 This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recorded by Kurt Wong, New York, February 2007. The Origin of Species by Means of Natural Selection or The Preservation of Favourite Races in the Struggle for Life 6. London Edition by Charles Darwin Chapter No. 8 Instinct Part 2 Slave-Making Instinct This remarkable instinct was first discovered in the Formica Polyurgis Rufusens by Pierre Ubert, a better observer even than his celebrated father. This ant is absolutely dependent on its slaves. Without their aid, the species would certainly become extinct in a single year. The males and fertile females do no work of any kind, and the workers or sterile females, though most energetic and courageous in capturing slaves, do no other work. They are incapable of making their own nests or of feeding their own larvae. When the old nest is found inconvenient and they have to migrate, this is the slaves which determine the migration and actually carry their masters in their jaws. So utterly helpless are the masters that when Ubert shut up thirty of them without a slave but with plenty of the food which they like best and with their larvae and pupae to stimulate them to work, they did nothing. They could not even feed themselves and many perished of hunger. Ubert then introduced a single slave, F. Fusca, who was instantly set to work, fed and saved the survivors, made some cells and tended the larvae, and put all to rights. What can be more extraordinary than these well-assertained facts? If we had not known of any other slave-making ant, it would have been hopeless to speculate how so wonderful an instinct could have been perfected. Another species, Formica sanguinea, was likewise first discovered by P. Ubert to be a slave-making ant. This species is found in the southern parts of England and its habits have been attended to by Mr. F. Smith of the British Museum, to whom I am much indebted for information on this and other subjects. Although fully trusting to the statements of Ubert and Mr. Smith, I tried to approach the subject in a skeptical frame of mind as anyone may well be excused for doubting the existence of so extraordinary an instinct as that of making slaves. Hence, I will give the observations which I made in some little detail. I opened fourteen nests of F. Sanguinea and found a few slaves in all. Males and fertile females of the slave species F. Fusca are found only in their own proper communities and have never been observed in the nest of Sanguinea. The slaves are black and not above half the size of their red masters so that the contrast in their appearance is great. When the nest is slightly disturbed, the slaves occasionally come out and like their masters are much agitated and defend the nest. When the nest is much disturbed, on the larvae and pupae are exposed, the slaves work energetically together with their masters in carrying them away to a place of safety. Hence, it is clear that the slaves feel quite at home. During the months of June and July on three successive years, I watched for many hours several nests in Syrian Sussex and never saw a slave either leave or enter a nest. As, during these months, the slaves are very few in number, I thought that they might behave differently when more numerous, but Mr. Smith informs me that he has watched the nest at various hours during May, June and August, both in Syria and Hampshire, and has never seen the slaves, though present in large numbers in August, either leave or enter the nest. Hence, he considers them as strictly household slaves. The masters, on the other hand, may be constantly seen bringing in materials for the nest and food of all kinds. During the year 1860, however, in the month of July, I came across a community with an unusually large stock of slaves, and I observed a few slaves mingled with their masters leaving the nest and marching along the same road to a tall scotch fir tree, 25 yards distant, which they had ascended together probably in search of aphids or kakai. According to Uber, who had ample opportunities for observation, the slaves in Switzerland habitually work with their masters in making the nest, and they alone open and close the doors in the morning and evening. And, as Uber expressly states, their principal office is to search for aphids. This difference in the usual habits of the masters and slaves in the two countries probably depends merely on the slaves being captured in greater numbers in Switzerland than in England. One day, I fortunately witnessed a migration of Efsanguinia from one nest to another, and it was a most interesting spectacle to behold the masters carefully carrying their slaves in their jaws, instead of being carried by them as in the case of F. Rufacens. Another day, my attention was struck by about a score of the slave makers haunting the same spot, and evidently not in search of food. They approached and were vigorously repulsed by an independent community of the slave species F. Fusca, sometimes as many as three of these ants clinging to the legs of the slave making Efsanguinia. The latter ruthlessly killed their small opponents and carried their dead bodies as food to their nest, 29 yards distant, but they were prevented from getting any pupae to rear as slaves. I then dug up a small parcel of the pupae of F. Fusca from another nest and put them down on a bare spot near the place of combat. They were eagerly seized and carried off by the tyrants who perhaps fancied that after all they had been victorious in their late combat. At the same time, I laid on the same place a small parcel of the pupae of another species, F. Flava, with a few of these little yellow ants still clinging to the fragments of their nest. This species is sometimes, though rarely, made into slaves and has been described by Mr. Smith. Although so small a species, it is very courageous and I have seen it ferociously attack other ants. In one instance I found to my surprise an independent community of F. Flava under a stone beneath a nest of the slave making Efsanguinia and when I had accidentally disturbed both nests, the little ants attacked their big neighbors with surprising courage. Now I was curious to ascertain whether Efsanguinia could distinguish the pupae of F. Fusca which they habitually make into slaves from those of the little and furious F. Flava which they rarely capture and it was evident that they did at once distinguish them for we have seen that they eagerly and instantly seized the pupae of F. Fusca whereas they were much terrified when they came across the pupae or even the earth from the nest of F. Flava and quickly ran away but in about a quarter of an hour shortly after all the little yellow ants had crawled away they took heart and carried off the pupae. One evening I visited another community of Efsanguinia and found a number of these ants returning home and entering their nests carrying the dead bodies of F. Fusca showing that it was not a migration and numerous pupae. I traced a long file of ants burdened with booty for about 40 yards back to a very thick clump of heath once I saw the last individual of Efsanguinia emerge carrying a pupae but I was not able to find the desolated nest in the thick heath. The nest however must have been close at hand for two or three individuals of F. Fusca were rushing