DARWINISMSTATED BY DARWIN HIMSELF.
The Power of Movement in Plants,page 1.
Themost widely prevalent movement is essentially of the same nature as that of the stem of a climbing plant, which bends successively to all points of the compass, so that the tip revolves. This movement has been called by Sachs “revolving nutation”; but we have found it much more convenient to use the termscircumnutationandcircumnutate. As we shall have to say much about this movement, it will be useful here briefly to describe its nature. If we observe a circumnutating stem, which happens at the time to be bent, we will say toward the north, it will be found gradually to bend more and more easterly, until it faces the east; and so onward to the south, then to the west, and back again to the north. If the movement had been quite regular, the apex would have described a circle, or rather, as the stem is always growing upward, a circular spiral. But it generally describes irregular elliptical or oval figures; for the apex, after pointing in any one direction, commonly moves back to the opposite side, not, however, returning alongthe same line. Afterward other irregular ellipses or ovals are successively described, with their longer axes directed to different points of the compass. While describing such figures, the apex often travels in a zigzag line, or makes small subordinate loops or triangles. In the case of leaves the ellipses are generally narrow.
Page 3.
Even the stems of seedlings before they have broken through the ground, as well as their buried radicles, circumnutate, as far as the pressure of the surrounding earth permits. In this universally present movement we have the basis or groundwork for the acquirement, according to the requirements of the plant, of the most diversified movements.
The Movements and Habits of Climbing Plants,page 202.
The most interesting point in the natural history of climbing plants is the various kinds of movement which they display in manifest relation to their wants. The most different organs—stems, branches, flower-peduncles, petioles, mid-ribs of the leaf and leaflets, and apparently aërial roots—all possess this power.
1. The first action of a tendril is to place itself in a proper position. For instance, the tendril ofCobæafirst rises vertically up, with its branches divergent and with the terminal hooks turned outward; the young shoot at the extremity of the stem is at the same time bent to one side, so as to be out of the way. The young leaves of clematis, on the other hand, prepare for action by temporarily curving themselves downward, so as to serve as grapnels.
2. If a twining plant or a tendril gets by any accident into an inclined position, it soon bends upward, thoughsecluded from the light. The guiding stimulus no doubt is the attraction of gravity, as Andrew Knight showed to be the case with germinating plants. If a shoot of any ordinary plant be placed in an inclined position in a glass of water in the dark, the extremity will, in a few hours, bend upward; and, if the position of the shoot be then reversed, the downward-bent shoot reverses its curvature; but if the stolon of a strawberry, which has no tendency to grow upward, be thus treated, it will curve downward in the direction of, instead of in opposition to, the force of gravity. As with the strawberry, so it is generally with the twining shoots of theHibbertia dentata, which climbs laterally from bush to bush; for these shoots, if placed in a position inclined downward, show little and sometimes no tendency to curve upward.
3. Climbing plants, like other plants, bend toward the light by a movement closely analogous to the incurvation which causes them to revolve, so that their revolving movement is often accelerated or retarded in traveling to or from the light. On the other hand, in a few instances tendrils bend toward the dark.
4. We have the spontaneous revolving movement which is independent of any outward stimulus, but is contingent on the youth of the part, and on vigorous health; and this again, of course, depends on a proper temperature and other favorable conditions of life.
5. Tendrils, whatever their homological nature may be, and the petioles or tips of the leaves of leaf-climbers, and apparently certain roots, all have the power of movement when touched, and bend quickly toward the touched side. Extremely slight pressure often suffices. If the pressure be not permanent, the part in question straightens itself and is again ready to bend on being touched.
6. Tendrils, soon after clasping a support, but notafter a mere temporary curvature, contract spirally. If they have not come into contact with any object, they ultimately contract spirally, after ceasing to revolve; but in this case the movement is useless, and occurs only after a considerable lapse of time.
With respect to the means by which these various movements are effected, there can be little doubt, from the researches of Sachs and H. de Vries, that they are due to unequal growth; but, from the reasons already assigned, I can not believe that this explanation applies to the rapid movements from a delicate touch.
