppose it is even possible that there is some neutral zone in which neither kind of response is made.
Physical parallels to the phenomena of regeneration are not easy to find and we still cannot penetrate beyond the empirical facts. Przibram has laid stress on the general resemblance between the new growth of an amputated part in an animal and the way in which a broken crystal repairs itself when placed in the mother-solution. That the two processes have interesting points of likeness cannot be denied. It must however never be forgotten that there is one feature strongly distinguishing the two; for I believe it is universally recognized by physicists that all the phenomena of geometrical regularity which crystals display are ultimately dependent on the forms of the particles of the crystalline body. This cannot in any sense be supposed to hold in regard to protoplasm or its constituents. The definiteness of crystals is also an unlikely guide for the reason that it is absolute and perfect, or in other words because this kind of regularity cannot be disturbed at all without a change so great that the substance itself is altered; whereas we know that the forms of living things are capable of such changes, great and small, that we must regard perfection of form, whether manifested in symmetry or in number, as an ideal which will only be produced in the absence of disturbance. The symmetry of the living things is like the symmetry of the concentric waves in a pool caused by a splash. Perfect circles are made only in the imaginary case of mathematical uniformity, but the system maintains an approximate symmetry though liable to manifold deformation.
Since the geometrical order of the living body cannot be a direct function of the materials it must be referred to some more proximate control. In renewing a part the body must possess the power of seizing particles of many dissimilar kinds, and whirl them into their several and proper places. The action in renewal, like that of original growth, may be compared—very crudely—with the action of a separator which simultaneously distributes a variety of heterogeneous materials in an orderly fashion; but in the living body the thing distributed must rather be theappetencyfor special materials, not the materials themselves.
If the analogy of crystals be set aside and we seek for other parallels to regeneration there are none very obvious. I have sometimes wondered whether it might not be possible to institute a fruitful comparison between the renewal of parts and the reformation of waves of certain classes after obliteration. In several respects, as I have already said, some curious resemblances with the repetitions formed by wave-motion are to be traced in our organic phenomena, and though admitting that I cannot develop these comparisons, I think nevertheless they may be worth bearing in mind. When, after obliteration, an eddy in a stream, or a ripple-mark (a more complex case of eddy-formation) in blown sand is re-formed, we have an example in which pattern is reconstituted and growth takes place not by virtue of the composition of the materials—in this case the water or the sand—but by the way in which they are acted upon by extraneous forces.
A feature in the actual mode by which ripple-marks are reconstituted may not be without interest in connexion with our phenomena of regeneration. When, for example, the wind is blowing steadily over a surface of fine, dry sand, the familiar ripple-marks are formed by a heaping of the sand in lines transverse to the direction of the wind. The heaping is due to the formation of eddies corresponding with positions of instability. When the wind is steady and the sand homogeneous, the distances between the ripples, or wave-lengths, are sensibly equal. If while the wind continues to blow, the ripples are obliterated with a soft brush they will quickly be re-formed over the whole area, but I have noticed that at first their wave-length is approximately half that of the ripples in the undisturbed parts of the system.[12]The normal wave-length is restored by the gradual accentuation of alternate ripples. Of course the sand-ripples are in reality slowly travelling forward in the direction towards which the wind is blowing, and for this our living segmentations afford no obvious parallel, but the appearances in the area ofreformation, and especially the forking of the old ridges where they join the new ones, are curiously reminiscent of the irregularities of segmentation seen in regenerated structures. The value of the considerations adduced in the chapter is, I admit, very small. The utmost that can be claimed for them is that mechanical segmentations, like those seen in ripple-mark, or in Leduc's osmotic growths, show how by the action of a continuous force in one direction, repeated and serially homologous divisions can be produced having features of similarity common to those repetitions by which organic forms and patterns are characterised. The analogy supplies a vicarious picture of the phenomena which in default of one more true may in a slight degree assist our thoughts. It suggests that the rhythms of segmentation may be the consequence of a single force definite in direction and continuously acting during the time of growth. The polarity of the organism would thus be the expression of the fact that this meristic force is definitely directed after it has once been excited, and the reversal seen in some products of regeneration suggest further that it is capable of being reflected. This polarity cannot be a property of the material, as such, but is determined by a force acting on that material, just as the polarity of a magnet is not determined by the arrangement of its particles, but by the direction in which the current flows.
To some it may appear that even to embark on such discussions as this is to enter into a perilous flirtation with vitalistic theories. How, they may ask, can any force competent to produce chemical and geometrical differentiation in the body be distinguished from the "Entelechy" of Driesch? Let me admit that in this reflexion there is one element of truth. If those who proclaim a vitalistic faith intend thereby to affirm that in the processes by which growth and division are effected in the body, a part is played by an orderly force which we cannotnowtranslate into terms of any known mechanics, what observant man is not a vitalist? Driesch's first volume, putting as it does into intelligible language that positive deduction from the facts—especially of regeneration—should carry a vivid realisation of this truth to any mind. If after their existence is realised, it isdesired that these unknown forces of order should have a name, and the word entelechy is proposed, the only objection I have to make is that the adoption of a term from Aristotelian philosophy carries a plain hint that we propose to relegate the future study of the problem to metaphysic.
From this implication the vitalist does not shrink. But I cannot find in the facts yet known to us any justification of so hopeless a course. It was but yesterday that the study ofEntwicklungsmechanikwas begun, and if in our slight survey we have not yet seen how the living machine is to be expressed in terms of natural knowledge that is poor cause for despair. Driesch sums up his argument thus:[13]
"It seems to me that there is only one conclusion possible. If we are going to explain what happens in our harmonious-equipotential systems by the aid of causality based upon the constellation of single chemical factors and events, theremustbe some such thing as a machine. Now the assumption of the existence of a machine proves to be absolutely absurd in the light of the experimental facts.Therefore there can be neither any sort of a machine nor any sort of causality based upon constellation underlying the differentiation of harmonious-equipotential systems."
"For a machine, typical with regard to the three chief dimensions of space, cannot remain itself if you remove parts of it or if you rearrange its parts at will."
To the last clause a note is added as follows:
"The pressure experiments and the dislocation experiments come into account here; for the sake of simplicity they have not been alluded to in the main line of our argument."
