§ 272. The motionless protococcoid forms of lower Algæ, which do not permanently expose any parts of their surfaces to actions unlike those which other parts are exposed to, have no parts of their surfaces unlike the rest in function and composition. This is what the hypothesis prepares us for. If physiological differentiations are determined by differences in the incidence of forces, then there will be no such differentiations where there are no such differences. Contrariwise, it is to be expected that the most conspicuous unlikeness of function and minute structure will arise between the most-dissimilarly circumstanced parts of the surface. We find that they do. The upper end and the lower end, or, more strictly speaking, the free end and the attached end, habitually present the strongest physiological contrasts.

Even aggregates of the first order illustrate this truth. Such so-called unicellular plants as those delineated in Figs. 4, 5, and 6, show us, on comparing the contents of their fixed ends and their loose ends, that different processes are going on in them, and that different functions are being performed by their limiting membranes. Caulerpa prolifera, which “consists of a little creeping stem with roots below and leaves above,” originating “in the growth of a body which may be regarded as an individual cell,” supplies a still-better example. Among aggregates of the second order a like connexion is displayed in more various modes but with equal consistency. As before, the Puff-ball served to exemplify the primary physiological differentiation of outer parts from inner parts; so, here, it supplies a simple illustration of the way in which the differentiated outer part is re-differentiated, in correspondence with the chief contrast in its relations to the environment. The only marked unlikeness which the cortical layer of the Puff-ball presents, is that between the portion next the ground and the opposite portion. The better-developed Fungi exhibit a more decided heterogeneity of parallel kind. Such incrusting Algæ as Ralfsia verrucosa furnish a kindred contrast; and in the higher Algæ it is uniformly repeated. Phænogams display this physiological differentiation very conspicuously. That earth and air are unlike portions of the environment, subjecting roots and leaves to unlike physical forces, which entail on them unlike reactions, and that the unlike functions and structures of their respective surfaces are fitted to these unlike physical forces, are familiar facts which it would be needless here to name, were it not that they must be counted as coming within a wider group of facts.