Finally, climbing plants are sufficiently numerous to form a conspicuous feature in the vegetable kingdom, more especially in tropical forests. America, which so abounds with arboreal animals, as Mr. Bates remarks, likewise abounds, according to Mohl and Palm, with climbing plants; and, of the tendril-bearing plants examined by me, the highest developed kinds are natives of this grand continent, namely, the several species ofBignonia,Eccremocarpus,Cobæa, andAmpelopsis. But even in the thickets of our temperate regions the number of climbing species and individuals is considerable, as will be found by counting them.
Page 206.
It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement. It should rather be said that plants acquire and display this power only when it is of some advantage to them; this being of comparatively rare occurrence, as they are affixed to the ground, and food is brought to them by the air and rain. We see how high in the scale of organization a plant may rise,when we look at one of the more perfect tendril-bearers. It first places its tendrils ready for action, as a polypus places its tentacula. If the tendril be displaced, it is acted on by the force of gravity and rights itself. It is acted on by the light, and bends toward or from it, or disregards it, whichever maybe most advantageous. During several days the tendrils or internodes, or both, spontaneously revolve with a steady motion. The tendril strikes some object, and quickly curls round and firmly grasps it. In the course of some hours it contracts into a spire, dragging up the stem, and forming an excellent spring. All movements now cease. By growth the tissues soon become wonderfully strong and durable. The tendril has done its work, and has done it in an admirable manner.
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The Power of Movement in Plants,page 571.
It is impossible not to be struck with the resemblance between the foregoing movements of plants and many of the actions performed unconsciously by the lower animals. With plants an astonishingly small stimulus suffices; and even with allied plants one may be highly sensitive to the slightest continued pressure, and another highly sensitive to a slight momentary touch. The habit of moving at certain periods is inherited both by plants and animals; and several other points of similitude have been specified. But the most striking resemblance is the localization of their sensitiveness, and the transmission of an influence from the excited part to another which consequently moves. Yet plants do not, of course, possess nerves or a central nervous system; and we may infer that with animals such structures serve only for the more perfect transmission of impressions, and for the more complete intercommunication of the several parts.
The Effects of Cross and Self Fertilization in the Vegetable Kingdom,page 443.
There are two important conclusions which may be deduced from my observations: 1. That the advantages of cross-fertilization do not follow from some mysterious virtue in the mere union of two distinct individuals, but from such individuals having been subjected during previous generations to different conditions, or to their having varied in a manner commonly called spontaneous, so that in either case their sexual elements have been in some degree differentiated; and, 2. That the injury from self-fertilization follows from the want of such differentiation in the sexual elements. These two propositions are fully established by my experiments. Thus, when plants of theIpomœaand of theMimulus, which had been self-fertilized for the seven previous generations, and had been kept all the time under the same conditions, were intercrossed one with another, the offspring did not profit in the least by the cross.
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Page 451.
The curious cases of plants which can fertilize and be fertilized by any other individual of the same species, but are altogether sterile with their own pollen, become intelligible, if the view here propounded is correct, namely, that the individuals of the same species growing in a state of nature near together have not really been subjected during several previous generations to quite the same conditions.
Page 446.
It is obvious that the exposure of two sets of plants during several generations to different conditions can lead to no beneficial results, as far ascrossing is concerned, unless their sexual elements are thus affected. That every organism is acted on to a certain extent by a change in its environment will not, I presume, be disputed. It is hardly necessary to advance evidence on this head; we can perceive the difference between individual plants of the same species which have grown in somewhat more shady or sunny, dry or damp places. Plants which have been propagated for some generations under different climates or at different seasons of the year transmit different constitutions to their seedlings. Under such circumstances, the chemical constitution of their fluids and the nature of their tissues are often modified. Many other such facts could be adduced. In short, every alteration in the function of a part is probably connected with some corresponding, though often quite imperceptible, change in structure or composition.