I doubt whether any man has sufficient knowledge of all possible machines to give reality to this statement. In spite also of the astonishing results of experiments in dislocation, doubt may further be expressed as to whether they have been tried in such variety or on such a scale as to justify the suggestion that the living organism remains itself if its parts are rearranged atwill. All we know is that it can "remain itself" when much is removed, and when much rearrangement has been affected, which is a different thing altogether.
I scarcely like to venture into a region of which my ignorance is so profound, but remembering the powers of eddies to re-form after partial obliteration or disturbance, I almost wonder whether they are not essentially machines which remain themselves when parts of them are removed.
Real progress in this most obscure province is not likely to be made till it attracts the attention of physicists; and though they for long may have to forego the application of exact quantitative methods, I confidently anticipate that careful comparison between the phenomena of repetition formed in living organisms and the various kinds of segmentation produced by mechanical agencies would be productive of illuminating discoveries.
We have now seen that among the normal physiological processes the phenomena of division form a recognisable, and in all likelihood a naturally distinct group. Variations in these respects may thus be regarded as constituting a special class among variations in general.
The substantive variations have only one property in common—the negative one that they are not Meristic. The work of classifying them and distinguishing them according to their several types demands a knowledge of the chemistry of life far higher than that to which science has yet attained. In reference to some of the simplest variations Garrod has introduced the appropriate term "Chemical sports." The condition in man known as Alkaptonuria in which the urine is red is due especially to the absence of the enzyme which decomposes the excretory substance, alkapton. The "chemical sport" here consists in the inability to break up the benzene ring. The chemical feature which distinguishes and is the proximate cause of several colour-varieties can now in a few cases be declared. The work of Miss Wheldale has shown that colour-varieties may be produced by the absence of the chromogen compound the oxidation of which gives rise to sap-colours, by differences in the completeness of this process of oxidation, and by a process of reduction supervening on or perhaps suppressing the oxidation. Some of these processes moreover may be brought about by the combined action of two bodies, the one an enzyme, for example an oxygenase, and the other a substance regarded as a peroxide, contributing the oxygen necessary for the oxidation to take place. Variation in colour may thus be brought about by the addition or omission of any one of the bodies concerned in the action.
Similar variations, or rather similar series of variations will undoubtedly hereafter be identified in reference to all the various kinds of chemical processes upon which the structure and functions of living things depend. The identification of these processes and of the bodies concerned in them will lead to a real classification of Substantive Variations.
To forecast the lines on which such classification will proceed is to look too far ahead. We may nevertheless anticipate with some confidence that future analysis will recognise among the contributing elements, some which are intrinsic and inalienable, and others which are extrinsic and superadded.
We already know that there may be such interdependence among the substantive characters that to disentangle them will be a work of extreme difficulty. The mere fact that in our estimation characters belong to distinct physiological systems is no proof of their actual independence. In illustration may be mentioned the sap-colour in Stocks and the development of hoariness on the leaves and stems, which Miss Saunders's experiments have shown to be intimately connected, so that in certain varieties no hoariness is produced unless the elements for sap-colour are already present in the individual plant.
The first step in the classification of substantive variations is therefore to determine which are due to the addition of new elements or factors, and which are produced by the omission of old ones.A priorithere is no valid criterion by which this can be known, and actual experiments in analytical breeding can alone provide the knowledge required. Some very curious results have by this method been obtained, which throw an altogether unexpected light on these problems. For example, in order that the remarkable development of mesoblastic black pigment characteristic of the Silky Fowl should be developed, it is practically certain that two distinct variations from such a type asGallus bankivamust have occurred. I assume, as is reasonable, thatG. bankivahas genetic properties similar to those of the Brown Leghorn breed which has been used in the experiments which Mr. Punnett and I have conducted.Gallus bankivawas not available but the Brown Leghorn agrees with it very closely in colouration, and probably in the general physiology of its pigmentation.Setting aside the various structural differences between the two breeds, the Silky is immediately distinguished from the Leghorn by the fact that the skin of the whole body including that of the face and comb appears to be of a deep purplish colour. The face and comb of the Leghorn are red and the skin of the body is whitish yellow. On examination it is found that the purple colour of the Silky is in reality due to the distribution of a deep black pigment in the mesoblastic membranes throughout the body. The somatopleura, the pleura,pia mater, the dermis, and in most organs the connective tissue and the sheaths of the blood-vessels, are thus impregnated with black. No such pigmentation exists in the Leghorn. As the result of an elaborate series of experimental matings we have proved that the distinction between the Leghorn and the Silky consists primarily in the fact that the Silky possesses a pigment-producing factor,P, which is not present in the Leghorn.
This variation must undoubtedly have been one ofaddition. But besides this there is another difference of an altogether dissimilar nature; for the Brown Leghorn possesses a factor which has the power of partially or completely restricting the operation of the pigment-producing factor,P. Moreover in respect of this pigment-restricting factor which we may callD, the sexes of the Brown Leghorn differ, for the male is homozygous orDD, but the female is heterozygous,Dd. Thus in order that the black-skinned breed could be evolved from such a type as a Brown Leghorn it must be necessaryboththatPshould be added and thatDshould drop out. We have not the faintest conception of the process by which either of these events have come to pass, but there is no reasonable doubt that in the evolution of the Silky fowl they did actually happen.
We may anticipate that numerous interdependences of this kind will be discovered.
Before any indisputable progress can be made with the problem of evolution it is necessary that we should acquire some real knowledge of the genesis of that class of phenomena which formed the subject of the last chapter. So long as the process of division remains entirely mysterious we can form no conceptioneven of the haziest sort as to the nature of living organisms, or of the proximate causes which determine their forms, still less can we attempt any answer to those remoter questions of origin and destiny which form the subject of the philosopher's contemplation. It is in no spirit of dogmatism that I have ventured to indicate the direction in which I look for a solution, though I have none to offer. It may well be that before any solution is attained, our knowledge of the nature of unorganised matter must first be increased. For a long time yet we may have to halt, but we none the less do well to prepare ourselves to utilise any means of advance that may be offered, by carefully reconnoitering the ground we have to traverse. The real difficulty which blocks our progress is ignorance of the nature of division, or to use the more general term, of repetition.
Let us turn to the more familiar problem of the causes of variation. Now since variation consists as much in meristic change as in alteration in substance or material, there is one great range of problems of causation from which we are as yet entirely cut off. We know nothing of the causation of division, and we have scarcely an observation, experiment or surmise touching the causes by which the meristic processes may be altered.