about in the greatest agitation and one was perched motionless with his own pupae in its mouth on the top of a spray of heath an image of despair over its ravaged home. Such are the facts though they did not need confirmation by me in regard to the wonderful instinct of making slaves let it be observed what a contrast the instinctive habits of Efsanguinia present with those of the continental F. Rufusans. The latter does not build its own nest does not determine its own migrations does not collect food for itself or its young and cannot even feed itself it is absolutely dependent on its numerous slaves. Formica sanguinia on the other hand possesses much fewer slaves and in the early part of the summer extremely few. The masters determine when and where a new nest shall be formed and when they migrate the masters carry the slaves. Both in Switzerland and England the slaves seem to have the exclusive care of the larvae and the masters alone go on slave making expeditions. In Switzerland the slaves and masters work together making and bringing materials for the nest. Both but chiefly the slaves tended milk as it may be called their aphids and thus both collect food for the community. In England the masters alone usually leave the nest to collect building materials and food for themselves their slaves and larvae so that the masters in this country receive much less service from their slaves than they do in Switzerland. By what steps the instinct of Efsanguinia originated I will not pretend to conjecture but as ants which are not slave makers will as I have seen carry off pupae of other species if scattered near their nests it is possible that such pupae originally stored as food might become developed and the foreign ants thus unintentionally reared would then follow their proper instincts and do what work they could. If their presence proved useful to the species which had seized them if it were more advantageous to the species to capture workers than to procreate them the habit of collecting pupae originally for food might by natural selection be strengthened and rendered permanent for the very different purpose of raising slaves when the instinct was once acquired if carried out to a much less extent even than our British Efsanguinia which as we have seen is less aided by its slaves than the same species in Switzerland natural selection might increase and modify the instinct always supposing each modification to be of use to the species until an ant was formed as abjectly dependent on its slaves as is the Formica Rufusens cell making instinct of the Hive bee I will not here enter on minute details on this subject but will merely give an outline of the conclusions at which I have arrived he must be a dull man who can examine the exquisite structure of a comb so beautifully adapted to its end without enthusiastic admiration we hear from mathematicians that bees have practically solved a recondite problem and have made their cells of the proper shape to hold the greatest possible amount of honey with the least possible consumption of precious wax in their construction it has been remarked that a skillful workman with fitting tools and measures would find it very difficult to make cells of wax of the true form attracted by a crowd of bees working in a dark hive granting whatever instincts you please it seems at first quite inconceivable how they can make all the necessary angles and planes or even perceive when they are correctly made but the difficulty is not nearly so great as it first appears all this beautiful work can be shown I think to follow from as few simple instincts I was led to investigate this subject by Mr. Waterhouse it was shown that the form of the cells stands in close relation to the presence of adjoining cells and the following view may perhaps be considered only as a modification of his theory let us look to the great principle of gradation and see whether nature does not reveal to us her method of work at one end of a short series we have humblebees which use their old cocoons to hold honey sometimes adding to them short tubes of wax otherwise making separate and very irregular rounded cells of wax at the other end of the series we have the cells of the hive bee placed in a double layer each cell as is well known is an hexagonal prism with the basal edges of its six sides beveled so as to join an inverted pyramid of three roms these roms have certain angles and the three which form the pyramidal base of a single cell on one side of the comb enter into the composition of the bases of three adjoining cells on the opposite side in the series between the extreme perfection of the cells of the hive bee and the simplicity of those of the humble bee we have the cells of the Mexican Malipina domestica carefully described and figured by Pierre Uber the Malipina itself is intermediate in structure between the hive and humble bee but more nearly related to the latter it forms a nearly regular wax and comb of cylindrical cells in which the young are hatched and in addition some large cells of wax for holding honey these latter cells are nearly spherical and of nearly equal sizes and are aggregated into an irregular mass but the important point to notice is that these cells are always made at that degree of nearness to each other that they would have intersected or broken into each other if the spheres had been completed but this is never permitted the bees building perfectly flat walls of wax between the spheres which thus tend to intersect hence each cell consists of an outer spherical portion and of two, three or more flat surfaces according as the cell adjoins two, three or more other cells when one cell rests in three other cells which from the spheres being nearly of the same size is very frequently and necessarily the case the three flat surfaces are united into a pyramid and this pyramid as Uber has remarked is manifestly a gross imitation of the three-sided pyramidal base of the cell of the hive bee as in the cells of the hive bee so here the three plain surfaces in any one cell necessarily enter into the construction of three adjoining cells it is obvious that the Milipinaw saves wax and what is more important, labor, by this manner of building for the flat walls between the adjoining cells are not double but are of the same thickness as the outer spherical portions and yet each flat portion forms a part of two cells reflecting on this case it occurred to me that if the Milipinaw had made its spheres at some given distance from each other and had made them of equal sizes and had arranged them symmetrically in a double layer the resulting structure would have been as perfect as the comb of the hive bee accordingly I wrote to Professor Miller of Cambridge and this geometer has kindly read over the following statement drawn up from his information and tells me that it is strictly correct if a number of equal spheres be described with their centers placed in two parallel layers with the center of each sphere at the distance of radius times square root 2 or radius times 1.41421 or at some lesser distance from the centers of the six surrounding spheres in the same layer and at the same distance from the centers of the adjoining spheres in the other and parallel layer then if planes of intersection between