Is this unlikeness between the outer tissues of the attached ends and those of the free ends in plants, determined by their converse with the unlike parts of the environment? That they result from an equilibration partly arising in the individual and partly arising by the survival of individuals in which it has been carried furthest, is inferable à priori; and this à priori argument may be adequately enforced by arguments of the inductive order. A few typical ones must here suffice. The gemmules of the Marchantia are little disc-shaped masses of cells composed of two or more layers. Their sides being alike, there is nothing to determine which side falls lowermost when one of them is detached. Whichever side falls lowermost, however, presently begins to send out rootlets, while the uppermost side begins to assume those characters which distinguish the face of the frond. When this differentiation has commenced, the tendency to its complete establishment becomes more and more decided; as is proved by the fact that if the positions of the surfaces be altered, the gemmule bends itself so as to re-adjust them: the change towards equilibrium with environing forces having been once set up, there is acquired, as it were, an increasing momentum which resists any counter-change. But the evidence shows that at the outset, the relations to earth and air alone determine the differentiation of the under surface from the upper. The experiences of the gardener, multiplying his plants by cuttings and layers, constitute another class of evidences not to be omitted: they are commonplace but instructive examples of physiological differentiation. While circumstanced as it usually is, the meristematic tissue of each branch in a Phænogam continues to perform its ordinary function—regularly producing on its outer side the cortical substances, and on its inner side the vascular and woody tissues. But change the conditions to those which the underground part of the plant is exposed to, and there begins another differentiation resulting in underground structures. Contact with water often suffices alone to produce this result, as in the branches of some trees when they droop into a pool, or as occasionally with a cutting placed in a bottle of water; and when the light is excluded by imbedding the end of the cutting, or the middle of the still-attached branch, in the earth, this production of tissues adapted to the function of absorbing moisture and mineral constituents proceeds still more readily. With such cases may be grouped those in which this development of underground organs by an above-ground tissue, is not exceptional but habitual. Creeping plants furnish good illustrations. From the shoots of the Ground-Ivy, rootlets are put out into the soil in a manner differing but little from that in which they are put out by an imbedded layer; save that the process follows naturally-induced conditions instead of following artificially-induced conditions. But in the common Ivy which, instead of running along the surface of the earth, runs up inclined or vertical surfaces, we see the process interestingly modified without being essentially changed. The rootlets, here differentiated by their conditions into organs of attachment much more than organs of absorption, still develop on that side of the shoot next the supporting surface, and do not develop where the shoot, growing away from the tree or wall, is surrounded equally on all sides by light and air: thus showing, undeniably, that the production of the rootlets is determined by the differential incidence of forces. Though survival of the fittest doubtless furthered this transition yet it clearly did not initiate it. That greenness which may be observed in these Ivy-branch rootlets while they are quite young, soft, and unshaded, introduces us to facts which are the converse of the foregoing facts; and proves that the parts ordinarily imbedded in the soil and adapted to its actions, acquire, often in very marked degrees, the superficial structures of the aërial parts, when they are exposed to light and air. This may be witnessed in Maize, which, when luxuriant, sends out from its nodes near the ground, clusters of roots that are thick, succulent, and of the same colour as the leaves. Examples more familiar to us in England occur in every field of turnips. On noting how green is the uncovered part of a turnip-root, and how manifestly the area over which the greenness extends varies with the area exposed to light, as well as with the degree of the exposure, it will be seen that beyond question, root-tissue assumes to a considerable extent the appearances and function of leaf tissue, when subject to the same agencies. Let us not forget, too, that where exposed roots do not approach in superficial character towards leaves, they approach in superficial character towards stems: becoming clothed with a thick, fissured bark, like that of the trunk and branches. But the most conclusive evidence is furnished by the actual substitutions of surface-structures and functions, that occur in aërial organs which have taken to growing permanently under ground, and in underground organs which have taken to growing permanently in the air. On the one hand, there is the rhizome exemplified by Ginger—a stem which, instead of shooting up vertically, runs horizontally below the surface of the soil, and assumes the character of a root, alike in colour, texture, and production of rootlets; and there is that kind of swollen underground axis, bearing axillary buds, which the Potato exemplifies—a structure which, though homologically an axis, simulates a tuberous root in surface-character, and when exposed to the air, manifests no greater readiness to develop chlorophyll than a tuberous root does. On the other hand, there are the aërial roots of certain Orchids which, habitually green at their tips, continue green throughout their whole lengths when kept moist; which have become leaf-like not only by this development of chlorophyll, but also by the acquirement of stomata; and which do not bury themselves in the soil when they have the opportunity.[46] Thus we have aërial organs so completely changed to fit underground actions, that they will not resume aërial functions; and underground organs so completely changed to fit aërial actions, that they will not resume underground functions.

That the physiological differentiation between the part of a plant’s surface which is exposed to light and air and the part which is exposed to darkness and moisture and solid matter, is primarily due to the unlike actions of these unlike parts of the environment, is, then, clearly implied by observed facts—more clearly, indeed, than was to be expected. Considering how strong must be the inherited tendency of a plant to assume those special characters, physiological as well as morphological, which have resulted from an enormous accumulation of antecedent actions, it may be even thought surprising that this tendency can be counteracted to so great an extent by changed conditions. Such a degree of modifiability becomes comprehensible only when we remember how little a plant’s functions are integrated, and how much, therefore, the functions going on in each part may be altered without having to overcome the momentum of the functions throughout the whole plant. But this modifiability being as great as it is, we can have no difficulty in understanding how, by the cumulative aid of natural selection, this primary differentiation of the surface in plants has become what we see it.

§ 273. We will leave now these contrasts between the free surfaces of plants and their attached or imbedded surfaces, and turn our attention to the secondary contrasts existing between different parts of their free surfaces. Were a full statement of the evidence practicable, it would be proper here to dwell on that which is furnished by the inferior classes. It might be pointed out in detail that where, as among the Algæ, the free surfaces are not dissimilarly conditioned, there is no systematic differentiation of them—that the frond of an Ulva, the ribbon-shaped divisions of a Laminaria, and the dichotomous expansions of the Fuci which clothe the rocks between tide-marks, are alike on both sides; because, swayed about in all directions as they are by the waves and tides, their sides are equally affected. Conversely, from the Fungi might be drawn abundant proof that even among Thallophytes, unlikenesses arise between different parts of the free surfaces when their circumstances are unlike. In such laterally-growing kinds as are shown in Fig. 196b, the honey-combed under surface and the smooth leathery upper surface, have their contrasts related to contrasted conditions; and in the adjacently-figured Agarics, and other stalked genera, the pileus exhibits a parallel difference, explicable in a parallel way. But passing over Cryptogams it must suffice if we examine more at length these traits as they are displayed by Phænogams. Let us first note the dissimilarities between the outer tissues of stems and leaves.