Of the way in which variations in the substantive composition of organisms are caused we have almost as little real evidence, but we are beginning to know in what such variations must consist. These changes must occur either by the addition or loss of factors.
We must not lose sight of the fact that though the factors operate by the production of enzymes, of bodies on which these enzymes can act, and of intermediary substances necessary to complete the enzyme-action, yet these bodies themselves can scarcely be themselves genetic factors, but consequences of their existence. What then are the factors themselves? Whence do they come? How do they become integral parts of the organism? Whence, for example, came the power which is present in a White Leghorn of destroying—probably reducing—the pigment in its feathers? That power is now a definite possession of the breed,present in all its germ-cells, male and female, taking part in their symmetrical divisions, and passed on equally to all as much as is the protoplasm or any other attribute of the breed. From the body of the bird the critical and efficient substance could in all likelihood be isolated by suitable means, just as the glycogen of the liver can be. But even when this extraction has been accomplished and the reducing body isolated, we shall know no more than we did before respecting the mode by which the power to produce it was conferred on the fowl, any more than we know how the walls of its blood-vessels acquired the power to form a fibrin-ferment.
It is when the scope of such considerations as this are fully grasped that we realise the fatuousness of the conventional treatment which the problem of the causes of variation commonly receives. Environmental change, chemical injury, differences in food supply, in temperature, in moisture, or the like have been proposed as "causes." Admitting as we must do, that changes may be produced—usually inhibitions of development—by subjecting living things to changes in these respects, how can we suppose it in the smallest degree likely that very precise, new, and adaptative powers can be conferred on the germs by such treatment? Reports of positive genetic consequences observed comparable with those I have mentioned, become from time to time current. We should I think regard them with the gravest doubt. Few, so far as I am aware, have ever been confirmed, though clear and repeated confirmation should be demanded before we suffer ourselves at all to build upon such evidence. In a subsequent chapter some of these cases will be considered in detail.
In no class of cases would the transmission of an acquired character superficially appear so probable as in those where power of resisting the attack of a pathogenic organism is acquired in the lifetime of the zygote. The possession of such a power is moreover a distinction comparable with those which differentiate varieties and species. It is due to the development in the blood of specific substances which pervade the whole fluid. This development is exactly one of those "appropriate responses tostimuli" which naturalists who incline to regard adaptation as a direct consequence of an environmental influence might most readily invoke as an illustration of their views. And yet all evidence is definitely unfavourable to the suggestion of an inheritance of the acquired power of resistance. Such change as can be perceived in the virulence of the attacks on successive generations may be most easily regarded as due to the extermination of the more susceptible strains, and perhaps in some measure to variation in the invading organisms themselves, an "acquired character" of quite different import.
The specific "anti-body" may have been produced in response to the stimulus of disease, but the power to produce it without this special stimulus is not included in the germ-cells any more than a pigment. All that they bear is thepower to producethe anti-bodies when the stimulus is applied.
If we could conceive of an organism like one of those to which disease may be due becoming actually incorporated with the system of its host, so as to form a constituent of its germ-cells and to take part in the symmetry of their divisions, we should have something analogous to the case of a species which acquires a new factor and emits a dominant variety. When we see the phenomenon in this light we realise the obscurity of the problem. The appearance of recessive varieties is comparatively easy to understand. All that is implied is the omission of a constituent. How precisely the omission is effected we cannot suggest, but it is not very difficult to suppose that by some mechanical fault of cell-division a power may be lost. Such variation by unpacking, or analysis of a previously existing complex, though unaccountable, is not inconceivable. But whence come the new dominants? Whether we imagine that they are created by some rearrangement or other change internal to the organism, or whether we try to conceive them as due to the assumption of something from without we are confronted by equally hopeless difficulty.
The mystery of the origin of a dominant increases when it is realised that there is scarcely any recent and authentic account of such an event occurring under critical observation, which canbe taken as a basis for discussion. The literature of horticulture for example abounds in cases alleged, but I do not think anyone can produce an illustration quite free from doubt. Such evidence is usually open to the suspicion that the plant was either introduced by some accident, or that it arose from a cross with a pre-existing dominant, or that it owed its origin to the meeting of complementary factors. In medical literature almost alone however, there are numerous records of the spontaneous origin of various abnormal conditions in man which habitually behave as dominants, and of the authenticity of some of these there can be no doubt.
When we know that such conditions as hereditary cataract or various deformities of the fingers behave as dominants, we recognize that those conditions must be due to the addition of some element to the constitution of the normal man. In the collections of pedigrees relating to such pathological dominants there are usually to be found alleged instances of the origin of the conditionde novo. Not only do these records occur with such frequency that they cannot be readily set aside as errors, but from general considerations it must be obvious that as these malformations are not common to normal humanity they must at some moment of time have been introduced. The lay reader may not be so much impressed with the difficulty as we are. He is accustomed to regard the origin ofanynew character as equally mysterious, but when once dominants are distinguished from recessives the problem wears a new aspect. Thus the appearance of high artistic gifts, whether as an attribute of a race or as a sporadic event among the children of parents destitute of such faculties, is not very surprising, for we feel fairly sure that the faculty is a recessive, due to the loss of a controlling or inhibiting factor; but thede novoorigin of brachydactylous fingers in a child of normal parents is of quite a different nature, and must indicate the action of some new specific cause.
Whether such evidence is applicable to the general problem of evolution may with some plausibility be questioned; but there is an obvious significance in the fact that it is among these pathological occurrences that we meet with phenomena mostnearly resembling the spontaneous origin of dominant factors, and I cannot see such pedigrees as these without recalling Virchow's aphorism that every variation owes its origin to some pathological accident. In the evolution of domestic poultry, ifGallus bankivabe indeed the parent form of all our breeds, at least some half dozen new factors must have been added during the process. Inbankivathere is, for example, no factor for rose comb, pea comb, barring on the feathers, or for the various dominant types of dark plumage. Whence came all these? It is, I think, by no means impossible that some other wild species now extinct did take part in the constitution of domestic poultry. It seems indeed to me improbable that the heavy breeds descend frombankiva. Both in regard to domestic races of fowls, pigeons, and some other forms, the belief in origin within the period of human civilization from one simple primitive wild type seems on a balance of probabilities insecurely founded, but allowing something for multiplicity of origin we still fall far short of the requisite total of factors. Elements exist in our domesticated breeds which we may feel with confidence have come in since their captivity began. Such elements in fowls are dominant whiteness, extra toe, feathered leg, frizzling, etc., so that even hypothetical extension of the range of origin is only a slight alleviation of the difficulty.
