The Life of the Plant

1 A recent discovery of a distinguished English botanist, Professor D. H, Scott, has proved, from the palaeontological point of view as well, the connexion between ferns and the Gymnosperms in which the antherozoids were found — a new triumph for Hofmeister. out of which other groups are formed, was called by Linnaeus a species. Out of species the genus is formed, out of genera families, and so on. E.g. a violet and a heart’ s-ease represent, according to Linnseus, two species of the genus Viola ; two poplars — the black and the white — are two species of the genus Populus ; a donkey and a horse fall, into the same genus Equus, the wolf and the dog into the genus Canis, and so on. The determina- tion of the groups called species marked a great advance in science : it rendered possible the strictly systematic classification of organisms. However, after having established this collective unit of their system, this group the species, the systematists, not so much Linnseus himself as his followers, declared that a species is some- thing actually stable, invariable in space or time ; that species have always been and will ever be what they are at present ; that the transformation of one species into another is out of the question altogether, and hence that the theory of the common origin of all organisms is quite inadmissible. So far we have been discussing data which have been gathered by the theory of metamorphosis, by comparative morphology, embryology, and palaeon- tology, and which testify to the possibility of the transi- tion of forms of one family into those of another (e.g. from Boragineae into Labiatae), the possibility of transition from a spore to a seed-plant, the impossibility of finding any line of demarcation between the vegetable and the animal world, etc.

But what can be the significance of all these facts if it is true that no transition is possible in the case of beings most closely related, in the case of species of the same genus ? If the violet and the heart’s-ease have always been so different from each other, if they are unable to vary, if species are immutable, then certainly all our considerations as to transition from one family to another, from one order to another in the vegetable kingdom, as also from one kindgom to another, are futile. Hence it is clear that the problem

of the common origin of organic beings (and therefore, as we have already seen, of the wider problem still, of the cause of their perfection) is bound up with the problem of the variability, or, speaking more generally, of the origin of species. That is how Darwin’s book, which caused a revolution in science, came to bear such a dry and technical title. Is it really true, as was steadily affirmed by a majority of naturalists, that species are variable ? In the first place we know that no two exactly similar forms exist in nature : plants grown from seeds of one and the same fruit vary one from another. Hence entire similarity is out of the question, and, as a matter of fact, no one has ever suggested it. Moreover, we know perfectly well that even within the limits of a species there may be narrower groups of beings still more closely resembling one another. Thus the species, serving as a unit for groups of a higher order, in its turn breaks up into units of a lower order. Everybody knows how different are the races of dogs, how diverse the kinds of wheat, how numerous the variety of flowers appearing year by year in gardeners’ catalogues. Where is the famous immutability of species ? The exponents of this theory have a ready answer. They say : * Within the limits of species

variation may certainly occur, but the range of these variations is limited ; the degree of difference between the varieties can never be as great as that between species ’ : in other words, new species cannot be formed in the same way as new varieties. Therefore the question as to the immutability of species resolves itself into the question of limitation in variation, or rather into the significance of variations. After such a categorical affirmation of the difference between a species and a variety, those who uphold the immutability of species might be expected to give an exact defini- tion of both terms, and to provide a criterion whereby we may know when we are dealing with two varieties

of a single species, and when with two independent species. Not at all : they not only give nothing of the kind, but neither can they do so, because species as well as varieties defy definition. We shall soon see why, starting with the a priori conviction that one species cannot arise from another, those who maintain their immutability generally follow a rule which says that two forms, connected by transitional forms, however unlike they may themselves be, cannot be acknow- ledged as independent species. Guided by the same rule, however, the opponents of the theory point to species between which transitional forms have been discovered. The others answer : ‘ This means that we have been mistaken, that we have taken for a species something which in the main is a variation.’ Obviously arguing in circles like this keeps up appearances. Affirm- ing that species are invariable, they may easily assign the term variety to every variable form, in the absence of any positive sign which would differentiate a species from a variety. There was a time, however, when those who maintained the immutability of species did cherish the hope of establishing the difference between species and varieties upon some positive physiological feature. A conviction arose that all the representatives of a species, all its variations, however different they may be amongst themselves, can intercross, producing hybrids capable of further reproduction ; and that, on the contrary, species are unable to produce hybrids, or, if they happen to do so, the hybrids are bound to be sterile. It has even been presumed that Nature has of set purpose made species invariable. After having produced a certain number of specific forms Nature has taken the trouble to preserve them for ever invariable, by preventing them from the possibility of varying as the result of their forming hybrids.