the several spheres in both layers be formed there will be result a double layer of hexagonal prisms united together by pyramidal bases formed with three roms and the roms and the sides of the hexagonal prisms will have every angle identically the same with the best measurements which have been made of the cells of the hive bee but I hear from Professor Wyman who has made numerous careful measurements that the accuracy of the workmanship of the bee has been greatly exaggerated so much so that whatever the typical form of the cells may be it is rarely if ever realized hence we may safely conclude that if we could slightly modify the instincts already possessed by the millipena and in themselves not very wonderful this bee would make a structure as wonderfully perfect as that of the hive bee we must suppose the millipena to have the power of forming her cells truly spherical and of equal sizes and this would not be very surprising seeing that she already does so to a certain extent and seeing what perfectly cylindrical burrows many insects make in wood apparently by turning around on a fixed point we must suppose the millipena to arrange her cells in level layers as she already does her cylindrical cells and we must further suppose and this is the greatest difficulty that she can somehow judge accurately at what distance to stand from her fellow laborers when several are making their spheres but she is already so far enabled to judge of a distance that she always describes her spheres so as to intersect to a certain extent and then she unites the points of intersection by perfectly flat surfaces by such modifications of instincts which in themselves are not very wonderful hardly more wonderful than those which guide a bird to make its nest I believe that the hive bee has acquired through natural selection her inimitable architectural powers but this theory can be tested by experiment following the example of Mr. Teggetmeier I separated two combs and put between them a long thick rectangular strip of wax the bees instantly began to excavate minute circular pits in it and as they deepened these little pits they made them wider and wider until they were converted into shallow basins appearing to the eye perfectly true or parts of a sphere and of about the diameter of a cell it was most interesting to observe that wherever several bees had begun to excavate their basins near together they had begun their work at such a distance from each other that by the time the basins had acquired the above stated width i.e. about the width of an ordinary cell and were in depth about one sixth of the diameter of the sphere of which they performed apart the rims of the basins intersected or broke into each other as soon as this occurred the bees ceased to excavate and began to build up flat walls of wax on the lines of intersection between the basins so that each hexagonal prism was built upon the scalloped edge of a smooth basin instead on the straight edges of a three-sided pyramid as in the case of ordinary cells i then put into the hive instead of a thick rectangular piece of wax a thin and narrow knife-edged ridge colored with vermilion the bees instantly began on both sides to excavate little basins near to each other in the same way as before but the ridge of wax was so thin that the bottoms of the basins if they had been excavated to the same depth as the former experiment would have broken into each other from the opposite sides the bees however did not suffer this to happen and they stopped their excavations in due time so that the basins as soon as they had been a little deepened came to have flat bases and these flat bases formed by thin little plates of the vermilion wax left unnod were situated as far as the eye could judge exactly along the plains of imaginary intersection between the basins on the opposite side of the ridge of wax in some parts only small portions in other parts large portions of a romping plate were thus left between the opposed basins but the work from the unnatural state of things had not been neatly performed the bees must have worked at very nearly the same rate in circularly gnawing away and deepening the basins on both sides of the ridge of vermilion wax in order to have thus succeeded in leaving flat plates between the basins by stopping work at the plains of intersection considering how flexible thin wax is I do not see that there is any difficulty in the bees whilst at work on the two sides of a strip of wax perceiving when they have nod the wax away to the proper thinness and then stopping their work in ordinary combs it has appeared to me that the bees do not always succeed in working at exactly the same rate from the opposite sides for I have noticed half completed roms at the base of a just commenced cell which were slightly concave on one side where I suppose that the bees had excavated too quickly and convex on the opposite side where the bees had worked less quickly in one well-marked instance I put the comb back into the hive and allowed the bees to go on working for a short time and again examined the cell and I found that the rhombic plate had been completed and had become perfectly flat it was absolutely impossible from the extreme thinness of the little plate that they could have effected this by gnawing away the convex side and I suspect that the bees in such cases stand in the opposed cells and push and bend the ductile and warm wax which as I have tried is easily done into its proper intermediate plane and thus flatten it from the experiment of the ridge of vermilion wax we can see that if the bees were to build for themselves a thin wall of wax they could make their cells of the proper shape by standing at the proper distance from each other by excavating at the same rate and by endeavoring to make equal spherical hollows but never allowing this fierce to break into each other now bees, as may be clearly seen by examining the edge of a growing comb do make a rough circumferential wall or rim all around the comb and they gnaw this away from the opposite sides always working circularly as they deepen each cell they do not make the whole three-sided pyramidal base of any one cell at the same time but only that of one rhombic plate which stands on the extreme growing margin with the two plates as the case may be and they never complete the upper edges of the rhombic plates until the hexagonal walls are commenced some of these statements differ from those made by the justly celebrated elder Uber but I am convinced of their accuracy and if I had space I could show that they are conformable with my theory Uber's statement that the very first cell is excavated out of a little parallel-sided wall of wax is not as far as I have seen strictly correct the first commencement having always been a little hood of wax but I will not here enter on details we see how important a part excavation plays in the construction of the cells but it would be a great error to suppose that the bees cannot build up a rough wall of wax in the proper position that is, along the plane of intersection between two adjoining spheres I have several specimens showing clearly that they can do this even in the rude circumferential rim or wall of wax around a growing comb flexures may be sometimes be observed corresponding