Somehow or other, therefore, we must recognize that dominant factors do arise. Whether they are created by internal change, or whether, as seems to me not wholly beyond possibility, they obtain entrance from without, there is no evidence to show. If they were proved to enter from without, like pathogenic organisms, we should have to account for the extraordinary fact that they are distributed with fair constancy to half the gametes of the heterozygote.
In proportion as the nature of dominants grows more clear so does it become increasingly difficult to make any plausible suggestion as to their possible derivation. On the other hand the origin of a recessive variety by the loss of a factor is a process so readily imagined that our wonder is rather that the phenomenon is not observed far more often. Some slip in the accurateworking of the mechanical process of division, and a factor gets left out, the loss being attested by the appearance of a recessive variety in some subsequent generation.
Consistently with this presentation of the facts we find that, as in our domesticated animals and plants, a diversity of recessives may appear within a moderately short period, and that when variations come they often do not come alone. Witness the cultural history of the Sweet Pea,Primula Sinensis,Primula obconica,Nemesia strumosaand many such examples in which variation when it did come was abundant. The fact cannot be too often emphasized that in the vast proportion of these examples of substantive variation under domestication, as well as of substantive variation in the natural state, the change has come about by omission, not by addition. To take, for example, the case of the Potato, in which so many spontaneous bud-variations have been recorded, East after a careful study of the evidence has lately declared his belief that all are of this nature, and the opinion might be extended to many other groups of cases whether of bud or seminal variation. Morgan draws the same conclusion in reference to the many varieties he has studied inDrosophila.
In the Sweet Pea, a form which is beyond suspicion of having been crossed with anything else, and has certainly produced all the multitude of types which we now possess by variations from one wild species, there is only one character of the modern types which could, with any plausibility, be referred to a factor not originally forming part of the constituents of the wild species. This is the waved edge, so characteristic of the "Spencer" varieties; for the cross between a smooth-edged and a waved type gives an intermediate not unfrequently. Nevertheless there is practically no doubt that this is merely an imperfection in the dominance of the smooth edge, and we may feel sure that any plant homozygous for smooth edge would show no wave at all. Hence it is quite possible that even the appearance of the original waved type, Countess Spencer, was due to the loss of one of the factors for smooth edge at some time in the history of the Sweet Pea.
In the case of the Chinese Primrose (Primula Sinensis) one dominant factor has been introduced in modern times, probably within the last six years at most. This is the factor which causes suppression of the yellow eye, giving rise to the curious type known as "Queen Alexandra." Mr. R. P. Gregory's experiments proved that this was a very definite dominant, and the element responsible for this development is undoubtedly an addition to the original ingredient-properties, with which the species was endowed. Unfortunately, as happens in almost every case of the kind, the origin of this important novelty appears to be lost. Its behaviour, however, when crossed with various other types is that of a simple dominant giving an ordinary 3:1 ratio. There is therefore no real doubt that it came into existence by the definite addition of a new factor, for if it was simply a case of the appearance of a new character made by combination of two previously existing complementary factors we should expect that when Queen Alexandra was self-fertilised a 9:7 ratio would be a fairly common result, which is not in practice found.
InOenotheraGates[1]has observed the appearance, in a large sowing of about 1,000Oenothera rubrinervis, of a single individual having considerably more red pigment in the calyx than is usual inrubrinervis. The whole of the hypanthium in the flowers of this plant was red instead of green as inrubrinervis, and the whole of the sepals were red in the bud-stage, except for small green areas at the base. This type behaved as a dominant overrubrinervis, but so far a pure-breeding individual was not found. Admittedly the variation of this plant from the type ofrubrinerviscan be represented as one of degree, though there is a very sensible gap in the series between the new form which Gates names "rubricalyx" and the reddestrubrinervisseen in his cultures. It must certainly be recognised as a new dominant. Gates, rightly as I consider, regards the distinction betweenrubrinervisandrubricalyxas a quantitative one, and the same remark applies to certain other types differing in the amount of anthocyanin which they produce. I do not understand the argumentwhich Gates introduces to the effect that the difference between such quantitative types cannot be represented in terms of presence and absence. We are quite accustomed to the fact that in the rabbit self-colour segregates from the Dutch-marked type. These two types differ in a manner which we may reasonably regard as quantitative. It is no doubt possible that the self-coloured type contains an ingredient which enables the colour to spread over the whole body, but it is, I think, perhaps more easy to regard the Dutch type as a form from which a part of the colour is absent. It may be spoken of in terms I have used, as asubtraction-stagein colour. Following a similar method we may regardrubricalyxas an addition-stage in colour-variation. The fact that crosses betweenrubrinervis, orrubricalyxandLamarckianagive a mixture of types in F1, does not I think show, as Gates declares, that there is any system here at work to which a factorial or Mendelian analysis does not apply; but that question may be more fitly discussed in connexion with the other problems raised by the behaviour ofOenotheraspecies in their crosses.
I do, however, feel that, interesting as this case must be admitted to be, we cannot quite satisfactorily discuss it as an illustration of thede novoorigin of a dominant factor. The difference between the novelty and the type is quantitative, and it is not unreasonable to think of such a difference being brought about by some "pathological accident" in a cell-division.
Recognition of the distinction between dominant and recessive characters has, it must be conceded, created a very serious obstacle in the way of any rational and concrete theory of evolution. While variations of all kinds could be regarded as manifestations of some mysterious instability of organisms this difficulty did not occur to the mind of evolutionists. To most of those who have taken part in genetic analysis it has become a permanent and continual obsession. With regard to the origin of recessive variations, there is, as we have seen, no special difficulty. They are negative and are due to absences, but as soon as it is understood that dominants are caused by an addition we are completely at a loss to account for their origin, for wecannot surmise any source from which they may have been derived. Just as when typhoid fever breaks out in his district the medical officer of health knows for certain that the bacillus of typhoid fever has by some means been brought into that district so do we know that when first dominant white fowls arose in the evolution of the domestic breeds, by some means the factor for dominant whiteness got into a bird, or into at least one of its germ-cells. Whence it came we cannot surmise.
