The frequency of the visits of bees is also sometimes shown by the manner in which the petals are scratched by their hooked tarsi; I have seen large beds of Mimulus, Stachys, and Lathyrus with the beauty of their flowers thus sadly defaced.
PERFORATION OF THE COROLLA BY BEES.
I have already alluded to bees biting holes in flowers for the sake of obtaining the nectar. They often act in this manner, both with endemic and exotic species, in many parts of Europe, in the United States, and in the Himalaya; and therefore probably in all parts of the world. The plants, the fertilisation of which actually depends on insects entering the flowers, will fail to produce seed when their nectar is stolen from the outside; and even with those species which are capable of fertilising themselves without any aid, there can be no cross-fertilisation, and this, as we know, is a serious evil in most cases. The extent to which humble-bees carry on the practice of biting holes is surprising: a remarkable case was observed by me near Bournemouth, where there were formerly extensive heaths. I took a long walk, and every now and then gathered a twig of Erica tetralix, and when I had got a handful all the flowers were examined through a lens. This process was repeated many times; but though many hundreds were examined, I did not succeed in finding a single flower which had not been perforated. Humble-bees were at the time sucking the flowers through these perforations. On the following day a large number of flowers were examined on another heath with the same result, but here hive-bees were sucking through the holes. This case is all the more remarkable, as the innumerable holes had been made within a fortnight, for before that time I saw the bees everywhere sucking in the proper manner at the mouths of the corolla. In an extensive flower-garden some large beds of Salvia grahami, Stachys coccinea, and Pentstemon argutus (?) had every flower perforated, and many scores were examined. I have seen whole fields of red clover (Trifolium pratense) in the same state. Dr. Ogle found that 90 per cent of the flowers of Salvia glutinosa had been bitten. In the United States Mr. Bailey says it is difficult to find a blossom of the native Gerardia pedicularia without a hole in it; and Mr. Gentry, in speaking of the introduced Wistaria sinensis, says "that nearly every flower had been perforated." (11/12. Dr. Ogle "Pop. Science Review" July 1869 page 267. Bailey "American Naturalist" November 1873 page 690.
Gentry ibid May 1875 page 264.)
As far as I have seen, it is always humble-bees which first bite the holes, and they are well fitted for the work by possessing powerful mandibles; but hive-bees afterwards profit by the holes thus made. Dr.
Hermann Muller, however, writes to me that hive-bees sometimes bite holes through the flowers of Erica tetralix. No insects except bees, with the single exception of wasps in the case of Tritoma, have sense enough, as far as I have observed, to profit by the holes already made.
Even humble-bees do not always discover that it would be advantageous to them to perforate certain flowers. There is an abundant supply of nectar in the nectary of Tropaeolum tricolor, yet I have found this plant untouched in more than one garden, while the flowers of other plants had been extensively perforated; but a few years ago Sir J. Lubbock"s gardener a.s.sured me that he had seen humble-bees boring through the nectary of this Tropaeolum. Muller has observed humble-bees trying to suck at the mouths of the flowers of Primula elatior and of an Aquilegia, and, failing in their attempts, they made holes through the corolla; but they often bite holes, although they could with very little more trouble obtain the nectar in a legitimate manner by the mouth of the corolla.
Dr. W. Ogle has communicated to me a curious case. He gathered in Switzerland 100 flower-stems of the common blue variety of the monkshood (Aconitum napellus), and not a single flower was perforated; he then gathered 100 stems of a white variety growing close by, and every one of the open flowers had been perforated. (11/13. Dr. Ogle "Popular Science Review" July 1869 page 267. Bailey "American Naturalist" November 1873 page 690. Gentry ibid May 1875 page 264.) This surprising difference in the state of the flowers may be attributed with much probability to the blue variety being distasteful to bees, from the presence of the acrid matter which is so general in the Ranunculaceae, and to its absence in the white variety in correlation with the loss of the blue tint.
According to Sprengel, this plant is strongly proterandrous (11/14. "Das Entdeckte" etc. page 278.); it would therefore be more or less sterile unless bees carried pollen from the younger to the older flowers.
Consequently the white variety, the flowers of which were always bitten instead of being properly entered by the bees, would fail to yield the full number of seeds and would be a comparatively rare plant, as Dr.
