Chapter XIX: THE GERM-PLASM THEORY (continued) 392 (7)

Again, in many plants the whole leaf-edge is transformed into a spiny wall, which may be enlarged by indentations and lobate projections, as in the holly, and, in a much higher degree, in the thistles (_Carduus_), in _Eryngium_, in _Acanthus_, and in many Solanaceæ. Often, too, there are barbed hooks on the leaf-edge, which work like a saw; or the leaf-edge, though without spines, may be made sharp by deposits of silicic acid, as in the sedges, whose sharp edges are moved to and fro in the mouths of ruminants, and thus injure the mucous membrane. The hook-bristles of the fig-cactus (_Opuntia_), which, though small, are abundantly provided with barbs, must also be mentioned; for they are to be found in great numbers surrounding the buds of these plants, and most effectively protect them from being eaten away by animals (Fig. 19).

To this category, too, belong the short, prickly bristles of the rough-leaved plants, which cover the whole plant as with an overcoat of sharp needles; of these we may mention the adder's tongue (_Echium vulgare_), the comfrey (_Symphytum officinale_), and the borage (_Borago officinalis_).

Very well known are the stinging-hairs of the Urticaceæ, long hairs (Fig. 20) with an elastic base, but with glass-like, brittle, rounded heads, which break off at the lightest touch, whereupon the sharp point of the broken hair penetrates the skin of the creature which has touched it, and the poisonous contents of the hair are poured into the wound. Even our large stinging-nettle (_Urtica dioica_) can cause intense irritation, and evoke the 'nettle-rash,' named after it, on the human skin; but there are many tropical species of nettle, e.g. _Urtica stimulata_ in Java, and others, which have an effect similar to that of snake-poison and produce tetanoid spasms, and so on. In addition to formic acid these hairs contain an undefined ferment, a so-called Enzyme. It need scarcely be said that these stinging-hairs must have much more severe effects on the mucous membrane of the mouth of grazing animals than on the human skin, and that they are therefore an excellent protection for the plants. As a matter of fact we never find our nettle patches eaten away, and even the donkey, which eats thistles freely, turns away from the stinging-nettle. But even these stinging-hairs, like all other protective devices, do not afford an _absolute_ protection. The caterpillars of several of our diurnal butterflies feed exclusively on the stinging-nettle, and they eat up the leaves, stinging-hairs and all. This is the case with five species of the genus _Vanessa_, namely: _Vanessa io_, the 'peacock,' _Vanessa urticæ_, the small tortoiseshell, _Vanessa prorsa_, _Vanessa C. album_, the C. butterfly, and _Vanessa atalanta_, the admiral.

We are all familiar with our mulleins (_Verbascum_), those beautiful flower-spikes with the thick, soft felt leaves, which grow on stony or sandy soil. Harmless as they look, they are much disliked by animals as food, for the thick hairy felt which covers them breaks up in the mouth, and sticks in the folds of the mucous membrane, causing burning sensations and other discomforts. They, too, are therefore spared by grazing animals, but they have smaller enemies, like the caterpillars of the genus _Cucullia_, which, however, never completely destroy them, but only eat large holes in their leaves.

Let us now consider in somewhat greater detail the true thorns, the most conspicuous protection of many plants. It is very remarkable that these are always so placed, and so regulated as to their length and character, as to afford protection to the most important and the most exposed parts of the plant. Thus many bushes, which would otherwise be in danger of being completely devoured by cattle, are stiff with thorns which are nothing else than pointed, hard twigs without, or with very little foliage. Among these are the sloes, the buckthorn (_Rhamnus_), the sea-buckthorn (_Hippophäe_), and the barberry (_Berberis_). In the last-named three thorns arise in a group, and protect the young bud from danger in three directions (Fig. 21).

The fine-leaved mimosas of the tropics have similar but very long and sharp thorns, and their leaves are movable and sensitive, so that, when they are touched, they shut up and draw back behind the rampart of stiff thorns, which are just of the right length to protect them.

In many thorny bushes only the young shoots of each spring remain green through the summer, and in autumn they become transformed into thorns, under whose protection the shoots of the following spring will develop. Sometimes, too, the leaf-stalks are modified in the course of the summer into thorns, as in Tragacanth (_Astragalus tragacantha_). In this case the young leaves are protected by a circle of thorns, consisting of the leaf-stalks of the preceding year which have not fallen off (Fig. 22, _A_, _B_, _C_).

I should have to go on for a long time with my exposition, even if I were to confine attention to the essential facts; we shall, therefore, only recall the well-known phenomenon of the Cactuses, in which the leaves are entirely transformed into spines, which may attain a length of eight centimetres, while the fleshy stem alone represents the green--that is, the assimilating parts of the plant. The species of Cactus are almost the only plants which grow on the stony, hard, and hot plateaux of Mexico, and they are protected from desiccation by the thickness of their epidermis. But, enticing as is the food promised by the juicy stem, animals rarely venture to approach them, and it is only when tortured by thirst that horses and asses occasionally knock off the spines with their hoofs, and so reach the soft tissues rich in water. For this attempt, however, as Alexander von Humboldt pointed out, they often suffer, as the sharp spines are apt to pierce the hoof. In any case, the cactuses are effectively protected from the danger of extermination by grazing animals.