Whether we look to the outer world or to some rearrangement within the organism itself, the prospect of finding a source of such new elements is equally hopeless.
Leaving this fundamental question aside as one which it is as yet quite unprofitable to discuss, we are on safe ground in foreseeing that the future classification of substantive variations, which genetic research must before long make possible, will be based on a reference to the modes of action of the several factors. Some will be seen to produce their effects by oxidation, some by reduction, some by generating substances of various types, sugars, enzymes, activators, and so forth. It may thus be anticipated that the relation of varieties to each other and to types from which they are derived will be expressible in terms of definite synthetical formulae. Clearly it will not for an indefinite time be possible to do this in practice for more than a few species and for characters especially amenable to experimental tests, but as soon as the applicability of such treatment is generally understood the influence on systematics must be immediate and profound, for the nature of the problem will at length be clear and, though the ideal may be unattainable, its significance cannot be gainsaid.
Note.—With hesitation I allow this chapter to appear in the form in which it was printed a year ago, but in passing it for the press after that interval I feel it necessary to call attention to a possible line of argument not hitherto introduced.
In all our discussions we have felt justified in declaring that the dominance of any character indicates that some factor ispresent which is responsible for the production of that character. Where there is no definite dominance and the heterozygote is of an intermediate nature we should be unable to declare on which side the factor concerned was present and from which side it was absent. The degree of dominance becomes thus the deciding criterion by which we distinguish the existence of factors. But it should be clearly realized that in any given case the argument can with perfect logic be inverted. We already recognize cases in which by the presence of an inhibiting factor a character may be suppressed and purely as a matter of symbolical expression we might apply the same conception of inhibition to any example of factorial influence whatever. For instance we say that in as much as two normal persons do not have brachydactylous children, there must be some factor in these abnormal persons which causes the modification. Our conclusion is based on the observed fact that the modification is a dominant. But it may be that normal persons are homozygous in respect of some factorN, which prevents the appearance of brachydactyly, and that in any one heterozygous,Nn, for this inhibiting factor, brachydactyly can appear. Similarly the round pea we say containsR, a factor which confers this property of roundness, without which its seeds would be wrinkled. But here we know that the wrinkled seed is in reality one having compound starch-grains, and that the heterozygote, though outwardly round enough, is intermediate in that starch-character. If we chose to say that the compoundness of the grains is due to a factorCand that two doses of it are needed to make the seed wrinkled, I know no evidence by which such a thesis could be actually refuted. That such reasoning is seemingly perverse must be conceded; but when we consider the extraordinary difficulties which beset any attempt to conceive the mode of origin of a new dominant factor, we are bound to remember that there is this other line of argument which avoids that difficulty altogether. In the case of the "Alexandra"-eye inPrimula, or the red calyx in Gates'sOenothera, inverting the reasoning adopted in the text, we may see that only thePrimulahomozygous for the yellow eye can develop it and that two doses of the factor for therubrinerviscalyx are required to prevent that part of the plant from being red.
We may proceed further and extend this mode of reasoning to all cases of genetic variation, and thus conceive of all alike as due to loss of factors present in the original complex. Until we can recognize factors by means more direct than are provided by a perception of their effects, this doubt cannot be positively removed. For all practical purposes of symbolic expression we may still continue to use in our analyses the modes of representation hitherto adopted, but we must not, merely on the ground of its apparent perversity, refuse to admit that the line of argument here indicated may some day prove sound.
When with the thoughts suggested in the last chapter we contemplate the problem of Evolution at large the hope at the present time of constructing even a mental picture of that process grows weak almost to the point of vanishing. We are left wondering that so lately men in general, whether scientific or lay, were so easily satisfied. Our satisfaction, as we now see, was chiefly founded on ignorance.
Every specific evolutionary change must represent a definite event in the construction of the living complex. That event may be a disturbance in the meristic system, showing itself in a change in the frequency of the repetitions or in the distribution of differentiation among them, or again it may be a chemical change, adding or removing some factor from the sum total.
If an attempt be made to apply these conceptions to an actual series of allied species the complexity of the problem is such that the mind is appalled. Ideas which in the abstract are apprehended and accepted with facility fade away before the concrete case. It is easy to imagine how Man was evolved from anAmoeba, but we cannot form a plausible guess as to howVeronica agrestisandVeronica politawere evolved, either one from the other, or both from a common form. We have not even an inkling of the steps by which a Silver Wyandotte fowl descended fromGallus Bankiva, and we can scarcely even believe that it did. The Wyandotte has its enormous size, its rose comb, its silver lacing, its tame spirit, and its high egg production. The tameness and the high egg production are probably enough both recessives, and though we cannot guess how the corresponding dominant factors have got lost, it is not very difficult to imagine that they were lost somehow. But the rose comb and the silver colour aredominants. The heavy weight also appears in the crosses with Leghorns, but we need not at once conclude that itdepends on a simple dominant factor, because the big size of the crosses may be a consequence of the cross and may depend on other elements.
Now no wild fowl known to us has these qualities. May we suppose that some extinct wild species had them? If so, may we again make the same supposition in all similar cases? To do so is little gain, for we are left with the further problem, whence did those lost wild species acquire those dominants? Suppositions of this kind help no more than did the once famous conjecture as to the origin of living things—that perhaps they came to earth on a meteorite. The unpacking of an original complex, the loss of various elements, and the recombination of pre-existing materials may all be invoked as sources of specific diversity. Undoubtedly the range of possibilities thus opened up is large. It will even cover an immense number of actual examples which in practice pass as illustrations of specific distinction. The Indian Rock pigeon which has a blue rump may quite reasonably be regarded as a geographically separated recessive form of our ownColumba livia, for as Staples-Browne has shown the white rump ofliviais due to a dominant factor. The various degrees to which the leaves of Indian Cottons are incised have, as Leake says, been freely used as a means of classification. The diversities thus caused are very remarkable, and when taken together with diversities in habit, whether sympodial or monopodial, the various combinations of points of difference are sufficiently distinctive to justify any botanist in making a considerable number of species by reference to them alone. Nevertheless Leake's work goes far to prove that all of these forms represent the re-combinations of a very small number of factors. The classical example ofPrimula Sinensisand its multiform races is in fact for a long way a true guide as to the actual interrelations of the species which systematists have made. That they did make them was due to no mistake in judgment or in principle, but simply to the want of that extended knowledge of the physiological nature of the specific cases which we now know to be a prime necessity.