Ogle informs me was the case.
Bees show much skill in their manner of working, for they always make their holes from the outside close to the spot where the nectar lies hidden within the corolla. All the flowers in a large bed of Stachys coccinea had either one or two slits made on the upper side of the corolla near the base. The flowers of a Mirabilis and of Salvia coccinea were perforated in the same manner; whilst those of Salvia grahami, in which the calyx is much elongated, had both the calyx and the corolla invariably perforated. The flowers of Pentstemon argutus are broader than those of the plants just named, and two holes alongside each other had here always been made just above the calyx. In these several cases the perforations were on the upper side, but in Antirrhinum majus one or two holes had been made on the lower side, close to the little protuberance which represents the nectary, and therefore directly in front of and close to the spot where the nectar is secreted.
But the most remarkable case of skill and judgment known to me, is that of the perforation of the flowers of Lathyrus sylvestris, as described by my son Francis. (11/15. "Nature" January 8, 1874 page 189.) The nectar in this plant is enclosed within a tube, formed by the united stamens, which surround the pistil so closely that a bee is forced to insert its proboscis outside the tube; but two natural rounded pa.s.sages or orifices are left in the tube near the base, in order that the nectar may be reached by the bees. Now my son found in sixteen out of twenty-four flowers on this plant, and in eleven out of sixteen of those on the cultivated everlasting pea, which is either a variety of the same species or a closely allied one, that the left pa.s.sage was larger than the right one. And here comes the remarkable point,--the humble-bees bite holes through the standard-petal, and they always operated on the left side over the pa.s.sage, which is generally the larger of the two. My son remarks: "It is difficult to say how the bees could have acquired this habit. Whether they discovered the inequality in the size of the nectar-holes in sucking the flowers in the proper way, and then utilised this knowledge in determining where to gnaw the hole; or whether they found out the best situation by biting through the standard at various points, and afterwards remembered its situation in visiting other flowers. But in either case they show a remarkable power of making use of what they have learnt by experience." It seems probable that bees owe their skill in biting holes through flowers of all kinds to their having long practised the instinct of moulding cells and pots of wax, or of enlarging their old coc.o.o.ns with tubes of wax; for they are thus compelled to work on the inside and outside of the same object.
In the early part of the summer of 1857 I was led to observe during some weeks several rows of the scarlet kidney-bean (Phaseolus multiflorus), whilst attending to the fertilisation of this plant, and daily saw humble- and hive-bees sucking at the mouths of the flowers. But one day I found several humble-bees employed in cutting holes in flower after flower; and on the next day every single hive-bee, without exception, instead of alighting on the left wing-petal and sucking the flower in the proper manner, flew straight without the least hesitation to the calyx, and sucked through the holes which had been made only the day before by the humble-bees; and they continued this habit for many following days. (11/16. "Gardeners" Chronicle" 1857 page 725.) Mr. Belt has communicated to me (July 28th, 1874) a similar case, with the sole difference that less than half of the flowers had been perforated by the humble-bees; nevertheless, all the hive-bees gave up sucking at the mouths of the flowers and visited exclusively the bitten ones. Now how did the hive-bees find out so quickly that holes had been made? Instinct seems to be out of the question, as the plant is an exotic. The holes cannot be seen by bees whilst standing on the wing-petals, where they had always previously alighted. From the ease with which bees were deceived when the petals of Lobelia erinus were cut off, it was clear that in this case they were not guided to the nectar by its smell; and it may be doubted whether they were attracted to the holes in the flowers of the Phaseolus by the odour emitted from them. Did they perceive the holes by the sense of touch in their proboscides, whilst sucking the flowers in the proper manner, and then reason that it would save them time to alight on the outside of the flowers and use the holes? This seems almost too abstruse an act of reason for bees; and it is more probable that they saw the humble-bees at work, and understanding what they were about, imitated them and took advantage of the shorter path to the nectar. Even with animals high in the scale, such as monkeys, we should be surprised at hearing that all the individuals of one species within the s.p.a.ce of twenty-four hours understood an act performed by a distinct species, and profited by it.