Whatever affects an organism in any way, likewise tends to act on its sexual elements. We see this in the inheritance of newly acquired modifications, such as those from the increased use or disuse of a part, and even from mutilations if followed by disease. We have abundant evidence how susceptible the reproductive system is to changed conditions, in the many instances of animals rendered sterile by confinement; so that they will not unite, or, if they unite, do not produce offspring, though the confinement may be far from close; and of plants rendered sterile by cultivation. But hardly any cases afford more striking evidence how powerfully a change in the conditions of life acts on the sexual elements than those already given, of plants which are completely self-sterile in one country, and, when brought to another, yield, even in the first generation, a fair supply of self-fertilized seeds.
But it may be said, granting that changed conditions act on the sexual elements, How can two or more plants growing close together, either in their native country or in a garden, be differently acted on, inasmuch as they appear to be exposed to exactly the same conditions?
Page 447.
In my experiments withDigitalis purpurea, some flowers on a wild plant were self-fertilized, and others were crossed with pollen from another plant growing within two or three feet distance. The crossed and self-fertilized plants raised from the seeds thus obtained produced flower-stems in number as 100 to 47, and in average height as 100 to 70. Therefore, the cross between these two plants was highly beneficial; but how could their sexual elements have been differentiated by exposure to different conditions? If the progenitors of the two plants had lived on the same spot during the last score of generations, and had never been crossed with any plant beyond the distance of a few feet, in all probability their offspring would have been reduced to the same state as some of the plants in my experiments—such as the intercrossed plants of the ninth generation ofIpomœa, or the self-fertilized plants of the eighth generation ofMimulus, or the offspring from flowers on the same plant; and in this case a cross between the two plants ofDigitaliswould have done no good. But seeds are often widely dispersed by natural means, and one of the above two plants, or one of their ancestors, may have come from a distance, from a more shady or sunny, dry or moist place, or from a different kind of soil containing other organic seeds or inorganic matter.
Page 449.
Seeds often lie dormant for several years in the ground, and germinate when brought near the surface by any means, as by burrowing animals. They would probably be affected by the mere circumstance of having long lain dormant; for gardeners believe that the production of double flowers, and of fruit, is thus influenced. Seeds, moreover, which were matured during different seasons will have been subjected during the whole course of their development to different degrees of heat and moisture.
It has been shown that pollen is often carried by insects to a considerable distance from plant to plant. Therefore, one of the parents or ancestors of our two plants ofDigitalismay have been crossed by a distant plant growing under somewhat different conditions. Plants thus crossed often produce an unusually large number of seeds; a striking instance of this fact is afforded by theBignonia, which was fertilized by Fritz Müller with pollen from some adjoining plants and set hardly any seed, but, when fertilized with pollen from a distant plant, was highly fertile. Seedlings from a cross of this kind grow with great vigor, and transmit their vigor to their descendants. These, therefore, in the struggle for life, will generally beat and exterminate the seedlings from plants which have long grown near together under the same conditions, and will thus tend to spread.
Page 458.
Under a practical point of view, agriculturists and horticulturists may learn something from the conclusions at which we have arrived. Firstly,we see that the injury from the close breeding of animals and from the self-fertilization of plants does not necessarily depend on any tendency to disease or weakness of constitution common to the related parents, and only indirectly on their relationship, in so far as they are apt to resemble each other in all respects, including their sexual nature. And, secondly, that the advantages of cross-fertilization depend on the sexual elements of the parents having become in some degree differentiated by the exposure of their progenitors to different conditions, or from their having intercrossed with individuals thus exposed; or, lastly, from what we call in our ignorance spontaneous variation. He therefore who wishes to pair closely related animals ought to keep them under conditions as different as possible.
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Page 459.
As some kinds of plants suffer much more from self-fertilization than do others, so it probably is with animals from too close interbreeding. The effects of close interbreeding on animals, judging again from plants, would be deterioration in general vigor, including fertility, with no necessary loss of excellence of form; and this seems to be the usual result.