But will such analysis cover all or even most of the ordinary cases of specific diversity between near allies? Postponing the problem of the interrelations of the larger divisions as altogether beyond present comprehension, can we suppose, that in general, closely allied species and varieties represent the various consequences of the presence or absence of allelomorphic factors in their several combinations? The difficulty in making a positive answer lies in the fact that in most of the examples in which it has been possible to institute breeding experiments with a view to testing the question, a greater or less sterility is encountered. Where, however, no such sterility is met with, as for instance in the crosses made by E. Baur among the species ofAntirrhinumthere is every reason to think that the whole mass of differences can and will eventually be expressed in terms of ordinary Mendelian factors. Baur has for example crossed species so unlike asAntirrhinum majusandmolle, forms differing from each other in almost every feature of organisation.[1]The F2generation from this cross presents an amazingly motley array of types which might easily if met with in nature be described as many distinct species. Yet all are fertile and there is not the slightest difficulty in believing that they can all be reduced to terms of factorial analysis.
If allowance be made for the complicating effects of sterility, is there anything which prevents us from supposing that such good species as those ofVeronicaor of any other genus comprising well-defined forms may not be similarly related? I do not know any reason which can be pointed to as finally excluding such a possibility. Nevertheless it has been urged with some plausibility that good species are distinguished bygroupsof differentiating characters, whereas if they were really related as the terms of a Mendelian F2family are, we should expect to find not groups of characters in association, but rather series of forms corresponding to the presence and absence of the integral factors composing the groups of characters. I am not well enough versed in systematic work to be able to decide with confidence how much weight should be attached to this consideration. Someweight it certainly has, but I cannot yet regard it as forming a fatal objection to the application of factorial conceptions on the grand scale. It may be recalled that we are no longer under any difficulty in supposing that differences of all classes may be caused by the presence or absence of factors. It seemed at first for example that such characters as those of leaf shape might be too subtle and complex to be reducible to a limited number of factors. But first the work of Gregory onPrimula Sinensisshowed that several very distinct types of leaves were related to each other in the simplest way. In that particular example, intermediates are so rare as to be negligible, but subsequently Shull dealing with such a complicated example asCapsella, and Leake in regard to Cottons, both forms in which intergrades occur in abundance, have shown that a simple factorial scheme is applicable. We need not therefore, to take an extreme case, doubt that if it were possible to examine the various forms of fruit seen in the Squashes by really comprehensive breeding tests, even this excessive polymorphism in respect of structural features would be similarly reducible to factorial order.
It must always be remembered also that in a vast number of cases, nearly allied forms which are distinct, occupy distinct ground. Moreover, by whatever of the many available mechanisms that end be attained, it is clear that nature very often does succeed in preventing intercrossing between distinct forms so far that the occurrence of that phenomenon is a rarity under natural conditions. The facts may, I think, fairly be summarized in the statement that species are on the whole distinct and not intergrading, and that the distinctions between them are usually such as might be caused by the presence, absence, or inter-combination of groups of Mendelian factors; but that they are so caused the evidence is not yet sufficient to prove in more than a very few instances.
The alternative, be it explicitly stated, is not to return to the view formerly so widely held, that the distinctions between species have arisen by the accumulation of minute or insensible differences. The further we proceed with our analyses the more inadequate and untenable does that conception of evolutionarychange become. If the differences between species have not come about by the addition or loss of factors one at a time, then we must suppose that the changes have been effected by even larger steps, and variations including groups of characters, must be invoked.
That changes of this latter order are really those by which species arise, is the view with which de Vries has now made us familiar by his writings on the Mutation Theory. In so far as mutations may consist in meristic changes of many kinds and in the loss of factors it is unnecessary to repeat that we have abundant evidence of their frequent occurrence. That they may also more rarely occur by the addition of a factor we are, I think, compelled to believe, though as yet the evidence is almost entirely circumstantial rather than direct. The evidence for the occurrence of those mutations of higher order, by which new species characterized by several distinct features are created, is far less strong, and after the best study of the records which I have been able to make, I find myself unconvinced. The facts alleged appear capable of other interpretations.
The most famous and best studied examples are of course the forms ofOenotheraraised by de Vries fromOenothera Lamarckianain circumstances well known to all readers of genetic literature. Whatever be the true significance of these extraordinary "mutations" there can be no question about the great interest which attaches to them, and the historical importance which they will long preserve. Apart also from these considerations it is becoming more and more evident that in their peculiarities they provide illustrations of physiological phenomena of the highest consequence in the study of genetics at large.
De Vries found, as is well known, thatOenothera Lamarckianagives off plants unlike itself. These mutational forms are of several distinct and recognizable types which recur, and several of them breed true from their first appearance. The obvious difficulty, which in my judgment should make us unwilling at present to accept these occurrences as proof of the genesis of new species by mutation, is that we have as yet no certainty that the appearance of the new forms is not an effect of the recombinationof factors, such as is to be seen in so many generations of plants derived from a cross involving many genetic elements. The first question is what isOenothera Lamarckiana? Is it itself a plant of hybrid origin? To this fundamental question no satisfactory answer has yet been given. All attempts to find it as a wild plant in America have failed. It existed in Europe in the latter half of the eighteenth century. Whence it came is still uncertain, but the view that it came into existence in Europe and perhaps in Paris, seems on the whole the most probable. The question has been debated by Macdougal, Gates, and Davis. From historical sources there is little expectation of further light. Those who favour the notion of a hybrid origin look onOenothera biennisas one of the putative parents. It has been conjectured that a species calledgrandifloralately re-discovered on the Alabama river was the other parent. Experiments have been instituted by Davis to discover whetherLamarckianacan be made artificially by crossing these two species. The results so far have shown that while plants approximating in various respects toLamarckianahave thus been produced, none agree exactly with that form. Davis, to whom reference should be made for a full account of the present state of the enquiry, points out that there are many strains ofbiennisin existence and that it is by no means impossible that by using others of these strains a still closer approximation can be made. None of Davis's artificial productions as yet breed at all true, asLamarckianaon the whole does. In such a case, however, where several characters are involved, this is perhaps hardly to be expected.