I have repeatedly observed with various kinds of flowers that all the hive and humble-bees which were sucking through the perforations, flew to them, whether on the upper or under side of the corolla, without the least hesitation; and this shows how quickly all the individuals within the district had acquired the same knowledge. Yet habit comes into play to a certain extent, as in so many of the other operations of bees. Dr.
Ogle, Messrs. Farrer and Belt have observed in the case of Phaseolus multiflorus that certain individuals went exclusively to the perforations, while others of the same species visited only the mouths of the flowers. (11/17. Dr. Ogle "Pop. Science Review" April 1870 page 167. Mr. Farrer "Annals and Magazine of Natural History" 4th series volume 2 1868 page 258. Mr. Belt in a letter to me.) I noticed in 1861 exactly the same fact with Trifolium pratense. So persistent is the force of habit, that when a bee which is visiting perforated flowers comes to one which has not been bitten, it does not go to the mouth, but instantly flies away in search of another bitten flower. Nevertheless, I once saw a humble-bee visiting the hybrid Rhododendron azaloides, and it entered the mouths of some flowers and cut holes into the others. Dr.
Hermann Muller informs me that in the same district he has seen some individuals of Bombus mastrucatus boring through the calyx and corolla of Rhinanthus alecterolophus, and others through the corolla alone.
Different species of bees may, however, sometimes be observed acting differently at the same time on the same plant. I have seen hive-bees sucking at the mouths of the flowers of the common bean; humble-bees of one kind sucking through holes bitten in the calyx, and humble-bees of another kind sucking the little drops of fluid excreted by the stipules.
Mr. Beal of Michigan informs me that the flowers of the Missouri currant (Ribes aureum) abound with nectar, so that children often suck them; and he saw hive-bees sucking through holes made by a bird, the oriole, and at the same time humble-bees sucking in the proper manner at the mouths of the flowers. (11/18. The flowers of the Ribes are however sometimes perforated by humble-bees, and Mr. Bundy says that they were able to bite through and rob seven flowers of their honey in a minute: "American Naturalist" 1876 page 238.) This statement about the oriole calls to mind what I have before said of certain species of humming-birds boring holes through the flowers of the Brugmansia, whilst other species entered by the mouth.
The motive which impels bees to gnaw holes through the corolla seems to be the saving of time, for they lose much time in climbing into and out of large flowers, and in forcing their heads into closed ones. They were able to visit nearly twice as many flowers, as far as I could judge, of a Stachys and Pentstemon by alighting on the upper surface of the corolla and sucking through the cut holes, than by entering in the proper way. Nevertheless each bee before it has had much practice, must lose some time in making each new perforation, especially when the perforation has to be made through both calyx and corolla. This action therefore implies foresight, of which faculty we have abundant evidence in their building operations; and may we not further believe that some trace of their social instinct, that is, of working for the good of other members of the community, may here likewise play a part?
Many years ago I was struck with the fact that humble-bees as a general rule perforate flowers only when these grow in large numbers near together. In a garden where there were some very large beds of Stachys coccinea and of Pentstemon argutus, every single flower was perforated, but I found two plants of the former species growing quite separate with their petals much scratched, showing that they had been frequently visited by bees, and yet not a single flower was perforated. I found also a separate plant of the Pentstemon, and saw bees entering the mouth of the corolla, and not a single flower had been perforated. In the following year (1842) I visited the same garden several times: on the 19th of July humble-bees were sucking the flowers of Stachys coccinea and Salvia grahami in the proper manner, and none of the corollas were perforated. On the 7th of August all the flowers were perforated, even those on some few plants of the Salvia which grew at a little distance from the great bed. On the 21st of August only a few flowers on the summits of the spikes of both species remained fresh, and not one of these was now bored. Again, in my own garden every plant in several rows of the common bean had many flowers perforated; but I found three plants in separate parts of the garden which had sprung up accidentally, and these had not a single flower perforated. General Strachey formerly saw many perforated flowers in a garden in the Himalaya, and he wrote to the owner to inquire whether this relation between the plants growing crowded and their perforation by the bees there held good, and was answered in the affirmative. Hence it follows that the red clover (Trifolium pratense) and the common bean when cultivated in great ma.s.ses in fields,--that Erica tetralix growing in large numbers on heaths,--rows of the scarlet kidney-bean in the kitchen-garden,--and ma.s.ses of any species in the flower-garden,--are all eminently liable to be perforated.