That these dissimilarities arose by degrees, as fast as the units of which the phænogamic axis is composed became integrated, is a conclusion in harmony with the truth that in every shoot of every plant, they are at first slight and become gradually marked. Already, in briefly tracing the contrasts between the outer and inner tissues of plants, some facts have been named showing, by implication, how the cessation of the leaf-function in axes is due to that change of conditions entailed by the discharge of other functions. Here we have to consider more closely facts of this class, together with others immediately to the point. On pulling off from a stem of grass the successive sheaths of its leaves, the more-inclosed parts of which are of a fainter green than the outer parts, it will be found that the tubular axis eventually reached is of a still fainter green; but when the axis eventually shoots up into a flowering stem, its exposed part acquires as bright a green as the leaves. In other Monocotyledons, the leaf-sheaths of which are successively burst and exfoliated by the swelling axis, it may be observed that where the dead sheaths do not much obstruct the light and air, the surface of the axis underneath is full of chlorophyll. Dendrobium is an example. But when the dead sheaths accumulate into an opaque envelope, the chlorophyll is absent, and also, we may infer, the function which its presence habitually implies. Carrying with us this evidence, we shall recognize a like relation in Dicotyledons. While its outer layer remains tolerably transparent, an exogenous stem or branch continues to show, by the formation of chlorophyll, that it shares in the duties of the leaves; but in proportion as a bark which the light cannot penetrate is produced by the adherent flakes of dead skin, or by the actual deposit of a protective substance, the differentiation of duties becomes more decided. Cactuses and Euphorbias supply us with converse facts having the same implication. The succulent axes so strangely combined in these plants, maintain for a long time the translucency of their outer layers and their greenness; and they so efficiently perform the offices of leaves that leaves are not produced. In some cases, axes that are not succulent participate largely in the leaf-function, or entirely usurp it—still, however, by fulfilling the same essential conditions. Occasionally, as in Statice brassicæfolia, stems become fringed; and the fringes they bear assume, along with the thinness of leaves, their darker green and general aspect. In the genus Ruscus, the flattened axis simulates so closely the leaf-structure, that were it not for the flower borne on its mid-rib, or edge, its axial nature would hardly be suspected. And let us not omit to note that where axes usurp the characters of leaves, in their attitudes as well as in their shapes and thicknesses, there are contrasts between their under and upper surfaces, answering to the contrasts between the relations of these surfaces to the light. Of this Ruscus androgynus furnishes a striking example. In it the difference which the unaided eye perceives is much less conspicuous than that disclosed by the microscope; for I find that while the face of the pseudo-leaf has no stomata, the back is abundantly supplied with them. One more illustration must be added. Equally for the morphological and physiological truths which it enforces, the Mühlenbeckia platyclada is one of the most instructive of plants. In it the simulation of forms and usurpation of functions, are carried out in a much more marvellous way than among the Cactaceæ. Imagine a growth resembling in outline a very long willow-leaf, but without a mid-rib, and having its two surfaces alike. Imagine that across this thin, green, semi-transparent structure, there are from ten to thirty divisions, which prove to be the successive nodes of an axis. Imagine that along the edges of this leaf-shaped aggregate of internodes, there arise axillary buds, some of which unfold into flowers, and others of which shoot up vertically into growths like the one which bears them. Imagine a whole plant thus seemingly composed of jointed willow-leaves growing from one another’s edges, and some conception will be formed of the Mühlenbeckia. The two facts which have meaning for us here are—first, that the performance of leaf-functions by these axes goes along with the assumption of a leaf-like translucency; and, second, that these flattened axes, retaining their upright attitudes, and therefore keeping their two faces similarly conditioned, have these two faces alike in colour and texture.