It must certainly strike every one that many districts, especially those which are dry, hot, and stony, are conspicuously rich in thorny plants, and it has often been supposed that the production of thorns must be a direct result of these peculiar conditions of life; indeed, the hard, thorny habit of many of these plants has even been regarded as a protection against desiccation. This, however, is contradicted by all those thorny plants which, like the cactuses, possess tissues extremely rich in sap, and in which desiccation is prevented, not by the thorns, but by the thick epidermis. The only satisfactory explanation is that afforded in terms of natural selection. In such hot, and at the same time dry regions, the plant-growth is often very scanty, and the food available for the grazing animals is, at least at times, very scarce; on this account, if the plants are to survive there at all, they must be armed with the most perfect means of protection possible against the attacks of hungry and thirsty animals. The struggle for existence in relation to such enemies is much more severe than in more luxuriant regions, and the protection by thorns has been developed to the highest possible pitch of perfection; species which were unable to develop this protection died out altogether. Hence the cactuses of Mexico, and the many thorny bushes and shrubs of the hot, and, in the summer, dried-up stony coast-lands of the Mediterranean in Spain, Corsica, Africa, and other countries. This so-called 'Prigana scrub' embraces a number of species, whose nearest relatives in our climate are not provided with spines, as, for instance, _Genista hispanica_, _Onobrychis cornuta_, _Sonchus cervicornus_, _Euphorbia spinosa_, _Stachys spinosa_, and others.

Why do so few thorny plants grow on the rich and well-watered Alpine pastures? Probably because there is to be found there a rich and luxuriant plant-growth which can never be wholly exterminated by the grazing of animals, so that an individual species would not, by developing thorns, have gained any advantage in the way of increased capacity for existence.

But these Alpine grazing grounds serve well to illustrate how great may be the advantage which protective devices give to a species. Much to the annoyance of the herdsmen, who endeavour to extirpate them as far as possible, enormous masses of rhododendrons often cover whole stretches, because their hard silicious leaves cannot be eaten, and many other plants despised of cattle flourish and increase on the grazing runs, like the repulsively bitter, large _Gentiana asclepiadea_, the malodorous _Aposeris fœtida_, and various ferns of disagreeable taste.

The advantage derived by plants from the possession of any kind of protective device against grazing animals is perhaps best of all seen in the 'shrubbery,' which on every Alp is to be found in the immediate neighbourhood of the herdsman's hut. There, where the cattle daily assemble, and where the soil is continually being richly manured by them, we always find a large, luxuriantly growing company of the poisonous aconite, the bitter goosefoot (_Chenopodium bonus henricus_), the stinging-nettle, the thistle (_Cirsium spinosissimum_), the ill-smelling _Atriplex_, and some other inedible species, while the palatable herbs are gradually exterminated by the cattle which daily gather round the hut (Kerner).

To sum up. We have seen that there is among plants an extraordinary diversity of protective adaptations, which secures them from extermination by the larger herbivores.

Since all useful contrivances, or, as we say, all adaptations, are capable of interpretation in terms of the process of selection, we must refer this great array of the most diverse protective devices to natural selection; and again, as among animals, we receive the impression that the organism is, to a certain extent, really capable of producing every variation necessary to its maintenance. Literally speaking, this would not be correct, but at any rate the number of adaptations possible to each form of life must be an enormous one, so great, indeed, that ultimately every species does secure protection for itself in some manner and in some degree, whether it be by the production of a poison or a nauseous substance within itself, or by surrounding itself with thorns or spines. And if it be, in a certain sense, a matter of 'chance' whether a plant has taken to one method of defence or to another, according as its innate constitution favoured the production of one rather than of any other, yet it would not be easy to prove, even in the case of the purely chemical means of protection, that these would have occurred in the same distribution and concentration as a necessary result of the metabolism of the plant, even if they had not been useful and consequently augmented by selection. But in the case of the mechanical means of protection this mode of explanation fails as utterly as that of the direct effect of the conditions of life. Why the holly should have spinose leaves beneath and smooth ones above can never be deduced from the constitution of the species.

While the protective adaptations of plants against the larger herbivores always point to natural selection, our appreciation of the adaptability of plants, and at the same time of the potency of natural selection, will be strengthened still more if we turn our attention for a little to the arrangements which prevent the extermination of plants by the lower and small animals.

It might indeed be supposed that extermination by these could hardly be an imminent danger, but if we think of the cockchafer blight, or of the destruction of whole woods by the caterpillar of the 'white nun,' or even of the destruction of several successive plantings of young salad plants which the snails often cause in our gardens, it cannot be doubted that all plants would be exterminated by insects and snails alone unless they were protected against them in some degree.

We owe our detailed knowledge of the means by which plants protect themselves against the menace of the greedy and prolific snails to the beautiful investigations of Stahl, Professor of Botany in the University of Jena.

In this case, too, both chemical and mechanical means are made use of. The minute quantity of tannic acid which is contained in the leaves of the clover prevents many snails from eating them, as, for instance, the garden snail (_Helix hortensis_). If the leaves be soaked so as to wash out the tannin the snail readily accepts them as food. It is true that the small, whitish field-slug (_Limax agrestis_) does not object to the presence of the tannin, and eats the fresh leaves of the clover; indeed, there is no such thing as absolute protection. In discussing the herbivorous mammals I have already mentioned that many trees and shrubs, mosses and ferns are effectively protected by the large amount of tannin they contain; this protection is effective also against snails, for all these plants are fairly free from their attacks; and the same is true of many other tannin-containing plants, species of saxifrage and sedum, the strawberry, many water-plants, like the pond-weeds (_Potamogeton_), the horn-nut (_Trapa_), the mare's tail (_Hippuris_). All these plants are only eaten by snails in case of necessity, or in the washed-out state.