The explanation of this fact is not difficult. Flowers growing in large numbers afford a rich booty to the bees, and are conspicuous from a distance. They are consequently visited by crowds of these insects, and I once counted between twenty and thirty bees flying about a bed of Pentstemon. They are thus stimulated to work quickly by rivalry, and, what is much more important, they find a large proportion of the flowers, as suggested by my son, with their nectaries sucked dry.
(11/19. "Nature" January 8, 1874 page 189.) They thus waste much time in searching many empty flowers, and are led to bite the holes, so as to find out as quickly as possible whether there is any nectar present, and if so, to obtain it.
Flowers which are partially or wholly sterile unless visited by insects in the proper manner, such as those of most species of Salvia, of Trifolium pratense, Phaseolus multiflorus, etc., will fail more or less completely to produce seeds if the bees confine their visits to the perforations. The perforated flowers of those species, which are capable of fertilising themselves, will yield only self-fertilised seeds, and the seedlings will in consequence be less vigorous. Therefore all plants must suffer in some degree when bees obtain their nectar in a felonious manner by biting holes through the corolla; and many species, it might be thought, would thus be exterminated. But here, as is so general throughout nature, there is a tendency towards a restored equilibrium.
If a plant suffers from being perforated, fewer individuals will be reared, and if its nectar is highly important to the bees, these in their turn will suffer and decrease in number; but, what is much more effective, as soon as the plant becomes somewhat rare so as not to grow in crowded ma.s.ses, the bees will no longer be stimulated to gnaw holes in the flowers, but will enter them in a legitimate manner. More seed will then be produced, and the seedlings being the product of cross-fertilisation will be vigorous, so that the species will tend to increase in number, to be again checked, as soon as the plant again grows in crowded ma.s.ses.
CHAPTER XII.
GENERAL RESULTS.
Cross-fertilisation proved to be beneficial, and self-fertilisation injurious.
Allied species differ greatly in the means by which cross-fertilisation is favoured and self-fertilisation avoided.
The benefits and evils of the two processes depend on the degree of differentiation in the s.e.xual elements.
The evil effects not due to the combination of morbid tendencies in the parents.
Nature of the conditions to which plants are subjected when growing near together in a state of nature or under culture, and the effects of such conditions.
Theoretical considerations with respect to the interaction of differentiated s.e.xual elements.
Practical lessons.
Genesis of the two s.e.xes.
Close correspondence between the effects of cross-fertilisation and self-fertilisation, and of the legitimate and illegitimate unions of heterostyled plants, in comparison with hybrid unions.
The first and most important of the conclusions which may be drawn from the observations given in this volume, is that cross-fertilisation is generally beneficial, and self-fertilisation injurious. This is shown by the difference in height, weight, const.i.tutional vigour, and fertility of the offspring from crossed and self-fertilised flowers, and in the number of seeds produced by the parent-plants. With respect to the second of these two propositions, namely, that self-fertilisation is generally injurious, we have abundant evidence. The structure of the flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc., renders the aid of insects almost indispensable for their fertilisation; and bearing in mind the prepotency of pollen from a distinct individual over that from the same individual, such plants will almost certainly have been crossed during many or all previous generations. So it must be, owing merely to the prepotency of foreign pollen, with cabbages and various other plants, the varieties of which almost invariably intercross when grown together. The same inference may be drawn still more surely with respect to those plants, such as Reseda and Eschscholtzia, which are sterile with their own pollen, but fertile with that from any other individual. These several plants must therefore have been crossed during a long series of previous generations, and the artificial crosses in my experiments cannot have increased the vigour of the offspring beyond that of their progenitors. Therefore the difference between the self-fertilised and crossed plants raised by me cannot be attributed to the superiority of the crossed, but to the inferiority of the self-fertilised seedlings, due to the injurious effects of self-fertilisation.
With respect to the first proposition, namely, that cross-fertilisation is generally beneficial, we likewise have excellent evidence. Plants of Ipomoea were intercrossed for nine successive generations; they were then again intercrossed, and at the same time crossed with a plant of a fresh stock, that is, one brought from another garden; and the offspring of this latter cross were to the intercrossed plants in height as 100 to 78, and in fertility as 100 to 51. An a.n.a.logous experiment with Eschscholtzia gave a similar result, as far as fertility was concerned.