in position to the planes of the rhombic basal plates of future cells but the rough wall of wax has in every case to be finished off by being largely gnawed away on both sides the manner in which the bees build is curious they always make the first rough wall from 10 to 20 times thicker than the excessively thin finished wall of the cell which will ultimately be left we shall understand how they work by supposing masons first to pile up a broad ridge of cement and then to begin cutting it away equally in both sides near the ground till a smooth, very thin wall is left in the middle the masons always piling up the cutaway cement and adding fresh cement on the summit of the ridge we shall thus have a thin wall steadily growing upward but always crowned by a gigantic coping from all the cells, both those just commenced and those completed being thus crowned by a strong coping of wax the bees can cluster and crawl over the comb without injuring the delicate hexagonal walls these walls, as Professor Miller has kindly ascertained for me very greatly in thickness being on average of 12 measurements made near the border of the comb one 350 second of an inch in thickness whereas the basal rhomboidal plates are thicker nearly in the proportion of three to two having a mean thickness from 21 measurements of one 229th of an inch by the above singular manner of building strength is continually given to the comb with the utmost ultimate economy of wax it seems at first to add to the difficulty of understanding how the cells are made that a multitude of beads all work together one bee, after working a short time at one cell going to another so that as uber has stated a score of individuals work even at the commencements of the first cell I was able practically to show this fact by covering the edges of the hexagonal walls of a single cell or the extreme margin of the circumferential rim of a growing comb with an extremely thin layer of melted vermilion wax and I invariably found that the color was most delicately diffused by the bees as delicately as a painter could have done it with his brush by atoms of the colored wax haven't been taken from the spot on which it had been placed and worked in the growing edges of the cells all round the work of construction seems to be a sort of balance struck between many bees all instinctively standing at the same relative distance from each other all trying to sweep equal spheres and then building up or leaving unnod the planes of intersection between these spheres it was really curious to note in cases of difficulty as when two pieces of comb met at an angle how often these bees would pull down and rebuild in different ways the same cell sometimes recurring to a shape which they had at first rejected when bees have a place on which they can stand in their proper positions for working for instance on a slip of wood placed directly under the middle of a comb growing downwards so that the comb has to be built over one face of the slip in this case the bees can lay the foundations of one wall of a new hexagon in its strictly proper place projecting beyond the other completed cells it suffices that the bees should be enabled to stand at their proper relative distances from each other and from the walls of the last completed cell and then by striking imaginary spheres they can build up a wall intermediate between two adjoining spheres but as far as I have seen they never know away and finish off the angles of a cell to a large part both of that cell and of the adjoining cells has been built this capacity in bees of laying down under certain circumstances a rough wall in its proper place between two just commenced cells is important as it bears on a fact which seems at first subversive of the foregoing theory namely that the cells on the extreme margin of wasp combs are sometimes strictly hexagonal but I have not space here to enter on this subject nor does there seem to me any great difficulty in a single insect as in the case of a queen wasp making hexagonal cells if she were to work alternately on the inside and outside of two or three cells commenced at the same time always standing at the proper relative distance from the parts of the cells just begun sweeping spheres or cylinders and building up intermediate planes as natural selection acts only by the accumulation of slight modifications of structure or instinct which is most profitable to the individual under its conditions of life it may reasonably be asked how a long and graduated succession of modified architectural instincts all tending towards the present perfect plan of construction could have profited the progenitors of the hive bee I think the answer is not difficult cells constructed like those of the bee of the wasp gain in strength and save much in labor and space in the materials of which they are constructed with respect to the formation of wax it is known that the bees are often hard-pressed to get sufficient nectar and I am informed by Mr. Tegetmayer that it has been experimentally proved that from 12 to 15 pounds of a dry sugar are consumed by a hive of bees for the secretion of a pound of wax so that a prodigious quantity of fluid nectar must be collected and consumed by the bees in a hive for the secretion of the wax necessary for the construction of their combs moreover many bees have to remain idle for many days during the process of secretion a large store of honey is indispensable to support a large stock of bees during the winter and the security of the hive is known mainly to depend on a large number of bees being supported hence the saving of wax by largely saving honey and the time consumed in collecting the honey must be an important element of success any family of bees of course the success of the species may be dependent on the number of its enemies or parasites or on quite distinct causes and so be altogether independent of the quantity of honey which the bees can collect but let us suppose that this latter circumstance determined as it probably often has determined whether a bee ally to our humble bees could exist in large numbers in any country and let us further suppose that the community lived through the winter and consequently required a store of honey there can in this case be no doubt that it would be an advantage to our imaginary humble bee if a slight modification of her instincts led her to make her waxen cells near together so as to intersect a little for a wall in common even to two adjoining cells would save some little labour and wax hence it would continually be more and more advantageous to our humble bees if they were to make the cells more and more regular nearer together and aggregated into a mass like the cells the malepina for in this case a large part of the bounding surface of each cell would serve to bound the adjoining cells and much labour and wax would be saved again from the same cause it would be advantageous to the malepina if she were to make her cells closer together and more regular in every way then at present for then as we have seen the spherical surfaces would wholly disappear and be replaced by plane surfaces and the malepina would make a comb as perfect as that of the hive bee beyond this stage of perfection in architecture natural selection could not lead for the comb of