One feature of theOenotherasis very curious. Not onlyLamarckiana, but all the allied species so far as I am aware, have a considerable proportion of bad and shrivelled pollen grains. This is undoubtedly true of species living in the wild state as well as of those in cultivation. I have had opportunities of verifying this for myself in the United States. No one looking at the pollen of anOenotherawould doubt that it was taken from some hybrid plant exhibiting partial sterility. On the other hand, it is difficult to suppose that numbers, perhaps all, of the"species" of the genus are really hybrids, and many of them breed substantially true. I regard this constant presence of bad pollen grains as an indication that the genetic physiology ofOenotherais in some way abnormal, and as we shall presently see, there are several other signs which point in the same direction.
Discussion of the whole series of phenomena is rendered exceedingly difficult first, by reason of the actual nature of the material. The characteristics of many of the types which de Vries has named are evasive. A few of these types, for instance,gigas,nanella,albida,brevistylis, and perhaps a few more are evidently clear enough, but we have as yet no figures and descriptions precise enough to enable a reader to appreciate exactly the peculiarities of the vast number of forms which have now to be considered in any attempt to gain a comprehensive view of the whole mass of facts. It is also not in dispute that the forms are susceptible of great variations due simply to soil and cultural influences.
The fact that no Mendelian analysis has yet been found applicable to this group ofOenotherasas a whole is perhaps largely due to the fact that until recently such analysis has not been seriously attempted. Following the system which he had adopted before the rediscovery of Mendelism, or at all events, before the development of that method of analysis, de Vries has freely appliednamesto special combinations of characters and has scarcely ever instituted a factorial analysis. Before we can get much further this must be attempted. It may fail, but we must know exactly where and how this failure comes about. There are several indications that such a recognition of factorial characters, could be carried some way. For example, the height, the size of the flowers, the crinkling of the leaves, the brittleness of the stems, perhaps even the red stripes on stems and fruits, and many more, are all characters which may or may not depend on distinct factors, but if such characters are really transmitted in unresolved groups, the limitations of those groups should be carefully determined. The free use of names for the several forms, rather than for the characters, has greatly contributed to deepen the obscurity which veils the whole subject.
I do not mean to suggest that theseOenotherasfollow a simple Mendelian system. All that we know of them goes to show that there are curious complications involved. One of these, probably the most important of all, has lately been recognized by de Vries himself, namely, that in certain types the characters borne by the female and the male germ-cells of the same plant are demonstrably different. There can be little doubt that further research will reveal cognate phenomena in many unsuspected places. The first example in which such a state of things was proved to exist is that of the Stocks investigated by Miss Saunders.[2]By a long course of analysis she succeeded in establishing in 1908 the fact that if a plant ofMatthiolais of that eversporting kind which gives a large proportion of double-flowered plants among its offspring (produced by self-fertilisation), then the egg-cells of such a plant are mixed in type, but the pollen of the same plant is homogeneous. Some of the egg-cells have in them the two factors for singleness, but some of them are short of one or both of these factors. The pollen-grains, however, are all recessives, containing neither of these factors. The egg-cells, in other words, are mixed, "singles" and "doubles," while the pollen-grains are all "doubles." The same is true of the factor differentiating "white," or colourless plastids from cream-coloured plastids inMatthiola, the egg-cells being mixed "whites" and "creams," while the pollen-grains are all "creams," viz: recessives. Later in the same year (1908) de Vries[3]announced a remarkable case which will be discussed in detail subsequently. It relates to certainOenotherasheterozygous for dwarfness, in which (p. 113) the ovules were mixed, tails and dwarfs, while the pollen is all dwarf.
Again inPetuniaMiss Saunders's[4]work has shown that a somewhat similar state of things exists, but with this remarkable difference, that though the egg-cells are mixed, singles and doubles, the pollen-grains are allsingles, viz: dominants. All thePetuniasyet examined have been in this condition, includingsome which in botanic gardens pass for original species. Whether actual wild plants from their native habitats are in the same state, is not yet known, but it is by no means improbable. The case may be compared with that of the mothAbraxas grossulariatastudied by Doncaster and Raynor, in which the females are all heterozygous, or we may almost say "hybrids" ofgrossulariataand the varietylacticolor. Similarly we may say that at least garden Petunias are heterozygous in respect of singleness. The proof of this is of course that when fertilised with the pollen of doubles they throw a mixture of doubles and singles. The statements which de Vries has published regarding the behaviour of several of theOenotherasgo far to show that they must have a somewhat similar organisation. On the present evidence it is still quite impossible to construct a coherent scheme which will represent all the phenomena in their interrelations, and among the facts are several which, as will appear, seem mutually incompatible. The first indication that theOenotherasmay have either mixed ovules or mixed pollen appears in the fact thatLamarckianaand several of its "mutants" used as males, with several other forms as females, give a mixed offspring. For example, de Vries (1907) found that
biennis♀ ×Lamarckiana♂biennis cruciata♀ ×Lamarckiana♂muricata♀ ×Lamarckiana♂biennis♀ ×rubrinervis♂biennis cruciata♀ ×rubrinervis♂
all give a mixture of two distinct types which he nameslaetaandvelutina, consisting of about equal numbers of each. On account of the fact that the two forms are produced in association de Vries has called these forms "twin hybrids," a designation which is not fortunate, seeing that it is impossible to imagine that any kind of twinning is concerned in their production. The distinction between these two seems to be considerable,laetahaving leaves broader, bright green in colour, and flat, with pollen scanty, whilevelutinahas leaves narrower, grayish green, more hairy, and furrow-shaped, with pollen abundant.
We next meet the remarkable fact that these two forms,laetaandvelutinabreed true to their respective types, and do not reproduce the parent-types among their offspring resulting from self-fertilisation. This statement must be qualified in two respects. Whenmuricata♂ is fertilised bybrevistylisthe formslaetaandvelutinaare produced, but each of them subsequently throws the short-styled form as a recessive (de Vries, 1907, p. 406). It may be remembered that de Vries's previous publications had already shown that the short style ofbrevistylis, one of theLamarckiana"mutants," behaves as a recessive habitually (Mutationstheorie, II, p. 178, etc.).