In neither of these cases were any of the plants the product of self-fertilisation. Plants of Dianthus were self-fertilised for three generations, and this no doubt was injurious; but when these plants were fertilised by a fresh stock and by intercrossed plants of the same stock, there was a great difference in fertility between the two sets of seedlings, and some difference in their height. Petunia offers a nearly parallel case. With various other plants, the wonderful effects of a cross with a fresh stock may be seen in Table 7/C. Several accounts have also been published of the extraordinary growth of seedlings from a cross between two varieties of the same species, some of which are known never to fertilise themselves; so that here neither self-fertilisation nor relationship even in a remote degree can have come into play. (12/1.
See "Variation under Domestication" chapter 19 2nd edition volume 2 page 159.) We may therefore conclude that the above two propositions are true,--that cross-fertilisation is generally beneficial and self-fertilisation injurious to the offspring.
That certain plants, for instance, Viola tricolor, Digitalis purpurea, Sarothamnus scoparius, Cyclamen persic.u.m, etc., which have been naturally cross-fertilised for many or all previous generations, should suffer to an extreme degree from a single act of self-fertilisation is a most surprising fact. Nothing of the kkind has been observed in our domestic animals; but then we must remember that the closest possible interbreeding with such animals, that is, between brothers and sisters, cannot be considered as nearly so close a union as that between the pollen and ovules of the same flower. Whether the evil from self-fertilisation goes on increasing during successive generations is not as yet known; but we may infer from my experiments that the increase if any is far from rapid. After plants have been propagated by self-fertilisation for several generations, a single cross with a fresh stock restores their pristine vigour; and we have a strictly a.n.a.logous result with our domestic animals. (12/2. Ibid chapter 19 2nd edition volume 2 page 159.) The good effects of cross-fertilisation are transmitted by plants to the next generation; and judging from the varieties of the common pea, to many succeeding generations. But this may merely be that crossed plants of the first generation are extremely vigorous, and transmit their vigour, like any other character, to their successors.
Notwithstanding the evil which many plants suffer from self-fertilisation, they can be thus propagated under favourable conditions for many generations, as shown by some of my experiments, and more especially by the survival during at least half a century of the same varieties of the common pea and sweet-pea. The same conclusion probably holds good with several other exotic plants, which are never or most rarely cross-fertilised in this country. But all these plants, as far as they have been tried, profit greatly by a cross with a fresh stock. Some few plants, for instance, Ophrys apifera, have almost certainly been propagated in a state of nature for thousands of generations without having been once intercrossed; and whether they would profit by a cross with a fresh stock is not known. But such cases ought not to make us doubt that as a general rule crossing is beneficial, any more than the existence of plants which, in a state of nature, are propagated exclusively by rhizomes, stolons, etc. (their flowers never producing seeds), (12/3. I have given several cases in my "Variation under Domestication" chapter 18 2nd edition volume 2 page 152.) (their flowers never producing seeds), should make us doubt that seminal generation must have some great advantage, as it is the common plan followed by nature. Whether any species has been reproduced as.e.xually from a very remote period cannot, of course, be ascertained.
Our sole means for forming any judgment on this head is the duration of the varieties of our fruit trees which have been long propagated by grafts or buds. Andrew Knight formerly maintained that under these circ.u.mstances they always become weakly, but this conclusion has been warmly disputed by others. A recent and competent judge, Professor Asa Gray, leans to the side of Andrew Knight, which seems to me, from such evidence as I have been able to collect, the more probable view, notwithstanding many opposed facts. (12/4. "Darwiniana: Essays and Reviews pertaining to Darwinism" 1876 page 338.)
The means for favouring cross-fertilisation and preventing self-fertilisation, or conversely for favouring self-fertilisation and preventing to a certain extent cross-fertilisation, are wonderfully diversified; and it is remarkable that these differ widely in closely allied plants,--in the species of the same genus, and sometimes in the individuals of the same species. (12/5. Hildebrand has insisted strongly to this effect in his valuable observations on the fertilisation of the Gramineae: "Monatsbericht K. Akad. Berlin" October 1872 page 763.) It is not rare to find hermaphrodite plants and others with separated s.e.xes within the same genus; and it is common to find some of the species dichogamous and others maturing their s.e.xual elements simultaneously.