When we discussed fertilisation, we stated facts which definitely contradicted this theory. We have seen that fertilisation by means of the pollen of the same or a similar plant is sometimes less produc- tive than that by means of pollen of another different plant, and even that fertilisation with the pollen of another species is sometimes more productive than that with the pollen of the same species. Realising that it is impossible to advance any positive criterion for differ- entiating species from variations, the exponents of this theory love to plead ‘ intuition ’ as a guide to investigators in the solution of this problem. The precarious nature, however, of this famous adjunct intuition is well proved by the following instances. It so happens that as long as a genus contains but few species botanists agree as to their number ; but as soon as a genus contains more than, say, four species they begin to disagree. How far this disagreement may carry them is shown by the following example : according to some botanists the genus Hieracium contains twenty species, according to others three hundred. The same disagreement holds with regard to the blackberry, willows, and many other plants.. Evidently some botanists take for a species what for others is only a variety. In view of these contradictions, systematists have coined the expression good species to differentiate the evident and universally acknowledged species from doubtful ones. These mani- fold contradictions unmistakably bring us to the con- clusion that it is impossible to draw a hard and fast line between a species and a variety, that it is impossible always and infallibly to apply to reality conceptions of this kind. This inference necessarily raises the question whether some logical fallacy has _not slipped into the argument, as was the case when we discussed the difference between plants and animals. Perhaps neither species nor variety exist in Nature as two qualitatively different categories.

Perhaps they are but typical conceptions, creations of our own mind. Let us try to make this clear by an example. We clearly realise the difference between a child and a grown-up man. Moreover, we differentiate between babies and adults, youths, mature and old men ; and these conceptions are entirely in accordance with reality, otherwise they would not have arisen in our mind. It does not, how- ever, follow that they must be applied without exception to all cases. Nobody would ever think of affirming that questions must or can be decided, in any and every case, such as : Have we before us an adult or a youth, a grown-up person or an old man ? and so on ; and yet it is with such problems that systematists struggle when they have to decide whether a doubtful species is to be considered a species or a variety. Species and variations are clearly differentiated in most cases ; but it does not follow that they should be two categories essentially different ; on the contrary, the difference between them is entirely one of quantity. They are two quantities passing gradually into one another : at one extreme we have slight individual variations, succeeded by sub-varieties, then obvious varieties, doubtful species, and, lastly, good species . 1 In a word, the only logical solution to this problem of species and varieties, so full of contradictions, consists in the acceptance of Darwin’s formulae : ‘A variety is an in- cipient species ’ ; ‘ a species is a strongly-marked variety ’ : just as a child is an undeveloped man, and a grown-up man is a developed child, nor can any line of demarcation be drawn where the child ends and the man begins. Let us carry our comparison further. Supposing that a being of some kind with a very short period of existence (a may- fly, for instance) were to raise the question whether a grown-up man develops from a child, or whether they are both quite independent beings. It would be impossible for our imaginary being to see this trans- formation because of the shortness of its own existence ;