It is a common practice with horticulturists to obtain seeds from another place having a very different soil, so as to avoid raising plants for a long succession of generations under the same conditions; but, with all the species which freely intercross by the aid of insects or the wind, it would be an incomparably better plan to obtain seeds of the required variety, which had been raised for some generations under as different conditions as possible, and sow them in alternate rows with seeds matured in the old garden. The two stocks would then intercross, with a thorough blending of their whole organizations, and withno loss of purity to the variety; and this would yield far more favorable results than a mere exchange of seeds. We have seen in my experiments how wonderfully the offspring profited in height, weight, hardiness, and fertility, by crosses of this kind. For instance, plants ofIpomœathus crossed were to the intercrossed plants of the same stock, with which they grew in competition, as 100 to 78 in height, and as 100 to 51 in fertility; and plants ofEschscholtziasimilarly compared were as 100 to 45 in fertility. In comparison with self-fertilized plants the results are still more striking; thus cabbages derived from a cross with a fresh stock were to the self-fertilized as 100 to 22 in weight.
Florists may learn, from the four cases which have been fully described, that they have the power of fixing each fleeting variety of color, if they will fertilize the flowers of the desired kind with their own pollen for half a dozen generations, and grow the seedlings under the same conditions. But a cross with any other individual of the same variety must be carefully prevented, as each has its own peculiar constitution. After a dozen generations of self-fertilization, it is probable that the new variety would remain constant even if grown under somewhat different conditions; and there would no longer be any necessity to guard against intercrosses between the individuals of the same variety.
Page 460.
With respect to mankind, my son George has endeavored to discover by a statistical investigation whether the marriages of first cousins are at all injurious, although this is a degree of relationship which would not be objected to in our domestic animals; and he has come to the conclusion from his own researches,and those of Dr. Mitchell, that the evidence as to any evil thus caused is conflicting, but on the whole points to its being very small. From the facts given in this volume we may infer that with mankind the marriages of nearly related persons, some of whose parents and ancestors had lived under very different conditions, would be much less injurious than that of persons who had always lived in the same place and followed the same habits of life. Nor can I see reason to doubt that the widely different habits of life of men and women in civilized nations, especially among the upper classes, would tend to counterbalance any evil from marriages between healthy and somewhat closely related persons.
Page 461.
Under a theoretical point of view it is some gain to science to know that numberless structures in hermaphrodite plants, and probably in hermaphrodite animals, are special adaptations for securing an occasional cross between two individuals; and that the advantages from such a cross depend altogether on the beings which are united, or their progenitors, having had their sexual elements somewhat differentiated, so that the embryo is benefited in the same manner as is a mature plant or animal by a slight change in its conditions of life, although in a much higher degree.
Another and more important result may be deduced from my observations. Eggs and seeds are highly serviceable as a means of dissemination, but we now know that fertile eggs can be produced without the aid of the male. There are also many other methods by which organisms can be propagated asexually. Why then have the two sexes been developed, and why do males existwhich can not themselves produce offspring? The answer lies, as I can hardly doubt, in the great good which is derived from the fusion of two somewhat differentiated individuals; and with the exception of the lowest organisms this is possible only by means of the sexual elements, these consisting of cells separated from the body, containing the germs of every part, and capable of being fused completely together.
It has been shown in the present volume that the offspring from the union of two distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advantage in height, weight, constitutional vigor and fertility over the self-fertilized offspring from one of the same parents. And this fact is amply sufficient to account for the development of the sexual elements, that is, for the genesis of the two sexes.
It is a different question why the two sexes are sometimes combined in the same individual, and are sometimes separated. As with many of the lowest plants and animals the conjugation of two individuals, which are either quite similar or in some degree different is a common phenomenon, it seems probable, as remarked in the last chapter, that the sexes were primordially separate. The individual which receives the contents of the other, may be called the female; and the other, which is often smaller and more locomotive, may be called the male; though these sexual names ought hardly to be applied as long as the whole contents of the two forms are blended into one. The object gained by the two sexes becoming united in the same hermaphrodite form probably is to allow of occasional or frequent self-fertilization, so as to insure the propagation of the species, more especially in the case of organisms affixed for life to the same spot.There does not seem to be any great difficulty in understanding how an organism, formed by the conjugation of two individuals which represented the two incipient sexes, might have given rise by budding first to a monœcious and then to an hermaphrodite form; and in the case of animals even without budding to an hermaphrodite form, for the bilateral structure of animals perhaps indicates that they were aboriginally formed by the fusion of two individuals.