That physiological differentiation of the surface which arises in Phænogams between axial organs and foliar organs, is thus traceable with tolerable clearness to those differences between their conditions which integration has entailed—partly in the way above described and partly in other ways still to be named. By its relative position, as being shaded by the leaves, the axis is less-favourably circumstanced for performing those assimilative actions effected by the aid of light. Further, that relatively-small ratio of surface to mass in the axis, which is necessitated by its functions as a support and a channel for circulation, prevents it from taking in, with the same facility as the leaves, those surrounding gases from which matter is to be assimilated. Both these special causes, however, in common with that previously assigned, fall within the general cause. And in the fact that where the differential conditions do not exist, the physiological differentiation does not arise, or is obliterated, we have clear proof that it is determined by unlikenesses in the relations of the parts to the environment.

§ 274. From this most general contrast between aërial surface-tissues—those of axes and those of folia—we pass now to the more special contrasts of like kind existing in folia themselves. Leaves present us with superficial differentiations of structure and function; and we have to consider the relations between these and the environing forces.

Over the whole surface of every phænogamic leaf, as over the fronds of the Pteridophyta, there extends a simple or compound epidermal layer, formed of cells that are closely united at their edges and devoid (in the Flowering Plants) of that granular colouring matter (chlorophyll) contained in the layers of cells they inclose: the result being that the membrane formed of them is comparatively transparent. On the submerged leaves of aquatic Phænogams, this outer layer is thin, delicate, and permeable by water; but on leaves exposed to the air, and especially on their upper surfaces, is comparatively strong, dense, often smooth and impermeable by water: being thus fitted to prevent the rapid escape of the contained juices by evaporation. Similarly, while the leaves of terrestrial plants which live in temperate climates, usually have comparatively thin coats thus composed, in climates that are both hot and dry, leaves are commonly clothed with a very thick cuticle. Nor is this all. The outside of an aërial leaf differs from that of a submerged leaf by containing a deposit of waxy substance. Whether this be exuded by the exposed surfaces of the cells, as some contend, or whether it is deposited within the cells, as thought by others, matters not in so far as the general result is concerned. In either case a waterproof coating is formed at the outermost sides of these outermost cells; and in many cases produces that polish by which the upper surface of the leaf is more or less distinguished from the under surface. This external pellicle presents us with another contrast of allied meaning. On the upper surfaces of leaves subject to the direct action of the sun’s rays, there are either few or none of those minute openings, or stomata, through which gases can enter or escape; but on the under surfaces these stomata are abundant: a distribution which, while permitting free absorption of the needful carbonic acid, puts a check on the exit of watery vapour. Two general exceptions to this arrangement may be noted. Leaves that float on the water have all their stomata on their upper sides, and leaves that are submerged have no stomata—modifications obviously appropriate to the conditions. What is to be said respecting the genesis of these differentiations? For the last there seems no direct cause: its cause must be indirect. The unlike actions to which the upper and under surfaces of leaves are subject, have no apparent tendency to produce unlikeness in the number of their breathing holes. Here the natural selection of spontaneous variations furnishes the only feasible explanation. For the first, however, there is a possible cause in the immediate actions of incident forces, which survival of the fittest continually furthers.

The fluid exhaling through the walls of the cells next the air, will be likely to leave behind suspended substances on their outer surfaces. On remembering the pellicle which is apt to form on thick solutions or emulsions as they dry, and how this pellicle as it grows retards the further drying, it will be perceived that the deposit of waxy matter next to the outer surfaces of the cuticular cells in leaves, is not improbably initiated by the evaporation which it eventually checks. Should it be so, there results a very simple case of equilibration. Where the loss of water is too great, this waxy pellicle left behind by the escaping water will protect most those individuals of the species in which it is thickest or densest; and by inheritance and continual survival of the fittest, there will be established in the species that thickness of the layer which brings the evaporation to a balance with the supply of water.

Another superficial differentiation, still more familiar, has to be noted. Every child soon learns to distinguish by its colour the upper side of a leaf from its under side, if the leaf is one that has grown in such way as to establish the relations of upper and under. The upper surfaces of leaves are habitually of a deeper green than the under. Microscopic examination shows that this deeper green results from the closer clustering of those parenchyma-cells full of chlorophyll that are in some way concerned with the assimilative actions; while beneath them are more numerous intercellular passages communicating with those openings or stomata through which is absorbed the needful air. Now when it is remembered that the formation of chlorophyll is clearly traceable to the action of light—when it is remembered that leaves are pale where they are much shaded and colourless when developed in the dark, as in the heart of a Cabbage—when it is remembered that succulent axes and petioles, like those of Sea-kale and Celery, remain white while the light is kept from them and become green when exposed; it cannot be questioned that this greater production of chlorophyll next to the upper surface of a leaf, is directly consequent on the greater amount of light received. Here, as in so many other cases, we must regard the differentiation as in part due to direct equilibration and in part to indirect equilibration. Familiar facts compel us to conclude that from the beginning, each individual foliar organ has undergone a certain immediate adaptation of its surfaces to the incidence of light; that when there arose a mode of growth which exposed the leaves of successive generations in similar ways, this immediately-produced adaptation, ever tending to be transmitted, was furthered by the survival of individuals inheriting it in the greatest degree; and that so there was gradually established that difference between the two surfaces which each leaf displays before it unfolds to the light, but which becomes more marked when it has unfolded.[47]