1 In fact, while systematists were able formerly to end their classifica- tion with species, to-day four subdivisions more are admitted within the limits of the species. yet by observing a whole series of slightly varying transitional forms between a suckling and an old man, by observing that all these beings vary before his eyes though to a very small degree, and vary in one direction only, i.e. become older, our imaginary being would come to the conclusion that the child he sees before him will in time become an old man, and that just as truly the old man has also been a child in his time. Let us also suppose that another similar imaginary being should criticise this conclusion by saying : * I maintain that a grown-up man has never been a child, and will maintain this statement until I see the transformation take place before my very eyes, which, as a matter of fact, could never happen.’ Tell me, pray, who is in the right ? Is it the one who reduces the whole of his experience to terms of a strictly logical inference, or is it the other who obstinately indulges in a kind of pseudo-philosophical scepticism, repudiating both the testimony of experience and the requirements of logic ? Yet this is exactly the position of the two opposite camps with regard to the question of species. Not only the life of a single man, but even many generations are as nothing when compared with the period of time necessary for the formation of a new species ; yet scientists who repudiate the immutability of species, seeing the variability of organic beings and taking into con- sideration the impossibility of establishing a difference between a species and a variety, inevitably come to the conclusion that species have arisen from varieties ; that varieties are only consecutive steps towards the for- mation of new species.

However conclusive this method of proof may be, there is no doubt that the actual observation of the formation of new species would have been more conclusive still. If species do vary, may they not within the confines of history have varied so much as to give rise to new species ? This cannot possibly be proved with regard to organisms in their natural conditions, since we cannot keep them under observation for a sufficiently long period of time. The solution of this problem, though still presenting great difficulties, is becoming a little simpler in relation to cultivated plants and domesticated animals, concerning which his- torical data do exist. Our greatest difficulty is to prove that different races of animals or plants, sharply differen- tiated one from the other, do actually descend from one and the same species. Darwin managed to prove it fully in some cases, particularly with regard to the breeds of pigeons. He proved that contemporary breeds vary so much that had they been discovered in their natural environment they would not only have been classified as different species but even in different genera, and yet they did undoubtedly arise from one and the same pigeon.

Those who maintain the hypothesis of the immuta- bility of species have one more argument in reserve, and also a very important one. They say : Granted that species descend one from another, how can we account for the absence of all the minute transitional forms which must have existed ? Why have they disappeared ? Why is it that species generally represent groups of beings entirely distinct from each other ? In passing let us remember that the very fact of the absence of transi- tional forms between species is in many cases doubtful, because, as we have already seen, whenever such transition is manifested between two species, the exponents of the immutability of species deny them specific rank. However, this argument holds good in the great majority of cases : really good species, as a matter of fact, are not connected with each other by transitional forms, and this fact was the main obstacle to all the early endeavours to prove the origin of species by means of variation ; they all collapsed in face of this crushing argument. As we proceed to study

the very essentials of the conception of the origin of species by variation, we shall see in what relation this argument stands to Darwin’s theory. At present we need only note the fact that this theory has employed in its own defence the very weapon of its adversaries ; it entirely accounts for the absence of transitional forms. Indeed their very existence would have served as a strong argument against it. This is one of its essential points, the reason for its ascendency over other attempts of the same kind.

So far we have brought forward arguments and done our best to refute those who have contradicted the fact that the organic world has a history. We have tried to prove by means of concordant evidence from all depart- ments of biology that the affinity of organisms, admitted by all naturalists without exception, can only be explained by their close relationship. Hence organisms have a genealogy, a history. Turning to the second half of our problem, we must now prove that this historical process necessarily leads to the perfection of organisms, implying by perfection the adaptation of the organ to its function, of the organism to its environment. Observ- ing that the organic world presents beings in all stages from the simplest to the more perfect, and realising that this perfection corresponds with the chronological order in the appearance of these beings upon the earth, many naturalists have even seen in this fact a proof that the organic world is endowed with a tendency to- wards perfection, and that this property needs no further explanation. Others again have tried to give a more rational explanation of the fact, though with ill success for the most part. Darwin was the first to point out the immediate causes, very general laws of Nature, which result in the progressive development, the evolution, of the organic world. He used for this purpose