In other plants protection is gained by means of some acid, especially oxalic acid, like the wood-sorrel (_Oxalis acetosella_), the sorrel (_Rumex_), and the species of Begonia. When Stahl smeared slices of carrot, which is a favourite food of snails, with a weak (one per cent.) solution of oxalate of potassium, they were refused by the snails, and this is not surprising when we remember that even the external skin of the snail is very sensitive, and the mucous membrane of the mouth is not likely to be less so.

Similarly, many plants develop ethereal oils in the hairs which cover them, as in the herb-Robert (_Geranium robertianum_). Even the almost omnivorous field-slug (_Limax agrestis_) does not touch this plant, and if it be placed upon it, escapes with all dispatch from the ethereal oil, which burns its naked skin, by covering itself with mucus and letting itself down to the ground by a thread. The mints (_Mentha_) and the dittany (_Dictamnus albus_) also produce such oils.

Among chemical means of protection must be named the pure bitter stuffs, such as are found in the species of gentian, the milkwort (_Polygala amara_), and in many other plants, and also the curious 'oil-bodies' of the liverworts.

But some plants also defend themselves against the attacks of snails by mechanical means.

First there are the various kinds of bristle arrangements, which prevent the snails from creeping up the stalks. We never find the comfrey (_Symphytum officinale_) of our meadows eaten by snails, for it is thickly covered over with stiff bristles, which are most disagreeable to the snail, and the stinging-nettle (_Urtica dioica_) is similarly protected by bristle hairs, while, as we have already seen, its stinging-hairs secure immunity from the attacks of larger animals.

And although it is true that the majority of plants do not prevent the snails from creeping up their stalks, yet they do not serve them in any great degree as food, since the green parts often offer resistance to mastication and digestion. Thus the lime encrustations which cover the stoneworts (_Chara_) prevent snails from eating them. If the lime be dissolved by means of acids, and the plants then offered to the snails, they will eat them greedily. The same is true of the silicifying of the cell-walls, so widely distributed among mosses and grasses, and when this occurs in a high degree it forms an effective protection even against the large herbivores. Our slightly siliceous grasses are secure from snails, and that it is really the presence of the silicic acid which deters them from an otherwise welcome kind of food is proved by Stahl's experiment of growing maize in pure water, and so obtaining plants poor in silica. These were devoured without ceremony by the snails.

Of the many other protective peculiarities which make it difficult for snails to eat plants I shall only recall the so-called 'Raphides,' those microscopic crystal-like needles of oxalate of lime, pointed at both ends, which lie close together in the tissues of many plants. Cuckoo pint (_Arum maculatum_), the narcissi, the snowdrops (_Leucojum_), the squill (_Scilla_), and the asparagus contain them, and all these plants are spared by snails obviously because during mastication they are unpleasantly affected by the raphides. Even the voracious field-slug rejects these.

Of course it cannot be said that these raphides protect against all other enemies. They are effective against rodents and ruminants, and also against locusts, but a number of caterpillars seek out by preference just those plants which contain raphides. Thus certain caterpillars of the Sphingidæ feed on species of _Galium_ and _Epilobium_, the leaves of the vine, and the wild balsam (_Impatiens_). The caterpillar of _Chærocampa elpenor_, which especially prefers _Vitis_ and _Epilobium_, has transferred its affections to the fuchsias in our gardens, which came from South America; the butterfly not infrequently lays its eggs on these plants, and the caterpillars devour them readily; but the fuchsias may also contain raphides.

We may say, indeed, that almost all wild Phanerogams are protected in some degree against snails, and this almost suggests the question: What then is left for the snails to feed on if everything is thus armed against them? But, in the first place, there remain our cultivated plants, which, like the garden lettuce (_Lactuca_), are quite without defence; and secondly, the snails often eat the plants only after they have been rooted up and lie rotting on the ground, that is, when the protective ingredient has been dissolved out by the rain; finally, no means of protection, as I have often said already, is absolute or effective against all snails. Many of these are, as Stahl calls them, 'specialists.' Thus, the large slug of our woods eats the poisonous fungi which are rejected by other snails, and in the same way there are many other specialists which, however, are not likely to eliminate unaided the plants to which they have adapted themselves. There are certainly also omnivorous forms, like the field-slug (_Limax agrestis_), to which we have referred so often, and _Arion empiricorum_, the red slug, but just because these eat so many kinds of plant they are less dangerous to any one species.