Page 463.
It is a more difficult problem why some plants, and apparently all the higher animals, after becoming hermaphrodites, have since had their sexes reseparated. This separation has been attributed by some naturalists to the advantages which follow from a division of physiological labor. The principle is intelligible when the same organ has to perform at the same time diverse functions; but it is not obvious why the male and female glands, when placed in different parts of the same compound or simple individual, should not perform their functions equally well as when placed in two distinct individuals. In some instances the sexes may have been reseparated for the sake of preventing too frequent self-fertilization; but this explanation does not seem probable, as the same end might have been gained by other and simpler means, for instance, dichogamy. It may be that the production of the male and female reproductive elements and the maturation of the ovules was too great a strain and expenditure of vital force for a single individual to withstand, if endowed with a highly complex organization; and that at the same time there was no need for all the individuals to produce young, and consequentlythat no injury, on the contrary, good, resulted from half of them, or the males, failing to produce offspring.
The Different Forms of Flowers,page 290.
Thirteen bushes (of the spindle-tree) growing near one another in a hedge consisted of eight females quite destitute of pollen, and of five hermaphrodites with well-developed anthers. In the autumn the eight females were well covered with fruit, excepting one which bore only a moderate number. Of the five hermaphrodites, one bore a dozen or two fruits, and the remaining four bushes several dozen; but their number was as nothing compared with those on the female bushes, for a single branch, between two and three feet in length, from one of the latter, yielded more than any one of the hermaphrodite bushes. The difference in the amount of fruit produced by the two sets of bushes is all the more striking, as from the sketches above given it is obvious that the stigmas of the polleniferous flowers can hardly fail to receive their own pollen; while the fertilization of the female flowers depends on pollen being brought to them by flies and the smallerHymenoptera, which are far from being such efficient carriers as bees.
I now determined to observe more carefully during successive seasons some bushes growing in another place about a mile distant. As the female bushes were so highly productive, I marked only two of them with the letters A and B, and five polleniferous bushes with the letters C to G. I may premise that the year 1865 was highly favorable for the fruiting of all the bushes, especially for the polleniferous ones, some of which werequite barren, except under such favorable conditions. The season of 1864 was unfavorable. In 1863 the female A produced “some fruit”; in 1864 only nine; and in 1865 ninety-seven fruit. The female B in 1863 was “covered with fruit”; in 1864 it bore twenty-eight; and in 1865 “innumerable very fine fruits.” I may add that three other female trees growing close by were observed, but only during 1863, and they then bore abundantly. With respect to the polleniferous bushes, the one marked C did not bear a single fruit during the years 1863 and 1864, but during 1865 it produced no less than ninety-two fruit, which, however, were very poor. I selected one of the finest branches with fifteen fruit, and these contained twenty seeds, or on an average 1·33 per fruit. I then took by hazard fifteen fruit from an adjoining female bush, and these contained forty-three seeds; that is, more than twice as many, or on an average 2·86 per fruit. Many of the fruits from the female bushes included four seeds, and only one had a single seed; whereas, not one fruit from the polleniferous bushes contained four seeds. Moreover, when the two lots of seeds were compared, it was manifest that those from the female bushes were the larger. The second polleniferous bush, D, bore in 1863 about two dozen fruit, in 1864 only three very poor fruit, each containing a single seed; and in 1865, twenty equally poor fruit. Lastly, the three polleniferous bushes, E, F, and G, did not produce a single fruit during the three years 1863, 1864, and 1865.
Page 293.
A tendency to the separation of the sexes in the cultivated strawberry seems to be much more strongly marked in the United States than in Europe;and this appears to be the result of the direct action of climate on the reproductive organs. In the best account which I have seen, it is stated that many of the varieties in the United States consist of three forms, namely, females, which produce a heavy crop of fruit; of hermaphrodites, which “seldom produce other than a very scanty crop of inferior and imperfect berries”; and of males, which produce none. The most skillful cultivators plant “seven rows of female plants, then one row of hermaphrodites, and so on throughout the field.” The males bear large, the hermaphrodites mid-sized, and the females small flowers. The latter plants produce few runners, while the two other forms produce many; consequently, as has been observed both in England and in the United States, the polleniferous forms increase rapidly and tend to supplant the females. We may therefore infer that much more vital force is expended in the production of ovules and fruit than in the production of pollen.