a method which appears paradoxical at first sight, and the logical significance of which is even yet misunder- stood ; or, rather, will never be understood by many of its opponents, whatever explanations may be offered for it. In order to find out how it is that by means of historical development the organic world has reached the degree of perfection we observe in it, Darwin started by inquiring how man reaches the same end, how he improves his artificial breeds of plants and animals — and he came to the conclusion that the main factor in the accomplishment of this end is selection, consisting, as we have already seen , 1 in the selection from every generation of only those organisms which correspond most closely to the ends in view. In its simplest and most perfect form selection consists in the extermination of all unfit individuals. For instance, when a gardener wishes to produce or to preserve a certain variety in a plant, he simply exterminates all the plants which do not correspond to his ideal.

Darwin next raises .another question : Does not Nature also advance towards perfection by means of a similar selection ? One has scarcely time to word this question before the opponents of the theory raise their voices in premature triumph and make exclamations such as, ‘ Can there be anything in common between a process directed by the rational will of man and the action of the blind forces of Nature ? You undertake to explain the origin of organic forms by physical laws, and yet you start by personifying Nature, by endowing it with rational activity, with a capacity for selection.’ Unmoved by exclamations of that kind, which are mere words, let us study facts in order to understand the great man’s idea. First of all Darwin dwells on cases of what he calls unconscious selection. In years of famine savages are obliged to kill some of their domestic animals. As a matter of course they preserve the best,

hence quite unconsciously improve the race. They do this, indeed, against their will, for had they the choice they would quite willingly preserve even the less satisfactory ones. By selecting individual animals they improve the race in course of time, and yet in respect of the result attained they work as a blind, unconscious elementary force. Can we then admit unconscious selection in Nature ? In order to make the question less startling let us put it a little differently, and look at it in another light* We have seen that in its simplest form the process of selection reduces itself to the extermination of un- satisfactory forms. Therefore we can substitute this question by another : Do the unsatisfactory forms become exterminated in Nature ? Such a process of extermination would be equivalent to improvement, a perfecting process. Science answers this question with a most decided affirmative : yes, it says, they do become exterminated in Nature to a very large extent, and with inexorable strictness. This phenomenon is based upon a property common to all organic beings. This property consists in the fact that in the reproduction of organisms is always involved their multiplication. This fact is so universal, so constant, that the two expressions are often interchangeable, multiplication being used as a synonym for reproduction. In fact we caftnot name a single organism which would normally produce only one single being during its life-time. On the contrary multiplication generally takes place in ever-increasing geometrical progression. This fact has very important consequences, the significance of which was first indicated by Darwin. We shall only fully realise the rapidity with which organic beings reproduce themselves if we take the trouble to calculate the whole posterity of a single organism in a given number of years. Thus, for instance, if all the posterity of a single dandelion were preserved during ten or twelve

years it would cover all the terra firma of our globe. Yet the dandelion is not particularly productive. According to Darwin our commonest orchid, the Spotted Orchis, produces no fewer than 180,000 seeds a year, so that even the grandchildren of a single plant would cover the earth with a close green carpet. Nor is this the limit to productiveness. There are orchids the seeds of which are counted by millions. Let us, too, recall the spores — the invisible grains of dust — formed on the under side of fern leaves ; each of these is able to produce a new plant.

What is the natural result of this enormous multiplica- tion of organisms without exception, this tendency of every one of them to occupy the whole earth ? It is obvious : the majority of these organisms perish. We may even say that the proportion which survives is insignificant as compared with that doomed to perish. A hard struggle sets in for the representatives of eveiy new generation, issuing in the arrival of a very small number of victors. What determines the survival of these selected forms ? What circumstances decide the result of the competition in their favour ? Obviously their own superiority, the perfection of their organisa- tion — implying by perfection, as has been already said, the adaptation of the organ to its function, of the organ- ism to its environment. In the majority of cases we are not even able to realise wherein that superior adaptation lies, because the advantage in the struggle for existence may depend on a variety of properties, sometimes even the opposite of one another. In one case the survival of the plant is due to the fact that it germinated before its fellows, appeared before them at the banquet of life, and had time to seize a place in the universe ; in another case, on the contrary, the selected, i.e. the surviving, plant will be the one which has germinated later than its fellows, and thus has happened to be preserved from late frosts which kill its too hasty rivals. The struggle