These manifold devices for protecting plants against the depredations of snails afford another proof that innumerable details in the organization of plants, as of animals, must be referred to natural selection, since they are capable of interpretation in no other way. If these protective devices were to be found only in isolated plants, we might perhaps talk of 'chance'; we might refer them to the inborn constitution of the plant, which made the production of bristles, or bitter stuffs, or the deposition of silicic acid a necessity, and which 'happened' to make the plants distasteful to certain snails. But as it appears that all plants are protected against snails, one in this way, another in that, this objection cannot be sustained. Furthermore, some of the beautiful experiments made by Stahl to prove the protective effect of these devices showed, at the same time, that they were not in themselves indispensable to the existence of the plant; maize, for instance, develops a plant perfectly capable of life, even though silicic acid be withheld, and the acid is, therefore, not an element essential to its constitution, but a means of protection against voracious animals. The clearest proof of this is afforded by plants like the lettuce (_Lactuca_), which formed protective stuffs in the wild state, but have lost them altogether under cultivation, through disuse, as we shall see more precisely later on. As the eyes of animals which live in darkness have degenerated, so the plants which have been taken under the protection of man have lost their natural means of defence, because these were no longer necessary to the maintenance of the species. Even the protective bitter substances (tannin-compounds) are not essential to the constitution of the genus _Lactuca_; their formation may be discontinued without the plant being otherwise affected. And in this case it is not a question of the withdrawal of something which has to be taken in from outside, it is the non-development of what is purely a product of the internal metabolism.

The adaptations of plants against snails are instructive in another way, namely, in their extraordinary diversity. Here again we see how great is the plasticity of organic forms, and how precisely, though in many very different ways, they adapt themselves to the conditions of their life, in this case the weaknesses of their greedy enemies, and all to attain the same end, the security of their existence as a species. We see at the same time that innumerable minute details in the structure and character of a species, which may appear unimportant, may yet have their definite uses--hairs, bristles, and raphides, as well as bitter substances, ethereal oils, acids, and tannin-compounds. But we must, of course, have minute and exhaustive investigations, like those of Stahl, in regard to the biological relations of these peculiarities before their utility can become clear to us.

LECTURE VII

CARNIVOROUS PLANTS

Introduction--The Bladderworts or Utriculariæ--Pitcher-plants, Nepenthes--The Toothwort, Lathræa--The Butterwort, Pinguicula--The Sundew, Drosera--The Flytrap--Aldrovandia--Conclusions.

THAT the principle of selection dominates, to a large extent at least, all the structural characters of plants, and moulds these in direct relation to the prospects of greater success which may be offered in the vicissitudes of the life-conditions of a single species or group of species, is nowhere more apparent than in the case of the so-called 'insectivorous' or 'carnivorous' plants. Here again it was Charles Darwin who led the way, for while many plants had long been known on the sticky leaves of which insects were often caught and killed, it had occurred to no one to regard this as of any special use for the plant, much less to look on the peculiar dispositions of such leaves as especially determined for this purpose. Darwin was the first to show that there is no small number of plants--we now know about 500--which secure only a portion of their nutritive material by the usual method of assimilation, and gain another and smaller portion by dissolving and utilizing animal protoplasm, especially nitrogenous muscle substance. The correctness of this interpretation was at first disputed, but Darwin showed that pieces of muscle, or any nitrogenous organic substance, were really dissolved by the relevant parts of the plant, and were afterwards absorbed. It can therefore no longer be doubted that the remarkable contrivances by which animals are laid hold of by plants--are in a certain sense caught and killed--have arisen with reference to this particular end; or, to speak less metaphorically, that existing structural and functional peculiarities in a plant which caused animals to be held fast were of advantage to the nutrition of the plant, and were therefore augmented and perfected by natural selection. That this was possible is obvious from the number of insectivorous plants which now live upon the earth, and that these processes of selection ran their courses quite independently of one another, and even that they started from different parts of the plant, is shown by the diversity of the contrivances which occur in plants of several different families. A few of these I wish to discuss in some detail.

The marshes of European countries, and also those of warmer lands, often contain bladderworts, or Utriculariæ (Fig. 23)--floating water-plants, without roots, and with horizontally spread, long-drawn-out, tendril-like shoots, in part thickly covered with whorls of delicate, needle-shaped leaves, in part bearing sparse leaves of quite peculiar structure. These are stalked, hollow bladders (Fig. 23 _A_, _FA_), with quite a narrow entrance at the apex, which is closed, as far as larger animals are concerned, by projecting bristle-like hairs (_B_). Small animals, such as water-fleas (_Daphnia_), species of _Cyclops_, and Ostracods, can swim in between the bristles, and they then come in contact with a valve which opens easily inwards (_B_, _kl_) and allows them to penetrate into the interior of the trap. Once inside they are captives, for the valve does not open outwards; therefore they soon die and decompose, and are then taken up by special absorptive cells (_B_, _C_, _sz_) and utilized as nourishment for the plants. In this way the Utriculariæ catch numerous little crustaceans and insect larvæ, which slip into their traps, presumably for concealment.