The dichogamous genus Saxifraga contains proterandrous and proterogynous species. (12/6. Dr. Engler "Botanische Zeitung" 1868 page 833.) Several genera include both heterostyled (dimorphic or trimorphic forms) and h.o.m.ostyled species. Ophrys offers a remarkable instance of one species having its structure manifestly adapted for self-fertilisation, and other species as manifestly adapted for cross-fertilisation. Some con-generic species are quite sterile and others quite fertile with their own pollen. From these several causes we often find within the same genus species which do not produce seeds, while others produce an abundance, when insects are excluded. Some species bear cleistogene flowers which cannot be crossed, as well as perfect flowers, whilst others in the same genus never produce cleistogene flowers. Some species exist under two forms, the one bearing conspicuous flowers adapted for cross-fertilisation, the other bearing inconspicuous flowers adapted for self-fertilisation, whilst other species in the same genus present only a single form. Even with the individuals of the same species, the degree of self-sterility varies greatly, as in Reseda. With polygamous plants, the distribution of the s.e.xes differs in the individuals of the same species. The relative period at which the s.e.xual elements in the same flower are mature, differs in the varieties of Pelargonium; and Carriere gives several cases, showing that the period varies according to the temperature to which the plants are exposed. (12/7. "Des Varieties" 1865 page 30.)
This extraordinary diversity in the means for favouring or preventing cross- and self-fertilisation in closely allied forms, probably depends on the results of both processes being highly beneficial to the species, but directly opposed in many ways to one another and dependent on variable conditions. Self-fertilisation a.s.sures the production of a large supply of seeds; and the necessity or advantage of this will be determined by the average length of life of the plant, which largely depends on the amount of destruction suffered by the seeds and seedlings. This destruction follows from the most various and variable causes, such as the presence of animals of several kinds, and the growth of surrounding plants. The possibility of cross-fertilisation depends mainly on the presence and number of certain insects, often of insects belonging to special groups, and on the degree to which they are attracted to the flowers of any particular species in preference to other flowers,--all circ.u.mstances likely to change. Moreover, the advantages which follow from cross-fertilisation differ much in different plants, so that it is probable that allied plants would often profit in different degrees by cross-fertilisation. Under these extremely complex and fluctuating conditions, with two somewhat opposed ends to be gained, namely, the safe propagation of the species and the production of cross-fertilised, vigorous offspring, it is not surprising that allied forms should exhibit an extreme diversity in the means which favour either end. If, as there is reason to suspect, self-fertilisation is in some respects beneficial, although more than counterbalanced by the advantages derived from a cross with a fresh stock, the problem becomes still more complicated.
As I only twice experimented on more than a single species in a genus, I cannot say whether the crossed offspring of the several species within the same genus differ in their degree of superiority over their self-fertilised brethren; but I should expect that this would often prove to be the case from what was observed with the two species of Lobelia and with the individuals of the same species of Nicotiana. The species belonging to distinct genera in the same family certainly differ in this respect. The effects of cross- and self-fertilisation may be confined either to the growth or to the fertility of the offspring, but generally extends to both qualities. There does not seem to exist any close correspondence between the degree to which their offspring profit by this process; but we may easily err on this head, as there are two means for ensuring cross-fertilisation which are not externally perceptible, namely, self-sterility and the prepotent fertilising influence of pollen from another individual. Lastly, it has been shown in a former chapter that the effect produced by cross and self-fertilisation on the fertility of the parent-plants does not always correspond with that produced on the height, vigour, and fertility of their offspring. The same remark applies to crossed and self-fertilised seedlings when these are used as the parent-plants. This want of correspondence probably depends, at least in part, on the number of seeds produced being chiefly determined by the number of the pollen-tubes which reach the ovules, and this will be governed by the reaction between the pollen and the stigmatic secretion or tissues; whereas the growth and const.i.tutional vigour of the offspring will be chiefly determined, not only by the number of pollen-tubes reaching the ovules, but by the nature of the reaction between the contents of the pollen-grains and ovules.