From the ordinary cases let us now pass to the exceptional cases. We will look first at those in which the two faces of the leaves differ but little, or not at all—their circumstances being similar or equal. Leaves that grow in approximately-upright attitudes, and attitudes which do not maintain the relative positions of the two surfaces with constancy, may be expected to display an unusual likeness between the two surfaces; and among them we see it. The Grasses may be named as a group exemplifying this relation; and if, instead of comparing them as a group with other groups, we compare those dwarf kinds of them which spread out their leaves horizontally, with the large aspiring kinds, such as Arundo, we trace a like antithesis: in the one the contrast of upper and under is very obvious, while in the other it is scarcely to be detected. Leaves of various other Monocotyledons that grow in a similar way, similarly show us a near approach to uniformity of the two surfaces; as instance the genus Clivia and the thinner-leaved kinds of Yucca. Where the contrast of upper and under is greatly diminished by the assumption of a rounded or cylindrical form, instead of a flattened form, the same thing happens. The genus Kleinia furnishes illustrations. It may be remarked, too, that even within the limits of this genus there are instructive variations; for while in Kleinia ficoides the leaves, shaped like pea-pods, are broadest in a vertical direction, and have their lateral surfaces alike in conditions and structure, in other species the leaves, broader horizontally than vertically, exhibit unlikeness between the upper and under sides. Equally to the point is the evidence furnished by vertically-growing leaves that are cylindrical, as those of Sanseviera cylindrica, or as those of the Rush-tribe: the similarly-placed surface has all around a similar character. Of kindred meaning, and still more conclusive, are the cases in which the under side of the leaf, being more exposed to light than the upper side, usurps the character and function of the upper side. If a common Flag be pulled to pieces, it will be seen that what answers to the face in other leaves, forms merely the inside of the sheath including the younger leaves, and is obliterated higher up. The two surfaces of the blade answer to the two under halves of a leaf that has been, as it were, folded together lengthways, with the two halves of its upper surface in contact. And here, in default of an upper surface, the under surface acquires its character and discharges its function. A like substitution occurs in Aristea corymbosa; and there are some of the Orchids, as Lockhartia, which display it in a very obvious way.

When joined with the foregoing evidence, the evidence which another kind of substitution supplies is of great weight. I refer to that which occurs in the Australian Acacias, already instanced as throwing light on morphological changes. In these plants the leaves properly so-called are undeveloped, and the foot-stalks, flattened out into foliaceous shapes, acquire veins and mid-ribs, and so far simulate leaves as ordinarily to be taken for them: a fact in itself of much physiological significance. But that which it concerns us especially to note, is the absence of distinction between the two faces of these phyllodes, as they are named, and the cause of its absence. These transformed petioles do not flatten themselves out horizontally, so as to acquire under and upper sides, as most true leaves do; but they flatten themselves out vertically: the result being that their two sides are similarly circumstanced with respect to light and other agencies; and there is consequently nothing to cause their differentiation. And then we find an analogous case where differential conditions arise, and where some differentiation results. In Oxalis bupleurifolia, Fig. 66, there is a similar flattening out of the petiole into a pseudo-leaf; but in it the flattening takes place in the same plane as the leaf, so as to produce an under and an upper surface; and here the two surfaces of the pseudo-leaf are slightly unlike—in contour if in nothing else.