Another example is found in the marsh plants of the genus _Nepenthes_, some species of which live as climbers on the outskirts of tropical forests, climbing up the trees and letting their long, thin tendrils hang downwards, often over ponds and stagnant pools, where swarms of small flying insects abound. These plants have developed exceedingly remarkable contrivances for catching insects and using them as food (Fig. 24). The long stalks (_St_) of their leaves (_Spr_) are first bent downwards, then they suddenly turn sharply upwards, and the upturned portion is modified into a pitcher-like structure, in the bottom of which a fluid gathers, acid in taste, containing pepsin, and therefore a digestive fluid. Nitrogenous substances, such as flesh, dissolve in this fluid, and insects which fall into the pitcher from the rim are killed and dissolved. There are many species of _Nepenthes_, but not all of them possess the trap-structure in equal perfection, so that we are able, to some extent, to follow the course of its evolution, from a broad leaf-stalk, somewhat bent over at the edges, to the marvellous closed pitcher shown by _Nepenthes villosa_ (Fig. 24) of Borneo. In this species the pitchers attain a length of fifty centimetres, and are beautifully coloured, resembling in that respect, as well as in their form, the tobacco-pipe-like flowers of the tropical Aristolochiæ. When we come to discuss the origin of flowers, we shall see that the bright, conspicuous colour possesses a very considerable value in attracting insects; and in the case of the pitcher-plant, too, the gorgeous colour probably allures insects to settle on the rim of the pitcher, and they are tempted to dally the longer since it secretes honey. But the thick, swollen rim of the pitcher is as smooth as if it were made of polished wax, and resembles the petals of those magnificent large orchids, the Stanhopeæ; the inner surface of the pitcher below the margin is also smooth, so that insects which creep about seeking honey are apt to slip and fall to the bottom. Even if many of them are not at once killed by the digestive fluid, but are able to climb up the smooth wall again, they cannot escape, for beneath the swollen rim, which projects inwards, there is a circle of strong bristles or teeth, with the points directed downwards, which, like thorns, prevent the captive's escape. Thus the pitchers of _Nepenthes_ secure and digest a large number of insects, and we can easily understand that the plant acquires a considerable amount of valuable nourishment in this way, for ready-made protoplasm is a convenient food to which the plant has to do but little in order to convert it into its own particular kind of living matter.

The toothwort (_Lathræa squamaria_) must also be briefly noticed here, because it does not catch insects through the medium either of air or of water, but through the earth. As is well known, this plant is parasitic on the roots of various foliage-trees. It is of a pale yellowish colour, and has no green assimilating parts. For such a plant it must be of particular value to be able to catch animals and to use them as food. To this end the short, pale leaves, which surround the creeping, underground stem in the form of closely appressed scales, have been modified into snares for minute animals. The leaves have their upper parts recurved downwards, and the edges have grown together, so that only a small opening is left at the base, and this leads into a system of tunnels. Aphides, rotifers, bear-animalcules, but especially springtails (Podurids), creep into these hollow leaves, are held fast by a sticky secretion, and are dissolved and absorbed.

Another example, also indigenous, is that graceful marsh plant, the butterwort (_Pinguicula vulgaris_), whose broad, tongue-shaped leaves, arranged in the form of a rosette, have been modified into an insect trap by the turning up of their edges, while the middle is deepened into a longitudinal groove (Fig. 25). The whole upper surface of the leaf is covered with an enormous number of little mushroom-shaped glands (_B_, _C_, _Dr_), which secrete a viscid slime. Insects which settle on the leaf stick fast, and as the glands continue to pour out more and more slime, while at the same time the edges of the leaf, stimulated by the struggling of the insect, curl over still farther, the victims are drowned in the slime, and ultimately absorbed; for this secretion is so powerful that even fragments of cartilage are dissolved by it in forty-eight hours. Midges and mayflies in particular fall victims to this plant, which is common in marshy places both in mountain and plain.

We must also mention the sundew (_Drosera rotundifolia_), which takes its name from the seeming dewdrops that sparkle in the sun on the leaves, or rather on the rounded extremities of long and rather thick cilia-like hairs which cover the whole upper surface of the leaf. In reality the apparent dewdrops consist of a sticky, clear, viscid slime, which is secreted by the glandular ends of the pin-shaped hairs or 'tentacles.' Insects which settle on the leaf are caught by the slime, and in this case also an acid, pepsin-containing fluid is secreted, which gradually effects the digestion of the soluble parts of the insect. It is especially noteworthy that it is not only those tentacles which are in contact with the insect that take part in its digestion and absorption, for all the others gradually alter their position from the moment when any nitrogenous body, be it a fragment of flesh or an insect, touches any of them. All begin to curve slowly towards the stimulating object (Fig. 27), so that, after one to three hours, all the tentacles have their heads towards it, and collectively pour out their digestive juice upon it.

The sundew grows in marshes, as, for instance, those of the Black Forest, and also on the moss-covered ridges there, and it is easy to observe that a leaf often shows not merely a single gnat, midge, or little dragon-fly, but several, sometimes as many as a dozen. In this case, again, the value of the arrangement from the point of view of nourishment can be no inconsiderable one.

In the case of the sundew we are obviously face to face with an exceedingly complex adaptation, for not only is there a secretion of the peculiar digestive juices, which occur only in carnivorous plants, but the secreting tentacles are actively motile. That the tentacles more remote from the captive may be excited to curve towards it, it is necessary that the stimulus exerted by it on the heads of the tentacles connected with it be conveyed to the base, and thence to the tips of the other tentacles, for they curve throughout their whole length. The utility of the contrivance is obvious, but that an arrangement so divergent from the ordinary dispositions of plants could be brought about points to the length of time that the processes of natural selection must have gone on, preserving every new little variation, and adding it to the rest.