the hive bee as far as we can see is absolutely perfect in economising labour and wax thus as I believe the most wonderful of all known instincts that of the hive bee can be explained by natural selection having taken advantage of numerous slight modifications of simpler instincts natural selection having by slow degrees more and more perfectly led to the bees to sweep equal spheres at a given distance from each other in a double layer and to build up and excavate the wax along the planes of intersection the bees of course no more knowing that they swept their spheres at one particular distance from each other than they know what are the several angles of the hexagonal prisms and of the basal rhombic plates the motive power of the process of natural selection having been the construction of cells of due strength and of the proper size and shape for the larvae thus being affected with the greatest possible economy of labour and wax that individual swarm which thus made the best cells with the least labour and least waste of honey in the secretion of wax having succeeded best and having transmitted their newly acquired economical instincts to new swarms which in their turn have had the best chance of succeeding in the struggle for existence End of chapter 8 part 2 This is a LibriVox recording All LibriVox recordings are in the public domain For more information or to volunteer please visit LibriVox.org Recorded by Anita Roy Dobbs Boston February 2007 The origin of species by means of natural selection or the preservation of favoured races in the struggle for life 6th London edition by Charles Darwin Chapter number 8 Instinct Part 3 Objections to the theory of natural selection as applied to instincts neuter and sterile insects It has been objected to the foregoing view of the origin of instincts that, quote the variations of structure and of instinct must have been simultaneous and accurately adjusted to each other as a modification in the one without an immediate corresponding change in the other would have been fatal end quote The force of this objection rests entirely on the assumption that the changes in the instincts and structure are abrupt To take as an illustration the case of the larger tit-mouse Paras Major alluded to in a previous chapter this bird often holds the seeds of the U with its beak till it gets at the kernel Now what special difficulty would there be in natural selection preserving all the slight individual variations in the shape of the beak which were better and better adapted to break open the seeds until a beak was formed as well constructed for this purpose as that of the nut hatch at the same time that habit or compulsion or spontaneous variations of taste led the bird to become more and more of a seed eater In this case the beak is supposed to be slowly modified by natural selection subsequently too but in accordance with slowly changing habits or taste but let the feed of the tit-mouse vary and grow larger from correlation with the beak or from any other unknown cause and it is not improbable that such larger feed would lead the bird to climb more and more until it acquired the remarkable climbing instinct and power of the nut hatch In this case a gradual change of structure is supposed to lead to changed instinctive habits to take one more case few instincts are more remarkable than that which leads the swift of the eastern islands to make its nest wholly of inspecated saliva Some birds build their nests of mud believed to be moistened with saliva and one of the swifts of North America makes its nest as I have seen of sticks agglutinated with saliva and even with flakes of this substance Is it then very improbable that the natural selection of individual swifts which secreted more and more saliva should at last produce a species with instincts leading it to neglect other materials and to make its nest exclusively of inspecated saliva and so in other cases it must however be admitted that in many instances we cannot conjecture whether it was instinct or the structure which first varied No doubt many instincts of very difficult explanation could be opposed to the theory of natural selection cases in which we cannot see how an instinct could have originated cases in which no intermediate gradations are known to exist cases of instincts of such trifling importance that they could hardly have been acted on by natural selection cases of instincts almost identically the same and almost so remote in the scale of nature that we cannot account for their similarity by inheritance from a common progenitor and consequently must believe that they were independently acquired through natural selection I will not here enter on these several cases but will confine myself to one special difficulty which at first appeared to me insuperable and actually fatal to the whole theory I allude to the neuters insect communities for these neuters often differ widely in instinct and in structure from both the males and fertile females and yet from being sterile they cannot propagate their kind the subject well deserves to be discussed at great length but I will here take only a single case that of working or sterile ants how the workers have been rendered sterile is a difficulty but not much greater than that of any other striking type of structure for it can be shown that some insects and other articulate animals in a state of nature occasionally become sterile and if such insects had been social and it had been profitable to the community that a number should have been annually born capable of work but incapable of procreation I can see no a special difficulty in this having been affected through natural selection but I must pass over this preliminary difficulty the great difficulty lies in the working ants differing widely from both the males and the fertile females in structure as in the shapes of the thorax and in being destitute of wings and sometimes of eyes and in instinct as far as instinct alone is concerned the wonderful difference in this respect between the workers and the perfect females would have been better exemplified by the hive bee if a working ant or other neuter insect had been an ordinary animal I should have unhesitatingly assumed that all its characters had been slowly acquired through natural selection namely by individuals having been born with slight profitable modifications which were inherited by the offspring and that these again varied and again were selected and so onwards but with the working ant we have an insect differing greatly from its parents yet absolutely sterile so that it could never have transmitted the acquired modifications of structure or instinct to its progeny it may well be asked how it is possible to reconcile this case with the theory of natural selection first let it be remembered that we have innumerable instances both in our domestic productions and in those in a state of nature of all sorts of differences of inherited structure which are correlated with certain ages and with either sex we have differences correlated not only with one sex but with that short period when the reproductive system is active as in the nuptial plumage of many birds and in the hooked jaws of the male salmon we have even slight differences in the horns of different breeds of cattle in