Also whennanella, the dwarf "mutant" ofLamarckianais used as male onmuricataas female,laetaandvelutinaare produced, but one only of these, namely,velutina, subsequently throws dwarfs on self-fertilisation. The dwarfs thus thrown are said to form about 50 per cent. of the families in which they occur (de Vries, 1908, p. 668). The fact that the two forms,laetaandvelutina, are produced by many matings in whichLamarckianaand its mutantrubrinervisare used as males is confirmed abundantly by Honing, who has carried out extensive researches on the subject. After carefully reading his paper, I have failed to understand the main purport of the argument respecting the "double nature" ofLamarckianawhich he founds on these results, but I gather that in some waylaetais shown to partake especially of the nature ofLamarckiana, whilevelutinais a form ofrubrinervis. The paper contains many records which will be of value in subsequent analysis of these forms.
Before considering the possible meaning of these facts we must have in our minds the next and most novel of the recent extensions of knowledge as to the genetic properties of theOenotheras. In the previous statement we have been concerned with the results of using eitherLamarckianaitself or one of its "mutants"rubrinervis,brevistylis, ornanellaas male, on one of the speciesbiennisormuricata. The new experiments relate to crosses between the two speciesbiennisandmuricatathemselves.
De Vries found:
1. That the reciprocal hybrids from these two species differed,biennis×muricataproducing one type of F1andmuricata×biennisproducing another. Each F1resembled the father more than the mother.
2. That each of the hybrids so produced breeds true on self-fertilisation.
3. That if we speak of the hybrid frombiennis×muricataasBMand of the reciprocal asMB, then
BM×MB
gives exclusively offspring ofbiennistype but that
MB×BM
gives exclusively offspring ofmuricatatype. Evidently, apart from all controversy as to the significance of the "mutants" ofLamarckiana, we have here a series of observations of the first importance.
The fact that reciprocal crossings give constantly distinct results must be taken to indicate that the male and female sides of one, if not of both, of the parents are different in respect of characters which they bear. This is de Vries's view, and he concludes rightly, I think, that the evidence from all the experiments shows that bothbiennisandmuricataare in this condition, having one set of characters represented in their pollen-grains and another in their ovules. The plants breed true, but their somatic structures are compounded of the two sets of elements which pass into them from their maternal and paternal sides respectively. This possibility that species may exist of which the males really belong to one form and the females to another, is one which it was evident from the first announcement of the discovery of Mendelian segregation might be found realised in nature.[5]
Oe. biennisandmuricatawere crossed reciprocally with each other and with a number of other species, and the behaviour of each, when used as mother, was consistently different from its behaviour when used as father. De Vries is evidently justifiedby the results of this series of experiments in stating that the "Bild," as he terms it, or composition of the male and female sides of these two species,biennisandmuricata, are distinct. On the evidence before us it is not, however, possible to form a perfectly clear idea of each, and until details are published, a reader without personal knowledge of the material cannot do more than follow the general course of the argument. For fuller comprehension a proper analysis of the characters with a clear statement of how they are distributed among the several types and crosses is absolutely necessary. According to de Vries the female ofbiennispossesses a group of characters which he defines as "conica" in allusion to the shape of the flower-buds. Besides the conical buds, this group of features includes imperfect development of wood, rendering the plant very liable to attacks ofBotrytis, and comparatively narrow leaves.
The female ofmuricatacarries a group of features which he calls "frigida," and, though this is not quite explicitly stated in a definition of that type, it is to be inferred[6]that its characteristics are regarded as greater height, strong development of wood with comparative resistance toBotrytis, and broad leaves.
The characters borne by the male parts of the two species are in general those by which they are outwardly distinguished. For example, the leaves ofOe. biennisare comparatively broad and are bright green, while those ofmuricataare much narrower and of a glaucous green, and I understand that de Vries regards these properties as contributed by the male side in each case and to be carried by the male cells of each species. The suggestion as regardsbiennisandmuricatacomes near the conception often expressed by naturalists in former times (e. g., Linnaeus) and not rarely entertained by breeders at the present day, that the internal structure is contributed by the mother and the external by the father.
On the other hand, the offspring of each species when used as mother is regarded as possessing in the main the features of the maternal "Bild," but the matter is naturally complicated by the introduction of features from the father's side, and it ishere especially that the account provided is at present unsatisfactory and inconclusive. There seems, however, to be no serious doubt thatbiennisandmuricataeach in their outward appearance exhibit on the whole the features which their pollens respectively carry, and that the features borne by their ovules are in many respects distinct.
Thetypesare thus "hybrids" which breed true. The results of intercrossing them each way are again "hybrids" which breed true. It will be remembered that on former occasions de Vries has formulated a general rule thatspecies-hybrids breed true, but that the cross-breds raised by interbreedingvarietiesdo not. One of these very cases was quoted[7]as an illustration of thisprinciple, viz:muricata×biennis. The grounds for this general statement have always appeared to me insufficient, and with the further knowledge which the new evidence provides we are encouraged to hope that when a proper factorial analysis of the types is instituted we shall find that the phenomenon of a constant hybrid will be readily brought into line with the systems of descent already worked out for such cases as that of the Stocks, and others already mentioned.
In further discussion of these facts de Vries makes a suggestion which seems to me improbable. Since the egg-cells ofmuricata, for instance, bear a certain group of features which are missing on the male side, and conversely the pollen bears features absent from the female side, he is inclined to regard thebad pollen grainsas the bearers of the missing elements of the male side and to infer that there must similarly be defective ovules representing the missing elements of the female side. No consideration is adduced in support of this view beyond the simple fact that the characters borne by male and female are dissimilar, whereas it would be more in accord with preconception if the same sets of combinations were represented in each—as in a normal Mendelian case. There is as yet no instance in which the absence of any particular class of gametes has been shown with any plausibility to be due to defective viability, though there are, of course, cases in which certain classes of zygotes do not surviveowing to defective constitution (e. g., the albinos ofAntirrhinumstudied by Baur, and the homozygous yellow mice). I am rather inclined to suppose that in these examples of hybrids breeding true we shall find a state of things comparable with that to which we formerly applied the terms "coupling" and "repulsion." In these cases certain of the possible combinations of factors occur in the gametic series with special frequency, being in excess, while the gametes representing other combinations are comparatively few. In a recent paper on these cases Professor Punnett and I have shown that these curious results vary according to the manner in which the factors are grouped in the parents. IfAandBare two factors which exhibit these phenomena we find that the gametic series of the double heterozygote differs according as the combination is made by crossingAB × ab, or by crossingAB × aB. In a normal Mendelian case the F1form,AaBb, produces gametesAB,Ab,aB,ab, in equal numbers; but in these peculiar cases those gametes which contain