Two plants remain to be noticed in conclusion, both possessing movable, closing traps for catching animals. The so-called Venus fly-trap (_Dionæa muscipula_) is a marsh plant of North America, the leaves of which, like those of _Pinguicula_ and _Drosera_, are arranged in a rosette on the ground. The individual leaf has a spatula-like stalk and a blade in two halves (Fig. 28, _A_), each edged with long and strong spinous processes, directed obliquely inwards. The halves of the blade, when the necessary stimulus is applied to the surface, can close together in a very short time, from 10 to 30 seconds. The two rows of marginal spines then cross, as the interlocking fingers of the hands do, and thus form a cage out of which the imprisoned insect cannot escape. The appropriate stimulus to set the mechanism in motion is a light touch, while a more violent shock, or strong pressure, or a current of air, does not cause the trap to close. But if a fly comes to creep about on the leaf, and in doing so touches one of six short jointed hairs rising erect from a minute cushion of cells, then the leaf closes, quickly indeed, but at the same time so gently and imperceptibly that the fly is unaware of danger and does not try to escape. Then numerous purple mucous glands begin to surround the victim with pepsin-containing, acid, digestive juice which gradually dissolves it.

One of the water-plants of Southern Europe, _Aldrovandia vesiculosa_, which is also to be found in swamps on the northern ridge of the Alps, possesses, in addition to the capturing and digesting apparatus proper, an active motile apparatus, which is set in motion through sensitive hairs. When I found the plant for the first time in a swamp at Lindau, on the Lake of Constance, I took it at first sight for an _Utricularia_, for the two plants resemble each other in external appearance (cf. Figs. 22 and 29), but the modification of the leaves into traps is quite different. On both halves of the leaf-blade there are numerous bristles (Fig. 30, _A_), and the lightest touch on these by a little water animal acts as a releasing stimulus to the motile elements of the leaf (_Stch_). As in the Venus fly-trap, the two halves of the leaf close together somewhat quickly, but quite quietly, and the animal is caught. Fig. 30 shows a section of one of these traps in its closed state. The captive animals cannot escape, because the margins of the leaf shut quite tightly on one another, and are beset with little teeth. Numerous little glands (_Dr_) secrete a digestive juice, and after some days, or even weeks, the insoluble remains of the minute animals may be found inside the trap.

Many more cases of animal-catching plants might be adduced, but it is far from my intention to try to describe all the existing contrivances; those already mentioned may suffice to give an idea of the diversity and of the detailed effectiveness of these adaptations. They amplify--so it seems to me--our conception of the scope of natural selection, by showing us that adaptations may arise which are quite foreign to the original mode of life of the organism in question, and stand, indeed, in apparent contradiction to its fundamental physiological processes. It is hardly necessary to enter into a special argument to show that they can only have been brought about in the course of natural selection, since every other interpretation of their occurrence fails. Neither climatic nor any other external direct influence could have effected these modifications of the parts of plants, which are all so different, yet all so well suited to their purpose; they are different even in plants growing quite close together, like the sundew and the butterwort. The Lamarckian principle of use and disuse hardly enters into the question at all, since plants do not possess a will, and we can hardly speak of 'chance' where we have to do with such complex and diversely combined transformations. A process of selection actually operative in each of these cases can easily be thought out, and I shall leave it to my readers themselves to do this, and shall only indicate that we have to do with increasing elaboration in two different directions: first, improvements in the ability to utilize animal substances which happened to stick to the leaves, and second, an increase in the probability of animals sticking to the leaves, and so becoming available. Thus there arose, on the one hand, dissolving and digestive juices, and arrangements for absorption; and, on the other hand, viscid slime, and traps of various kinds to secure the animals, as well as honey and bright colours to attract them.

But it is not merely transformations in the form of the stems and leaves which have come about; there are also important physiological changes. The sensitiveness to stimulus of various parts of the leaf is greatly increased, to a certain extent in the butterwort, the edges of whose leaves turn inwards in response to stimulus, still more in the sundew, in which the stimulus is conveyed from the tentacles touched to all the others, but most wonderfully of all in the Venus fly-trap and _Aldrovandia_, whose sensitive hairs so transmit the stimulus that the whole leaf is affected by it, and is set in motion, in a manner quite comparable to the effects of a nerve-stimulus in animals.

Thus the case of carnivorous or insectivorous plants shows us that, in the course of natural selection, quite new organs can be produced in a plant by a thoroughgoing transformation of old ones, as, for instance, the pitchers of _Nepenthes_, and that, furthermore, even the physiological capacities of the plant may be changed in the most far-reaching manner, increasing and varying until they come to resemble the functions of the animal body.

LECTURE VIII

THE INSTINCTS OF ANIMALS

The robber-wasp--Statement of the problem--Material basis of instincts--Instincts are not 'inherited habits'--Instinct of self-preservation--Fugitive instinct: death-feigning--Masking of crabs--Nutritive instinct--Monophagous caterpillars--Diverse modes of acquiring food: May-flies, sea-cucumbers, fishes that snare--'Aberration' of instinct--Change of instinct during metamorphosis: Eristalis, Sitaris--Imperfection of adaptation points to origin through natural selection--Instinct and will--Instincts and protective coloration--Leisurely flight of Heliconiidæ--Rapid flight of Papilionidæ--Instincts which act only once in a lifetime--Pupation of butterflies--Pupation of the Longicorns--Pupation of the silk-moth--The emperor moth--The cocoons of Atlas--Oviposition of butterflies.