relation to an artificially imperfect state of the male sex for oxen of certain breeds have longer horns than the oxen of other breeds relatively to the length of the horns in both the bulls and cows of these same breeds hence I can see no great difficulty in any character becoming correlated with the sterile condition of certain members of insect communities the difficulty lies in understanding how such correlated modifications of structure could have been slowly accumulated by natural selection this difficulty though appearing insuperable is lessened or as I believe disappears when it is remembered that selection may be applied to the family as well as to the individual and may thus gain the desired end breeders of cattle wish the flesh and fat to be well marbled together an animal thus characterized has been slaughtered but the breeder has gone with confidence to the same stock and has succeeded such faith may be placed in the power of selection that a breed of cattle always yielding oxen with extraordinarily long horns could, it is probable be formed by carefully watching which individual bulls and cows when matched produce oxen with the longest horns and yet no one ox would ever have propagated its kind here is a better and real illustration according to M. Verlo some varieties of the double annual stock from having been long and carefully selected to the right degree always produce a large proportion of seedlings bearing double and quite sterile flowers but they likewise yield some single and fertile plants these latter by which alone the variety can be propagated may be compared with the fertile male and female ants and the double sterile plants with the neuters of the same community as with the varieties of the stock so with social insects selection has been applied to the family and not to the individual for the sake of gaining a serviceable end hence we may conclude that slight modifications of structure of instinct correlated with the sterile condition of certain members of the community have proved advantageous consequently the fertile males and females have flourished and transmitted to their fertile offspring a tendency to produce sterile members with the same modifications this process must have been repeated many times until that prodigious amount of difference between the fertile and sterile females of the same species has been produced which we see in many social insects and not as yet touched on the acme of the difficulty namely the fact that the neuters of several ants differ not only from the fertile females and males but from each other sometimes to an almost incredible degree and are thus divided into two or even three casts the casts moreover do not generally graduate into each other but are perfectly well defined being as distinct from each other as are any two species of the same genus or rather as any two genera of the same family thus in acetone there are working in soldier neuters with jaws and instincts extraordinarily different in cryptoceros the workers of one cast alone carry a wonderful sort of shield on their heads the use of which is quite unknown in the mexican mermicocystis the workers of one cast never leave the nest they are fed by the workers of another cast and they have an enormously developed abdomen which secretes a sort of honey supplying the place of that which is excreted by the affidies or the domestic cattle as they may be called which are European ants guard and imprison it will indeed be thought that I have an overweening confidence in the principle of natural selection when I do not admit that such wonderful and well established facts at once annihilate the theory in the simpler case of neuter insects all of one cast which as I believe have been rendered different from the fertile males and females we may conclude from the analogy of ordinary variations that the successive slight profitable modifications did not first arise in all the neuters in the same nest but in some few alone and that by the survival of the communities with females which produced most neuters having the advantageous modification all the neuters ultimately came to be thus characterized according to this view we ought occasionally to find in the same nest insects presenting gradations of structure and this we do find even not rarely considering how few neuter insects out of Europe have been carefully examined Mr. F. Smith has shown that the neuters of several British ants differ surprisingly from each other in size and sometimes in color and that the extreme forms can be linked together by individuals taken out of the same nest I have myself compared perfect gradations of this kind it sometimes happens that the larger or the smaller sized workers are the most numerous or that both large and small are numerous while those of an intermediate size are scanty in numbers Formica flava has larger and smaller workers with some few of intermediate size and in this species as Mr. F. Smith has observed the larger workers have simple eyes, ocelli which though small can be plainly distinguished whereas the smaller workers have their ocelli rudimentary having carefully dissected several specimens of these workers I can affirm that the eyes are far more rudimentary in the smaller workers than can be accounted for merely by their proportionately lesser size and I fully believe though I dare not assert so positively that the workers of intermediate size have their ocelli in an exactly intermediate condition so that here we have two bodies of sterile workers in the same nest differing not only in size but in their organs of vision yet connected by some few members in an intermediate condition I may digress by adding that if the smaller workers had been the most useful to the community and those males and females had been continually selected which produced more and more of the smaller workers until all the workers were in this condition we should then have had a species of ant with neuters in nearly the same condition as those of mermica for the workers of mermica have not even rudiments of ocelli though the male and female ants of this genus have well developed ocelli I may give one other case so confidently did I expect occasionally to find gradations of important structures between the different casts of neuters in the same species that I gladly availed myself of Mr. F. Smith's offer of numerous specimens from the same nest of the driver ant in Africa the reader will perhaps best appreciate the amount of difference in these workers by my giving not the actual measurements but a strictly accurate illustration the difference was the same as if we were to see a set of workmen building a house of whom many were 5 feet 4 inches high and many 16 feet high and we must in addition suppose that the larger workmen had heads 4 instead of 3 times as big as those of the smaller men and jaws nearly 5 times as big the jaws moreover of the working ants of the several sizes differed wonderfully in shape and in the form and number of the teeth but the important fact for us is that though the workers can be grouped into casts of different sizes yet they graduate insensibly into each other as does the widely different structure of their jaws I speak confidently on this later point as Sir. J. Lubbock made drawings for me with the