The Different Forms of Flower,page 345.
If the sexual elements belonging to the same form are united, the union is an illegitimate one, and more or less sterile. With dimorphic species two illegitimate unions, and with trimorphic species twelve are possible. There is reason to believe that the sterility of these unions has not been specially acquired, but follows as an incidental result from the sexual elements of the two or three forms having been adapted to act on one another in a particular manner, so that any other kind of union is inefficient, like that between distinct species. Another and still more remarkable incidental result is that the seedlings from an illegitimate union are often dwarfed and more or less completelybarren, like hybrids from the union of two widely distinct species.
Fertilization of Orchids by Insects,page 245.
It is interesting to look at one of the magnificent exotic species (orchids), or, indeed, at one of our humblest forms, and observe how profoundly it has been modified, as compared with all ordinary flowers—with its great labellum, formed of one petal and two petaloid stamens; with its singular pollen-masses, hereafter to be referred to; with its column formed of seven cohering organs, of which three alone perform their proper function, namely, one anther and two generally confluent stigmas; with the third stigma modified into the rostellum and incapable of being fertilized; and with three of the anthers no longer functionally active, but serving either to protect the pollen of the fertile anther or to strengthen the column, or existing as mere rudiments, or entirely suppressed. What an amount of modification, cohesion, abortion, and change of function do we here see! Yet hidden in that column, with its surrounding petals and sepals, we know that there are fifteen groups of vessels, arranged three within three, in alternate order, which probably have been preserved to the present time from being developed at a very early period of growth, before the shape or existence of any part of the flower is of importance for the well-being of the plant.
Can we feel satisfied by saying that each orchid was created, exactly as we now see it, on a certain “ideal type”; that the omnipotent Creator, having fixed on one plan for the whole order, did not depart from this plan; that he, therefore, made the same organ to perform diversefunctions—often of trifling importance compared with their proper function—converted other organs into mere purposeless rudiments, and arranged all as if they had to stand separate, and then made them cohere? Is it not a more simple and intelligible view that all theOrchideæowe what they have in common to descent from some monocotyledonous plant, which, like so many other plants of the same class, possessed fifteen organs, arranged alternately, three within three, in five whorls; and that the now wonderfully changed structure of the flower is due to a long course of slow modification—each modification having been preserved which was useful to the plant, during the incessant changes to which the organic and inorganic world has been exposed?
Fertilization of Orchids,page 282.
It has, I think, been shown that theOrchideæexhibit an almost endless diversity of beautiful adaptations. When this or that part has been spoken of as adapted for some special purpose, it must not be supposed that it was originally always formed for this sole purpose. The regular course of events seems to be, that a part which originally served for one purpose becomes adapted by slow changes for widely different purposes. To give an instance: in all theOphreæ, the long and nearly rigid caudicle manifestly serves for the application of the pollen-grains to the stigma, when the pollinia are transported by insects to another flower; and the anther opens widely in order that the pollinium should be easily withdrawn; but, in theBee ophrys, the caudicle, by a slight increase in length and decrease in its thickness, and by the anther opening a little more widely, becomes specially adapted for the very different purposeof self-fertilization, through the combined aid of the weight of the pollen-mass and the vibration of the flower when moved by the wind. Every gradation between these two states is possible—of which we have a partial instance inO. aranifera.