for existence and its necessary outcome, the survival of the fittest, or, as Darwin metaphorically puts it, natural selection , is the necessary logical outcome of the law of the rapid multiplication of organic forms. 1 Such deductive proofs are not the only ones that can be brought forward in support of the fact of the struggle for existence and selection ; immediate observation brings us to the same inference. We have only to take a mixture of flower seeds, say of sweet peas of different colour, and gather all the seeds each year and sow them again on the same bed, to find in a few years' time that some colours will oust the others from the bed. This means that even such an insignificant character as colour (most likely some property correlated with it, which escapes our immediate observation) can decide the victory in the struggle for existence. The same result is observed in experiments in manuring natural meadows. We have seen that nitrogenous manures and mineral salts containing phosphoric acid and potassium constitute beyond doubt useful and indispensable food for every plant. But if we manure a natural meadow containing a certain percentage of grasses and a certain percentage of leguminous plants, we notice that when we use exclusively nitrogenous manures the cereals get the upper hand of the leguminous plants. On the other hand, by using manures without nitrogen the advantage is all on the side of the leguminous plants. Both manures are useful for both kinds of plants, although to a different degree, and according to that difference the success in the struggle for existence falls to the one or to the other. Lastly, as has been rightly observed,

1 The metaphorical use of the word selection has led many critics astray (as has been already mentioned) : they said that the very expression * selection * points to the fact that Darwin was obliged to ascribe conscious activity to Nature. If formerly it was only slow people who could be led into this error, to-day, after the explanation given by Darwin, it is only people unscrupulous in their choice of methods of argument who can have recourse to such quibbling.

we have but to recall the pains taken by a farmer to save his fields from being overrun with weeds, to realise the struggle our cultivated plants have to maintain, and how easily they would perish if left to themselves. Hence the fact of the struggle for existence, as the out- come of the law of geometrical progression in the multiplication of organic beings, obvious as any mathe- matical truth, is proved by direct experiment. This struggle in the same logically inevitable way leads to natural selection, i.e. to more and more perfect adapta- tion, although it may be liable to escape our notice in any single generation. If we take the testimony of geology into account as to the almost immeasurable space of time that has elapsed since organisms appeared upon the earth, we shall readily agree that the process of selection, acting with such inexorable severity and during such a lengthy period of time, can fully account for both the variety of organic forms and the perfection of their adaptation.

Thus the evolution of organic forms and their infallible trend towards perfection may be considered as a neces- sary logical outcome of the three fundamental properties of organisms. These are the capacity for variation, the capacity for transmitting variations to posterity, i.e. heredity, and the capacity for multiplication, which is invariably connected with reproduction. The capacity of organisms for variation is indisput- able. We do not know of any two absolutely similar beings. Yet the causes of variation, and the relation which variation bears to selection, need some further explanation. The primary cause which produces changes in an organism must lie in the indirect or immediate influence of their external conditions ; and then comes the influence of secondary causes, such as correlation in the development of parts, the exercise of organs, and so on. In most cases it is, however, very difficult to discover the connexion between a variation

and its cause. We then call the variation accidental, though science cannot admit accidental phenomena in the literal sense of the word. We call it accidental in so far as its primary cause is concealed from us. The difficulties we meet in trying to unravel the con- nexion between a variation and its cause depend mainly upon two circumstances : in the first place it is already rather late to look for a cause when the variation has already manifested itself: secondly, external influences very seldom produce a lasting effect upon a fully developed organism ; this probably happens more frequently in the case of organisms in the embryonic stage of their development, for it is obvious that the earlier the influence is exerted the more far-reaching its consequences. The influence of any disturbing factor is generally very strong in the earliest stages of develop- ment. This can be proved, for instance, by the fact that it is impossible to propagate some sports otherwise than by asexual reproduction, because the influence of the second parent in the process of sexual reproduction is strong enough to affect the whole organism to such an extent as to interfere with the transmission of the desired character. One of the secondary causes of variation must be looked for in the action of a law called the correlation of growth, which consists for instance in the excessive development of one part being accompanied by the under-development of another. Having at a given time only a certain quantity of nutrient substances to dispose of, Nature, as Goethe says, 1 in order to be lavish in one direction is forced to economise in another.’ Lastly, once formed, the organ is apparently able to go on developing precisely in so far as it is being used. 1