§ 275. We now come to such physiological differentiations among the outer tissues of plants, as are displayed in the contrasts between foliar organs on the same axis, or on different axes—contrasts between the seed-leaves and the leaves subsequently formed, between submerged and aërial leaves in certain aquatic plants, between leaves and bracts, and between bracts and sepals. To deal even briefly with these implies information which even a professed botanist would have to increase by special inquiries, before attempting interpretations. Here it must suffice to say something respecting those marked unlikenesses existing between the tissues of the more characteristic parts of flowers, and the tissues of the homologous foliar organs.

It was pointed out in § 196, that the terminal folia of a phænogamic axis have sundry characters in common with such fronds as those out of which we concluded that the phænogamic axis has arisen by integration—common characters of a kind to be expected. In their simple cellular composition, comparative want of chlorophyll, and deficiency of vascular structures, the undeveloped ends of leaf-shoots and the developed ends of flower-shoots, approach to the fronds of the simpler Archegoniates. We also noted between them another resemblance. It is said of the Jungermanniaceæ, that “though under certain circumstances of a pure green, they are inclined to be shaded with red, purple, chocolate, or other tints;” and answering to this we have the facts that such colours commonly occur in the terminal folia of a phænogamic axis, when arrest of its development leads to the formation of a flower, and that very frequently they are visible at the ends of leaf-axes. In the unfolding parts of shoots, more or less of red, or copper-colour, or chocolate-colour, may generally be seen: often, indeed, it characterizes the leaves for some time after they are unfolded. Occasionally the traces of it are permanent; and, as in the scarlet terminal leaves of Poinsettia pulcherrima, we see that it may become, and continue, extremely conspicuous. The question, then, now to be asked is—has this colouring by which the immature part of the phænogamic axis is characterized, anything to do with the colouring of flowers? Has this difference between undeveloped folia and folia that are further developed, been increased by natural selection where an advantage accrued from it, until it has ended in the strong contrast we now see? I think we may not irrationally infer that this has happened.

Facts, very numerous and varied, united to warrant us in concluding that gamogenesis commences where the forces which conduce to growth are nearly equilibrated by the forces which resist growth (§ 78); and the induction that in plants, fertilized germs are produced at places where there is an approach towards this balance, we found to be in harmony with the deduction that an advantage to the species must be gained by sending off migrating progeny from points where nutrition is failing. Other things equal, failure of nutrition may be expected in parts which have the most remote or most indirect access to the materials furnished by the roots—materials which have to be carried great distances by a very imperfect apparatus. The ends of lateral axes are therefore the probable points of fructification, in aggregates of the third order that have taken to growing vertically. But if these points at which nutrition is failing, are also the points at which the colours inherited from lower types are likely to recur in more marked degrees than elsewhere; then we may infer that the organs of fructification will not unfrequently co-exist with such colours at the ends of such axes. How may the resulting contrast between the older fronds and the fronds next the germ-producing organs be increased? If uninterfered with it would be likely to diminish. These traits inherited from remote ancestry might be expected slowly to fade away. How, then, is the intensification of them to be explained?