Again, the elasticity of the pedicel of the pollinium in someVandeæis adapted to free the pollen-masses from their anther-cases; but, by a further slight modification, the elasticity of the pedicel becomes specially adapted to shoot out the pollinium with considerable force, so as to strike the body of the visiting insect. The great cavity in the labellum of manyVandeæis gnawed by insects, and thus attracts them; but inMormodes igneait is greatly reduced in size, and serves in chief part to keep the labellum in its new position on the summit of the column. From the analogy of many plants we may infer that a long, spur-like nectary is primarily adapted to secrete and hold a store of nectar; but in many orchids it has so far lost this function that it contains fluid only in the intercellular spaces. In those orchids in which the nectary contains both free nectar and fluid in the intercellular spaces, we can see how a transition from the one state to the other could be effected, namely, by less and less nectar being secreted from the inner membrane, with more and more retained within the intercellular spaces. Other analogous cases could be given.
Although an organ may not have been originally formed for some special purpose, if it now serves for this end, we are justified in saying that it is specially adapted for it. On the same principle, if a man were to make a machine for some special purpose, but were to use old wheels, springs, and pulleys, only slightly altered, the whole machine, with all its parts, might be said to be specially contrived for its present purpose. Thus throughoutnature almost every part of each living being has probably served, in a slightly modified condition, for diverse purposes, and has acted in the living machinery of many ancient and distinct specific forms.
In my examination of orchids, hardly any fact has struck me so much as the endless diversities of structure—the prodigality of resources—for gaining the very same end, namely, the fertilization of one flower by pollen from another plant. This fact is to a large extent intelligible on the principle of natural selection. As all the parts of a flower are co-ordinated, if slight variations in any one part were preserved from being beneficial to the plant, then the other parts would generally have to be modified in some corresponding manner. But these latter parts might not vary at all, or they might not vary in a fitting manner, and these other variations, whatever their nature might be, which tended to bring all the parts into more harmonious action with one another, would be preserved by natural selection.
Page 284.
To give a simple illustration: in many orchids the ovarium (but sometimes the foot-stalk) becomes for a period twisted, causing the labellum to assume the position of a lower petal, so that insects can easily visit the flower; but from slow changes in the form or position of the petals, or from new sorts of insects visiting the flowers, it might be advantageous to the plant that the labellum should resume its normal position on the upper side of the flower, as is actually the case withMalaxis paludosa, and some species ofCatasetum, etc. This change, it is obvious, might be simply effected by the continued selection of varieties which had their ovaria less and less twisted; but, if theplant only afforded varieties with the ovarium more twisted, the same end could be attained by the selection of such variations, until the flower was turned completely round on its axis. This seems to have actually occurred withMalaxis paludosa, for the labellum has acquired its present upward position by the ovarium being twisted twice as much as is usual.
Again, we have seen that in mostVandeæthere is a plain relation between the depth of the stigmatic chamber and the length of the pedicel, by which the pollen-masses are inserted; now, if the chamber became slightly less deep from any change in the form of the column, or other unknown cause, the mere shortening of the pedicel would be the simplest corresponding change; but, if the pedicel did not happen to vary in shortness, the slightest tendency to its becoming bowed from elasticity, as inPhalænopsis, or to a backward hygrometric movement, as in one of theMaxillarias, would be preserved, and the tendency would be continually augmented by selection; thus the pedicel, as far as its action is concerned, would be modified in the same manner as if it had been shortened. Such processes carried on during many thousand generations in various ways, would create an endless diversity of co-adapted structures in the several parts of the flower for the same general purpose. This view affords, I believe, the key which partly solves the problem of the vast diversity of structure adapted for closely analogous ends in many large groups of organic beings.
Page 285.
The more I study nature, the more I become impressed, with ever-increasing force, that the contrivances and beautiful adaptations slowlyacquired through each part occasionally varying in a slight degree but in many ways, with the preservation of those variations which were beneficial to the organism under complex and ever-varying conditions of life, transcend in an incomparable manner the contrivances and adaptations which the most fertile imagination of man could invent.
The use of each trifling detail of structure is far from a barren search to those who believe in natural selection. When a naturalist casually takes up the study of an organic being, and does not investigate its whole life (imperfect though that study will ever be), he naturally doubts whether each trifling point can be of any use, or, indeed, whether it be due to any general law. Some naturalists believe that numberless structures have been created for the sake of mere variety and beauty—much as a workman would make different patterns. I, for one, have often and often doubted whether this or that detail of structure in many of theOrchideæand other plants could be of any service; yet, if of no good, these structures could not have been modeled by the natural preservation of useful variations; such details can only be vaguely accounted for by the direct action of the conditions of life, or the mysterious laws of correlated growth.