camera lucida of the jaws which I dissected from the workers of the several sizes Mr. Bates and his interesting naturalist on the Amazons has described analogous cases with these facts before me I believe that natural selection by acting on the fertile ants or parents could form a species which should regularly produce neuters all of large size with one form of a different jaw or all of small size with widely different jaws or lastly and this is the greatest difficulty one set of workers of one size and structure and simultaneously another set of workers of a different size and structure a graduated series having first been formed as in the case of the driver ant and then the extreme forms having been produced in greater and greater numbers through the survival of parents which generated them until none of the intermediate structure were produced an analogous explanation has been given by Mr. Wallace of the equally complex case of certain Malayan butterflies regularly appearing under two or even three distinct female forms and by Fritz Muller of certain Brazilian crustaceans likewise appearing under two widely distinct male forms but this subject need not here be discussed I have now explained how I believe the wonderful fact of two distinctly defined castes of sterile workers existing in the same nest both widely different from each other and from their parents has originated we can see how useful their production may have been to a social community of ants on the same principle that the division of labor is useful to civilized man ants however work by inherited and by inherited organs or tools while man works by acquired knowledge and manufactured instruments but I must confess that with all my faith in natural selection I should never have anticipated that this principle could have been efficient in so high a degree had not the case of these neuter insects led me to this conclusion I have therefore discussed this case at some little but holy insufficient length in order to show the power of natural selection and likewise because this is by far the most serious special difficulty which my theory has encountered the case also is very interesting as it proves that with animals as with plants any amount of modification may be affected by the accumulation of numerous slight spontaneous variations which are in any way profitable without exercise or habit having been brought into play for peculiar habits I am not inclined to the workers of sterile females however long they might be followed could not possibly affect the males and fertile females which alone leave descendants I am surprised that no one has advanced this demonstrative case of neuter insects against the well known doctrine of inherited habit as advanced by Lamarck Summary I have endeavored in this chapter briefly to show that the mental qualities of our domestic animals vary but the variations are inherited still more briefly I have attempted to show that instincts vary slightly in a state of nature no one will dispute that instincts are of the highest importance to each animal therefore there is no real difficulty under changing conditions of life in natural selection accumulating to any extent slight modifications of instinct which are in any way useful in many cases habit or use and disuse have probably come into play I do not pretend that the facts given in this chapter strengthen in any great degree my theory but none of the cases of difficulty to the best of my judgment annihilate it On the other hand the fact that instincts are not always absolutely perfect and are liable to mistakes that no instinct can be shown to have been produced for the good of other animals though animals take advantage of the instincts of others the canon in natural history of natura non facet saltum is applicable to instincts as well as to corporeal structure and is plainly explicable on the foregoing views but is otherwise inexplicable all tend to corroborate the theory of natural selection this theory is also strengthened by some few other facts in regard to instincts as by that common case of closely allied but distinct species when inhabiting distant parts of the world and living under considerably different conditions of life yet often retaining nearly the same instincts for instance we can understand on the principle of inheritance how it is that the thrush of tropical south america lines its nest with mud in the same peculiar manner as does our british thrush how it is that the hornbills of africa and india have the same extraordinary instinct of plastering up and imprisoning than a hole in a tree with only a small hole left in the plaster through which the males feed them and they're young when hatched how it is that the male wrens troglodytes of north america build cocknests to roost in like the males of our kitty wrens a habit wholly unlike that of any other known bird finally it may not be a logical deduction but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo rejecting its foster brothers ants making slaves the larvae of ignumina day feeding within the live bodies of caterpillars not as specially endowed or created instincts but as small consequences of one general law leading to the advancement of all organic beings namely multiply vary let the strongest live and the weakest die end of chapter 8 this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org the origin of species by means of natural selection or the preservation of favoured races in the struggle for life sixth London edition by Charles Darwin chapter number nine hybridism section one of two of the contents of this chapter distinction between the sterility of first crosses and of hybrids sterility, various in degree not universal affected by close interbreeding removed by domestication laws governing the sterility of hybrids sterility not a special endowment but incidental on other differences not accumulated by natural selection causes of the sterility of first crosses and of hybrids parallelism between the effects of change conditions of life and of crossing dimorphism and trimorphism fertility of varieties when crossed and of their mongrel offspring not universal hybrids and mongrels compared independently of their fertility summary the view commonly entertained by naturalists is that species when intercrossed have been specially endowed with sterility in order to prevent their confusion this view certainly seems at first highly probable for species living together could hardly have been kept distinct had they been capable of freely crossing the subject is in many ways important for us more especially as the sterility of species when first crossed and that of their hybrid offspring cannot have been acquired as I shall show by the preservation of successive profitable degrees of sterility it is an incidental result of differences in the reproductive systems of the parent species in treating this subject two classes of facts to a large extent fundamentally different have generally been confounded namely the sterility of species when first crossed and the sterility of the hybrids produced from them pure species have of course their organs of reproduction in a perfect condition yet when intercrossed they produce either few or no offspring hybrids on the other hand have their reproductive organs