There are two other important conclusions which may be deduced from my observations: firstly, that the advantages of cross-fertilisation 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 s.e.xual elements have been in some degree differentiated. And secondly, that the injury from self-fertilisation follows from the want of such differentiation in the s.e.xual elements. These two propositions are fully established by my experiments. Thus, when plants of the Ipomoea and of the Mimulus, which had been self-fertilised 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. Mimulus offers another instructive case, showing that the benefit of a cross depends on the previous treatment of the progenitors: plants which had been self-fertilised for the eight previous generations were crossed with plants which had been intercrossed for the same number of generations, all having been kept under the same conditions as far as possible; seedlings from this cross were grown in compet.i.tion with others derived from the same self-fertilised mother-plant crossed by a fresh stock; and the latter seedlings were to the former in height as 100 to 52, and in fertility as 100 to 4. An exactly parallel experiment was tried on Dianthus, with this difference, that the plants had been self-fertilised only for the three previous generations, and the result was similar though not so strongly marked. The foregoing two cases of the offspring of Ipomoea and Eschscholtzia, derived from a cross with a fresh stock, being as much superior to the intercrossed plants of the old stock, as these latter were to the self-fertilised offspring, strongly supports the same conclusion. A cross with a fresh stock or with another variety seems to be always highly beneficial, whether or not the mother-plants have been intercrossed or self-fertilised for several previous generations. The fact that a cross between two flowers on the same plant does no good or very little good, is likewise a strong corroboration of our conclusion; for the s.e.xual elements in the flowers on the same plant can rarely have been differentiated, though this is possible, as flower-buds are in one sense distinct individuals, sometimes varying and differing from one another in structure or const.i.tution. Thus the proposition that the benefit from cross-fertilisation depends on the plants which are crossed having been subjected during previous generations to somewhat different conditions, or to their having varied from some unknown cause as if they had been thus subjected, is securely fortified on all sides.
Before proceeding any further, the view which has been maintained by several physiologists must be noticed, namely, that all the evils from breeding animals too closely, and no doubt, as they would say, from the self-fertilisation of plants, is the result of the increase of some morbid tendency or weakness of const.i.tution common to the closely related parents, or to the two s.e.xes of hermaphrodite plants.
Undoubtedly injury has often thus resulted; but it is a vain attempt to extend this view to the numerous cases given in my Tables. It should be remembered that the same mother-plant was both self-fertilised and crossed, so that if she had been unhealthy she would have transmitted half her morbid tendencies to her crossed offspring. But plants appearing perfectly healthy, some of them growing wild, or the immediate offspring of wild plants, or vigorous common garden-plants, were selected for experiment. Considering the number of species which were tried, it is nothing less than absurd to suppose that in all these cases the mother-plants, though not appearing in any way diseased, were weak or unhealthy in so peculiar a manner that their self-fertilised seedlings, many hundreds in number, were rendered inferior in height, weight, const.i.tutional vigour and fertility to their crossed offspring.
Moreover, this belief cannot be extended to the strongly marked advantages which invariably follow, as far as my experience serves, from intercrossing the individuals of the same variety or of distinct varieties, if these have been subjected during some generations to different conditions.
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 as crossing is concerned, unless their s.e.xual 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 const.i.tutions to their seedlings. Under such circ.u.mstances, the chemical const.i.tution of their fluids and the nature of their tissues are often modified. (12/8.
Numerous cases together with references are given in my "Variation under Domestication" chapter 23 2nd edition volume 2 page 264. With respect to animals, Mr. Brackenridge "A Contribution to the Theory of Diathesis"
Edinburgh 1869, has well shown that the different organs of animals are excited into different degrees of activity by differences of temperature and food, and become to a certain extent adapted to them.) 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 s.e.xual 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. (12/9. "Variation under Domestication" chapter 12 2nd edition volume 1 page 466.) 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 s.e.xual 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-fertilised seeds.