If a contrast of the kind described favours the propagation of a race in which it exists, it will be maintained and increased; and if we take into account an agency of which Mr. Darwin has shown the great importance—the agency of insects—we shall have little difficulty in understanding how such a contrast may facilitate propagation. We cannot, of course, here assume the agency of insects so specialized in their habits as Bees and Butterflies; for their specialized habits imply the pre-existence of the contrast to be explained. But there is an insect-agency of a more general kind which may be fairly counted upon as coming into action. Various small Flies and Beetles wander over the surfaces of plants in search of food. It is a legitimate assumption that they will frequent most those parts in which they find most food, or food most to their liking—especially if at the same time they gain the advantage of concealment. Now the ends of axes, formed of young, soft, and closely-packed folia, are the parts which more than any others offer these several advantages. They afford shelter from enemies; they frequently contain exuded juices; and when they do not, their tissues are so tender as to be easily pierced in search of the sap. If, then, from the first, as at present, these ends of axes have been favourite haunts of small insects; and if, where the closely-clustered folia contained the generative organs, the insects frequenting them occasionally carried adherent fructifying cells from one plant to another, and so aided fertilization; it would follow that anything which made such terminal clusters more attractive to such insects, or more conspicuous to them, or both, would further the multiplication of the race, and would so be continually increased by the extra multiplication of individuals in which it was greatest. Here we find the clue. This contrast of colour between the folia next to the fructifying parts and all other folia, must constantly have facilitated insect-agency; supposing the insects to have had the power of distinguishing between colours. That Bees and Butterflies have this power is manifest. They may be watched flying from flower to flower, disregarding all other parts of the plants. And if the less-specialized insects possessed some degree of such discrimination, then the initial contrasts of colour above described would be maintained and increased. Let such a connexion be once established, and it must tend to become more decided. Insects most able to discern the parts of plants which afford what they seek, will be those most likely to survive and leave offspring. Plants presenting most of the desired food, and showing most clearly where it lies, will have their fertilization and multiplication furthered in the greatest degree. And so the mutual adaptation will become ever closer; while it is rendered at the same time more varied by the special requirements of the insects and of the plants in each locality, under each change of conditions. Of course, the genesis of the sweet secretions and the odours of flowers, has a parallel interpretation. The simultaneous production of honey, or some kindred substance, is implied above; since, unless a bait co-existed with the colour, the colour would not attract insects, and would not be maintained and intensified by natural selection. Gums, and resins, and balsams, are familiar products of plants; apparently, in many cases, excreted as useless matters from various parts of their surfaces. These substances, admitting of wide variations in quality, as they do, afford opportunities for the action of natural selection wherever any of them, attractive to insects, happen to be produced near the organs of fructification. And this action of natural selection once set up, may lead to the establishment of a local excretion, to the production of an excretion more and more attractive, and to the disposal of the organ containing it in such a way as most to facilitate the carrying away of pollen. Similarly and simultaneously with odours. Odours, like colours, draw insects to flowers. After observing how Bees come swarming into a house where honey is largely exposed, or how Wasps find their way into a shop containing much ripe fruit, it cannot be questioned that insects are to a considerable extent guided by scent. Being thus sensitive to the aromatic substances which flowers exhale, they may, when the flowers are in large masses, be attracted by them from distances at which the flowers themselves are invisible. And manifestly, the flowers which so attract them from the greatest distances, increasing thereby their chances of efficient fertilization, will be most likely to perpetuate themselves. That is to say, survival of the fittest must tend to produce perfumes that are both more powerful and more attractive.

These physiological differentiations, then, which mark off the foliar organs constituting flowers from other foliar organs, are the consequences of indirect equilibration. They are not due to the immediate actions of unlike incident forces on the parts of the individual plant; but they are due to the actions of such unlike incident forces on the aggregate of individuals, generation after generation.[48]

§ 276. The unity of interpretation which we here find for phenomena of such various orders, could hardly be found were the phenomena otherwise caused. That the stronger and the feebler contrasts among the different parts of the outer tissues in plants, should so constantly occur along with stronger and feebler contrasts among the incident forces, is in itself weighty evidence that unlike outer actions have caused unlike inner actions, and correspondingly-unlike structures; either by changing the functional equilibrium in the individual, or by changing it in the race, or by both.

Even in the absence of more direct proof, there would be great significance in the marked differences that habitually exist between the exposed and imbedded parts of plants, between the stems and the leaves, and between the upper and under surfaces of the leaves. The significance of these differences is increased when we discover that they vary in degree as the differences in the conditions vary in degree. Still greater becomes the force of the evidence on finding that these strongly-contrasted parts may, when placed in one another’s conditions, and kept in them from generation to generation, permanently assume one another’s functions, and, in a great degree, one another’s structures. Even more conclusive yet is the argument rendered, by the discovery that, where these substitutions of function and structure take place, the superinduced modifications differ in different circumstances; just as the original modifications do. The fact that a flattened stem simulating a vertically-growing leaf has its two surfaces alike, while when it simulates a horizontally-growing leaf its upper and under surfaces differ, is a fact which, standing alone, might prove little, but proves much when joined with all the other evidence. And its profound meaning becomes the more obvious on discovering that the same thing happens with petioles when they usurp leaf-functions.

Finally, when we remember how rapidly analogous modifications of function and structure arise in the superficial tissues of individual plants, the general inference can scarcely be resisted. When we meet with so striking a case as that of the Begonia-leaf, a fragment of which stuck in the ground produces roots from its under surface and leaves from its upper surface—when we see that though, in this case, the typical structure of the plant presently begins to control the organizing process, yet the initial differentiations are set up by the differential actions of the environment; the presumption becomes extremely strong that the heterogeneities of surface which we have considered, result, as alleged, directly or indirectly from heterogeneities in the incident forces.