Fertilization of Orchids,page 2.
This treatise affords me also an opportunity of attempting to show that the study of organic beings may be as interesting to an observer who is fully convinced that the structure of each is due to secondary laws as to one who views every trifling detail of structure as the result of the direct interposition of the Creator.
The Power of Movement in Plants,page 280.
The so-called sleep of leaves is so conspicuous a phenomenon that it was observed as early as the time of Pliny; and since Linnæus published his famous essay, “Somnus Plantarum,” it has been the subject of several memoirs. Many flowers close at night, and these are likewise said to sleep; but we are not here concerned with their movements, for although effected by the same mechanism as in the case of young leaves, namely, unequal growth on the opposite sides (as first proved by Pfeffer), yet they differ essentially in being excited chiefly by changes of temperature instead of light; and in being effected, as far as we can judge, for a different purpose. Hardly any one supposes that there is any real analogy between the sleep of animals and that of plants, whether of leaves or flowers. It seems, therefore, advisable to give a distinct name to the so-called sleep-movements of plants. These have also generally been confounded, under the term “periodic,” with the slight daily rise and fall of leaves, as described in the fourth chapter; and this makes it all the more desirable to give some distinct name to sleep-movements. Nyctitropism and nyctitropic, i. e., night-turning, may be applied both to leaves and flowers, and will be occasionally used by us; but it would be best to confine the term to leaves.
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Page 281.
Leaves, when they go to sleep, move either upward or downward, or, in the case of the leaflets of compound leaves, forward, that is, toward the apex of the leaf, or backward, that is, toward its base; or, again, they may rotate on their own axis without moving either upward or downward. But in almost every case the plane of the blade is so placed as to stand nearlyor quite vertically at night. Therefore the apex, or the base, or either lateral edge, may be directed toward the zenith. Moreover, the upper surface of each leaf, and more especially of each leaflet, is often brought into close contact with that of the opposite one; and this is sometimes effected by singularly complicated movements. This fact suggests that the upper surface requires more protection than the lower one. For instance, the terminal leaflet in trifolium, after turning up at night so as to stand vertically, often continues to bend over until the upper surface is directed downward, while the lower surface is fully exposed to the sky; and an arched roof is thus formed over the two lateral leaflets, which have their upper surfaces pressed closely together. Here we have the unusual case of one of the leaflets not standing vertically, or almost vertically, at night.
Considering that leaves in assuming their nyctitropic positions often move through an angle of 90°; that the movement is rapid in the evening; that in some cases it is extraordinarily complicated; that with certain seedlings, old enough to bear true leaves, the cotyledons move vertically upward at night, while at the same time the leaflets move vertically downward; and that in the same genus the leaves or cotyledons of some species move upward, while those of other species move downward—from these and other such facts, it is hardly possible to doubt that plants must derive some great advantage from such remarkable powers of movement.
Page 284.
The fact that the leaves of many plants place themselves at night in widely different positions from what they hold during the day, but withthe one point in common, that their upper surfaces avoid facing the zenith, often with the additional fact that they come into close contact with opposite leaves or leaflets, clearly indicates, as it seems to us, that the object gained is the protection of the upper surfaces from being chilled at night by radiation. There is nothing improbable in the upper surface needing protection more than the lower, as the two differ in function and structure. All gardeners know that plants suffer from radiation. It is this, and not cold winds, which the peasants of Southern Europe fear for their olives. Seedlings are often protected from radiation by a very thin covering of straw; and fruit-trees on walls by a few fir-branches, or even by a fishing-net, suspended over them. There is a variety of the gooseberry, the flowers of which, from being produced before the leaves, are not protected by them from radiation, and consequently often fail to yield fruit. An excellent observer has remarked that one variety of the cherry has the petals of its flowers much curled backward, and after a severe frost all the stigmas were killed; while, at the same time, in another variety with incurved petals, the stigmas were not in the least injured.