Variations when occurring under the influence of physical conditions may equally well be useful or harm- 1 It has not yet been explained what variations obtained as the result of exercise are inherited, and which are not. ful to the organism ; it is only the struggle for existence and selection which control variation in a definite way by destroying harmful deviations and preserving useful ones ; so that scarcely perceptible variations, when accumulated during a long series of generations, in the end are considerably accentuated. Let us try to explain by means of an illustration what part of the phenomenon may belong to variation as such, and what part to the subsequent action of selection. A while back we tried to explain the course by which a symmetrical flower is derived from a regular one. We pointed out a whole series of transitional forms, but of course this does not explain the primary origin of bilateral symmetry, the primary deviation from regularity. We may conjecture with a certain degree of probability that this transfor- mation has taken place under the influence of the force of gravity acting upon the branches as they developed. We have seen 1 that growing organs change the direction of their growth under the influence of the force of gravity, and this change of direction depends upon irregularity of growth in the upper and lower parts of an organ. The same effect is also manifested in another way : horizontal branches become unlike on their upper and lower sides ; the leaves, uniformly distributed on all sides of the erect main stem, in horizontal branches distribute themselves in one horizontal plane, and so on.

A considerable number of similar facts concerning flowers apparently justify this opinion. Flowers of one and the same plant have been observed to be regular or slightly symmetrical, according to the place they occupy on the flower axis. Thus, for instance, plants with regular flowers, having lateral flowers either in an almost horizontal or in a drooping position, often acquire a slightly bilateral form ; while the apical flowers of the same cluster, or erect flowers, preserve an

entirely regular form. This phenomenon is observed in campanulas and other plants. On the other hand, in plants with entirely symmetrical flowers, such as those of the Labiateae, Orchidaceae, and other families, the apical flower often acquires a regular form. This is the case in the Sage. All the lateral flowers have a characteristic two-lipped form, while the apical flower is sometimes quite regular. Recent experiments have adduced direct evidence in support of this suggestion . 1 On removing the effect of the earth’s attraction by means described in chapter vii., it was found possible to transform bilaterally symmetrical flowers artificially into regular, radially symmetrical flowers. Thus the first appearance of longitudinal symmetry in a flower may be ascribed to the action of the force of gravity, the rest will follow as the effect of selection. Since it is beyond doubt on the one hand that cross-fertilisation effected by insects is useful to plants and gives rise to a more vigorous and healthy generation, and on the other hand that for insects, which visit the flowers for their honey, the lower lip of the flower presents a convenient platform, it is clear that in every generation amid plants struggling for existence those which possess this bilabiate form more sharply differentiated have more chance of survival. Similarly, partly owing to the force of gravity and partly on the strength of the law we have just mentioned of correlation of growth, first one, then all the three upper stamens become atrophied, while the two lower ones increase in size, and under the influence of selection develop their peculiar shape, which is that most useful to the plant.

We gather from this example that in order to explain the origin of a given form, be it even a very complicated one, we have but to settle the following three points : that the original variation might have arisen under the influence of physical forces (acting upon the organism generally in its embryonic stage, and only seldom when it is fully developed), that there exists a series of transitional forms, and — the most important of the three — that this transformation is useful to the organism itself. Then it becomes obvious that under the influence of natural selection such a form not only might, but necessarily must, have arisen.