WE have hitherto considered animals with especial regard to the variation and re-adaptation of morphological characters, e.g. modifications of form and colour; and we have now to ask whether their behaviour also is to be referred as to its origin, in whole or in part, to the principle of selection. All around us we can see that animals know how to use their parts or organs in a purposeful manner: the duckling swims at once upon the water; the chicken which has just been hatched from the egg pecks at the seeds lying on the ground; the butterfly but newly emerged from the pupa, as soon as its wings have dried and hardened, knows how to use them in flight; and the predatory wasp requires no instruction to recognize her victim, a particular caterpillar, a grasshopper, or some other definite insect; she knows how to attack it, to paralyse it by stings, and then hesitates not a moment as to what she has to do next; she drags it to her nest, deposits it in one of the cells already prepared for her future brood, lays a single egg upon it, and roofs the cell carefully over. It is only because all these complex acts are so precisely performed, as precisely as if the wasp knew why she performed them, that the species is able to maintain its existence, for only thus can the rearing of the next generation be secured. Out of the egg there slips a little larva, which at once makes for the paralysed victim, feeds upon it, and grows thereby, then, within the shelter of the closed cell, passes through the pupa stage and is transformed into a perfect wasp. Many species of these predatory wasps do not lay the egg directly beside or upon their prey, but lest its movements should endanger their offspring, they hang the egg above it by a silken thread. It is thus in security, and the young larva, too, when it appears, can withdraw to its safely swinging resting-place as soon as danger threatens from the convulsive struggles of the unfortunate victim at whose body it is gnawing.

Every animal has a great many such 'instincts,' which lead it, indeed force it, to act appropriately towards an end, without having any consciousness of that end. For how should the butterfly know what flying is, or that it possessed the power of flight at all, or who could have shown the predatory wasp, when she wakened from the pupa sleep to quite a new kind of life, all that she had to do in order to procure food for herself and to secure shelter and nourishment for the brood which was still enclosed within her ovary? Since species have developed from other species, these regulators of the body, the instincts, cannot have been the same in earlier times; they must have evolved out of the instincts of ancestors, and the questions we have to ask are: By what factors? In what way? Has the principle of selection been operative here too, or can we refer instincts to the inherited effects of use and disuse?

Before I enter upon this question it is necessary to consider for a little the physiological basis of instinct. We can distinguish three kinds of actions: purely reflex, purely instinctive, and purely conscious actions. In the case of the first, we see most clearly that they depend on an existing mechanism, for they follow of necessity on a particular stimulus, and cannot always be suppressed. Bright light striking our eye makes the pupil narrower by a contraction of the iris, and in the same way our eyelids close if a finger be thrust suddenly towards them. We know, too, the principle of these reflex mechanisms; they depend on nerve connexions. Sensory nerves are so connected in the nerve-centres with motor nerves, that a stimulus affecting the former at the periphery of the body, as at the eye, is carried to certain nerve-cells of the brain, and from these it excites to activity certain motor centres, so that definite movements are set up. It is rarely only one muscle that is thus excited to activity, there are usually several, and here we have the transition to instinctive action, which consists in a longer or shorter series of actions, that is, of motor combinations. These, too, are originally, at least, set a-going by a sense impression, an external stimulus which affects a sensory nerve exactly in the same way as in the reflex mechanism, and this stimulus is carried to a particular group of sensory nerve-cells in the central nervous organ, and from these transmitted by very fine inter-connexions to motor centres. There are extraordinarily complex instinctive actions, and in these the completion of one action is obviously the stimulus to the second, the completion of the second to the third, and so on, until the entire chain of inter-dependent movements which make up the whole performance has been completed.

Instincts have thus a material basis in the cells and fibres of the nervous system, and through variations in the connexions and irritability of these nervous parts they too can be modified, like any of the other characters of the body, such as form and colour.

Conscious actions depend directly on the will, and they have a close connexion with instinctive actions in as far as these also can be controlled by the will, that is, can be set a-going or inhibited, and also, on the other hand, in as far as purely voluntary actions may become instinctive through frequent repetition. The first case is illustrated, for instance, when the suckling of a child at the mother's breast is continued into the second year of life, as not infrequently happens in the southern countries of Europe. Such a child knows exactly why it wants the breast, and its action is a conscious one, while the newborn child seeks about with the mouth instinctively, and when it has found what it sought performs the somewhat complex sucking movements automatically. The second case is illustrated, when, for instance, we have made a habit of winding up a watch on going to bed, and do it when we happen to change our clothes through the day, although it is then purposeless and would have been omitted if the action had required a conscious effort of will. One can often observe on oneself in how short a time a conscious action may become instinctive. I once sent my keyless watch to a watchmaker for repairs, and received from him for the time an ordinary watch, which had to be wound with a key, which key I kept for safety in my purse. At the end of eight days I got back my own watch, and on undressing the first night I found myself 'instinctively' taking my purse from my pocket in order to get the key, which, as I very well knew, I no longer needed. And that a long series of complex movements, originally performed only consciously, may be gone through instinctively, is shown by the fact that pieces of music which have been learnt by heart can often be played without mistake from beginning to end while the player is thinking of quite other things. The complex instinctive actions of animals are performed in quite a similar manner.

There is thus no sharp boundary line between reflex and instinctive actions, nor between instinctive and conscious actions, but one passes over into the other, and the thought suggests itself, that in the phyletic development also transitions from one kind of action to the other must have taken place. As long as one believes the Lamarckian principle to be really operative one can suppose that actions, which were originally dependent on the will, when they were often repeated, became instinctive, or, in other words, that instincts, many of them at least, are inherited habits.