But it may be said, granting that changed conditions act on the s.e.xual 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? Although this question has been already considered, it deserves further consideration under several points of view. In my experiments with Digitalis purpurea, some flowers on a wild plant were self-fertilised, and others were crossed with pollen from another plant growing within two or three feet"s distance. The crossed and self-fertilised 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 s.e.xual 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 of Ipomoea,--or the self-fertilised plants of the eighth generation of Mimulus,--or the offspring from flowers on the same plant,--and in this case a cross between the two plants of Digitalis would 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 or inorganic matter. We know from the admirable researches of Messrs. Lawes and Gilbert that different plants require and consume very different amounts of inorganic matter. (12/10. "Journal of the Royal Agricultural Society of England" volume 24 part 1.) But the amount in the soil would probably not make so great a difference to the several individuals of any particular species as might at first be expected; for the surrounding species with different requirements would tend, from existing in greater or lesser numbers, to keep each species in a sort of equilibrium, with respect to what it could obtain from the soil. So it would be even with respect to moisture during dry seasons; and how powerful is the influence of a little more or less moisture in the soil on the presence and distribution of plants, is often well shown in old pasture fields which still retain traces of former ridges and furrows.
Nevertheless, as the proportional numbers of the surrounding plants in two neighbouring places is rarely exactly the same, the individuals of the same species will be subjected to somewhat different conditions with respect to what they can absorb from the soil. It is surprising how the free growth of one set of plants affects others growing mingled with them; I allowed the plants on rather more than a square yard of turf which had been closely mown for several years, to grow up; and nine species out of twenty were thus exterminated; but whether this was altogether due to the kinds which grew up robbing the others of nutriment, I do not know.
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 circ.u.mstance 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 was shown in the last chapter 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 of Digitalis may 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 the Bignonia, previously mentioned, which was fertilised by Fritz Muller with pollen from some adjoining plants and set hardly any seed, but when fertilised with pollen from a distant plant, was highly fertile. Seedlings from a cross of this kind grow with great vigour, and transmit their vigour 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.
When two varieties which present well-marked differences are crossed, their descendants in the later generations differ greatly from one another in external characters; and this is due to the augmentation or obliteration of some of these characters, and to the reappearance of former ones through reversion; and so it will be, as we may feel almost sure, with any slight differences in the const.i.tution of their s.e.xual elements. Anyhow, my experiments indicate that crossing plants which have been long subjected to almost though not quite the same conditions, is the most powerful of all the means for retaining some degree of differentiation in the s.e.xual elements, as shown by the superiority in the later generations of the intercrossed over the self-fertilised seedlings. Nevertheless, the continued intercrossing of plants thus treated does tend to obliterate such differentiation, as may be inferred from the lessened benefit derived from intercrossing such plants, in comparison with that from a cross with a fresh stock. It seems probable, as I may add, that seeds have acquired their endless curious adaptations for wide dissemination, not only that the seedlings would thus be enabled to find new and fitting homes, but that the individuals which have been long subjected to the same conditions should occasionally intercross with a fresh stock. (12/11. See Professor Hildebrand"s excellent treatise "Verbreitungsmittel der Pflanzen" 1873.)
From the foregoing several considerations we may, I think, conclude that in the above case of the Digitalis, and even in that of plants which have grown for thousands of generations in the same district, as must often have occurred with species having a much restricted range, we are apt to over-estimate the degree to which the individuals have been subjected to absolutely the same conditions. There is at least no difficulty in believing that such plants have been subjected to sufficiently distinct conditions to differentiate their s.e.xual elements; for we know that a plant propagated for some generations in another garden in the same district serves as a fresh stock and has high fertilising powers. The curious cases of plants which can fertilise and be fertilised 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.
Some naturalists a.s.sume that there is an innate tendency in all beings to vary and to advance in organisation, independently of external agencies; and they would, I presume, thus explain the slight differences which distinguish all the individuals of the same species both in external characters and in const.i.tution, as well as the greater differences in both respects between nearly allied varieties. No two individuals can be found quite alike; thus if we sow a number of seeds from the same capsule under as nearly as possible the same conditions, they germinate at different rates and grow more or less vigorously. They resist cold and other unfavourable conditions differently. They would in all probability, as we know to be the case with animals of the same species, be somewhat differently acted on by the same poison, or by the same disease. They have different powers of transmitting their characters to their offspring; and many a.n.a.logous facts could be given.
(12/12. Vilmorin as quoted by Verlot "Des Varieties" pages 32, 38, 39.) Now, if it were true that plants growing near together in a state of nature had been subjected during many previous generations to absolutely the same conditions, such differences as those just specified would be quite inexplicable; but they are to a certain extent intelligible in accordance with the views just advanced.