Hence the explanation of the harmony, of the per- fection of the organic world, suggested by Darwin does not turn out ' to involve any a priori endow- ment of the plant with a tendency towards perfec- tion, with any inborn progressive activity. On the contrary, according to that theory, variations as such may equally well be useful or harmful. But owing to selection every harmful variation, precisely because of its harmfulness, is doomed to extermination sooner or later, whereas every useful adaptation is transmitted to future generations. The general pro- gression, the drift towards perfection, is effected by exterminating everything that is harmful, and by accumulating slowly and gradually useful properties. Thus the perfection of the organic world no longer appears in itself as an incomprehensible end, but as an eminently conceivable result of authentic natural causes well known to everybody.

Curiously enough another great thinker came to a similar conclusion before Darwin ; with the difference, however, that according to the current ideas of his time he could not admit the theory of the mutability of species. Auguste Comte writes as follows in the third volume of his Positive Philosophy : ‘ Without doubt every organ- ism finds itself necessarily related to a certain combina- tion of external conditions. It does not at all follow, however, that the former of these two correlated forces has been produced by, any more than it could have itself produced, the other. We have simply to deal with two forces in a state of equilibrium, totally independent of each other as also essentially different. If we imagine

all sorts of organisms to be consecutively exposed to the action of all sorts of external conditions in the course of a sufficiently long lapse of time, we shall clearly see that the great majority of these organisms ought necessarily to disappear ; only those that satisfy the fundamental law of equilibrium mentioned above should survive. In all probability such a system of elimination has established the biological harmony, which we observe on our planet, and which goes on changing before our eyes.’ The similarity of both opinions consists in that biological harmony, for both Comte and Darwin, is the result of the elimination of all that is inharmonious, and inconsistent with the funda- mental law of equilibrium between the organism and its environment. Comte does not indicate the mode of this elimination of unsatisfactory organisms, nor the reason of its inevitable and fatal necessity ; and besides, for him, adherent as he was of the immutability of species, this harmony ought to have appeared as something stable, something that had already attained its end ; while for Darwin, the advocate of the unlimited variation of organic forms, this equilibrium is unstable, a harmony ever progressing and never reaching its end. Now, if this harmony is unstable it cannot be absolute ; and this is altogether consistent with reality. We never meet with absolute perfection in Nature. The eye is rightly considered the most perfect of organs, and yet it is of the eye that Helmholtz, the greatest authority in his subject, and at the same time an enemy of all idle talk, said that had he received from an optician an apparatus with similar defects he would have sent it back to be repaired !

We see, therefore, that Darwin’s theory explains the reason of the perfection of organisms by starting from fundamental properties of bodies well known to every one, and without having recourse to arbitrary premises. Upon this rests its superiority to all former attempts of the kind. Its other great advantage consists in turning to its own account one of the strongest objections which former supporters of the theory of the variability of organic forms were unable to over- come. This objection consists in the absence of transi- tional forms between really good species. In fact, if species are related to each other there must exist links between them, some transitional forms. To this Dar- win’s theory says : such forms must indeed have existed, but they have disappeared in course of time, and their disappearance is one of the necessary results of the struggle for existence and of selection. Before entering into this question let us observe that an entirely false representation is very often made of the transitional form which connects two other forms. Such a form is generally considered to be the middle form in the literal sense of the word, a form which contains in itself the attributes of both the forms which it links together ; whereas in reality it may be almost entirely lacking in the characteristic attributes of either of them. Very often objections of the following kind are raised: if the birch and the oak are related to each other show us an organism that would be half one and half the other. In all probability such an organism has never existed in Nature. Organisms that exist to-day are related to each other not because they may have originated the one from the other, but because they proceed from common ancestors ; and very probably had we found the real link between two contemporary forms, i.e. the form of the ancestor upon which their relationship is founded, we should not have recognised it as such, because it would have presented in very slight degree, if indeed at all, the characteristic attributes of its two descendants. Let us take an example from among cultivated plants.