I shall endeavour to show later on that this assumption, plausible as it seems at first sight, cannot be correct; but in the meantime I must confine myself to saying that there are a great number of instincts which must be referred to the process of selection, and that the rest can be similarly interpreted in their essentials at least.

The instinct of self-preservation is universally distributed, and it is exhibited in many animals by flight from their enemies. The hare flees from the fox and from men; the bird flies away at the approach of the cat; the butterfly flies from even the shadow of the net spread to catch it. These might be regarded as purely conscious actions, and in the case of the hare and the bird experience and will have undoubtedly some part in them, but even in these the basis of the action is an organic impulse; this, and not reflection, causes the animal to flee at sight of an enemy. In the butterfly, indeed, this must be purely instinctive, since it is done with the same precision immediately on leaving the pupa state, before the animal has had any experience. But even in the case of the hare and the bird, taking to flight would in most cases come too late if reflection were necessary first; if it is to be effective it must take place as instantaneously as the shutting of the lids when danger threatens the eye.

The hermit-crab (Fig. 34, p. 163), which conceals its soft abdomen in an empty mollusc shell, and drags that about with it on the floor of the sea, withdraws with lightning-like rapidity into its house as soon as any suspicious movement catches its eye, and it is very difficult to grip one of its legs with the forceps in time to draw it out of its shell. The same is the case with the so-called Serpulids, worms of the genus _Serpula_, and its allies; it is not easy to seize them, because, however quick one is with the forceps, their instinct of fugitive self-preservation acts more quickly still, and they shoot back into their protecting tubes before one has had time to grasp them. But this impulse to flee from enemies, though it seems almost a matter of course, is by no means common to all animals, for in quite a large number the instinct of self-preservation finds expression in an exactly contrary manner, namely, in the so-called 'death-feigning,' that is, remaining absolutely motionless in a definite position precisely prescribed to the animal by its instinct. In speaking of protective colouring, I drew attention to the 'wood-moth' (_Xylina_), which resembles a broken fragment of half-decayed wood so deceptively, and I pointed out that the colour-resemblance to wood would be in itself of but little use to the insect if it were not combined with the instinct to remain motionless in danger, to 'feign death.' The antennæ and legs are drawn close to the body, so that they rather heighten the disguise, and, instead of running away, the insect does not move a muscle until the danger is past. This instinct must have evolved hand in hand with the resemblance to a piece of wood, and, just as we sought to interpret the latter from the fact that the moths which most resembled the wood had always the best chance of surviving, so we maintain that those moths would profit most by their resemblance which drew in their legs and antennæ closely and lay most perfectly still. Thus the brain-mechanism, which effected the keeping still whenever the senses announced danger, would be more and more firmly established and perfected in the course of selection.

Even nearly related animals may have quite different instincts which secure them against danger. Thus in the group of pocket crabs (Notopoda) there are some species which run away when danger threatens, but others which anticipate the risk of discovery by masking themselves to a certain extent. With the last pair of legs they hold over themselves a large piece of sponge, which then grows till it often leaves only the limbs and frontal region uncovered. Of course there can be no question of consciousness in what the crab does, as is proved by the fact that these crabs will, in case of necessity, take a transparent piece of glass instead of the sponge; but the impulse to cover themselves with something is strong in them, and finds expression not only when they see a really protective substance, but even when they see one which is transparent and therefore wholly useless for the purpose. Crabs from which the sponge has been taken away wander about until they find another; the impulse is thus set up not only by the sight of the sponge or of a stone, but also by the feeling that their back is uncovered. The large spider-crab of the Mediterranean (_Maja squinado_) effects its disguise in a somewhat different manner. It has peculiar hooked bristles on the back, and on these it hooks little bunches of seaweed, often many of them, so that it is entirely covered and looks like a bunch of wrack rather than like an animal. Here again a bodily variation has gone hand in hand with the development of the instinct to cover itself: the bristles of the back have become hooked. Many instincts are accompanied by structural modifications, and in the crabs which cover themselves with sponge or stone this is the case, for the last pair of thoracic legs is turned towards the back, instead of being set at the side of the body, as is usual among crabs. They are thus enabled to hold the sponge much better and more permanently, and as this is advantageous we may well ascribe the change to natural selection.

Let us now turn our attention to another category of instincts, the most common and most indispensable of all, those which lead to the seeking and devouring of food.

The chicken just emerged from the egg picks up the seeds thrown to it with no experience of what eating is, or what can be made to serve it as food; its instinct for food expresses itself in picking up, and it is awakened or stimulated to action by sight of the seeds. As Lloyd Morgan in his excellent book on _Habit and Instinct_ well says, 'It does not pick at the seeds because instinct says to it that this is something to be picked up and tested, but because it cannot do anything else.'

In the same way the instinct to seek for food wakes in the kitten at the sight of a mouse. I once set before a kitten which had never seen a mouse a living one in a trap. The kitten became greatly excited, and when I opened the trap and let the mouse run away she overtook and caught it in a few bounds. The instinct in this case does not express itself as in the chicken by the rapid lowering of the head and seizing the food, but in quite a different combination of movements, in running after and grasping the fleeing victim. But that is not all that is included in the instinctive action in the case of the cat, for there is also the whole wild and gruesome play, the familiar letting go and catching again, the passionate growling of satisfaction which, in its wildness, reminds us much more of a blood-thirsty tiger than of a tame domestic animal.