functionally impotent as may be clearly seen in the state of the male element both plants and animals though the formative organs themselves are perfect in structure as far as the microscope reveals in the first case the two sexual elements which go to form the embryo are perfect in the second case they are either not at all developed or are imperfectly developed this distinction is important when the cause of the sterility which is common to the two cases is to be considered the distinction probably has been slurred over owing to the sterility in both cases being looked on as a special endowment beyond the province of our reasoning powers the fertility of varieties that is of the forms known or believed to be descended from common parents when crossed and likewise the fertility of their mongrel offspring is with reference to my theory of equal importance with the sterility of species for it seems to make a broad and clear distinction between varieties and species degrees of sterility first for the sterility of species when crossed and of their hybrid offspring it is impossible to study the several memoirs and works of those two conscientious and admirable observers Colroyter and Gardner who almost devoted their lives to this subject without being deeply impressed with the high generality of some degree of sterility Colroyter makes the rule universal but then he cuts the knot for in ten cases in which he found two forms considered by most authors as distinct species quite fertile together he unhesitatingly ranks them as varieties Gardner also makes the rule equally universal and he disputes the entire fertility of Colroyter's ten cases but in these and in many other cases Gardner is obliged carefully to count the seeds in order to show that there is any degree of sterility he always compares the maximum number of seeds produced by two species when first crossed the maximum produced by the hybrid offspring with the average number produced by both pure parent species in a state of nature but causes of serious error here intervene a plant to be hybridized must be castrated and what is often more important must be secluded in order to prevent pollen being brought to it by insects from other plants nearly all the plants that were invented on by Gardner were potted and were kept in a chamber in his house that these processes are often injurious to the fertility of a plant cannot be doubted for Gardner gives in his table about a score of cases of plants which he castrated and artificially fertilized with their own pollen and excluding all cases such as leguminose in which there is an acknowledged accuracy in the manipulation half of these twenty plants had their fertility in some degree impaired moreover as Gardner repeatedly crossed some forms such as the common red and blue pimpenels anigalis arvencis and corulia which the best botanist rank as varieties and found them absolutely sterile we may doubt whether many species are really so sterile when intercrossed as he believed it is certain on the one hand that the sterility of various species when crossed is so different in degree and graduates away so insensibly and on the other hand that the sterility of pure species is so easily affected by various circumstances that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins I think no better evidence of this can be required than that the two most experienced observers who have ever lived namely Col. Reuter and Gardner arrived at diametrically opposite conclusions in regard to some of the very same forms it is also most instructive to compare but I have not space here to enter on details the evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties with the evidence from fertility adduced by different hybridisers or by the same observer from experiments made during different years it can thus be shown that neither sterility nor fertility affords any certain distinction between species and varieties the evidence from this source graduates away and is doubtful in the same degree as is the evidence derived from other constitutional and structural differences in regard to the sterility of hybrids in successive generations though Gardner was enabled to rear some hybrids carefully guarding them from across with either pure parent for six or seven and in one case for ten generations yet he asserts positively that their fertility never increases but generally decreases greatly and suddenly with respect to this decrease it may first be noticed that when any deviation in structure or constitution is common to both parents this is often transmitted in an augmented degree to the offspring and both sexual elements in hybrid plants are already affected in some degree but I believe that their fertility has been diminished in nearly all these cases by an independent cause namely by too close interbreeding I have made so many experiments and collected so many facts showing on the one hand that an occasional cross with a distinct individual or variety increases the vigor and fertility of the offspring and on the other hand at very close interbreeding lessens their vigor and fertility that I cannot doubt the correctness of this conclusion hybrids are seldom raised by experimentalists in great numbers and as the parent species or other allied hybrids generally grow in the same garden the visits of insects must be carefully prevented during the flowering season hence hybrids if left to themselves will generally be fertilized during each generation by pollen from the same flower and this would probably be injurious to their fertility already lessened by their hybrid origin I am strengthened in this conviction by a remarkable statement repeatedly made by Gartner namely that if even the less fertile hybrids be artificially fertilized with hybrid pollen of the same kind their fertility notwithstanding the frequent ill effects from manipulation sometimes decidedly increases and goes on increasing now in the process of artificial fertilization pollen is as often taken by chance as I know from my own experience from the anthers of another flower as from the anthers of the flower itself which is to be fertilized so that a cross between two flowers though probably often on the same plant would be thus effected moreover whenever complicated experiments are in progress so careful an observer as Gartner would have castrated his hybrids and this would have ensured in each generation a cross with pollen from a distinct flower either from the same plant of the same hybrid nature thus the strange fact of an increase in fertility in the successive generations of artificially fertilized hybrids in contrast with those spontaneously self fertilized may as I believe be accounted for by too close interbreeding having been avoided now let us turn to the results arrived at by a third most experienced hybridizer namely the honourable and reverend W. Herbert he is as emphatic in his conclusion that some hybrids are perfectly fertile as fertile as the pure parent species as our colloiter and Gartner that some degree of sterility between distinct species is a universal law of nature he experimented on some of the very same species as did Gartner the difference in their results may I think be in part accounted for by Herbert's great horticultural skill and by his having hot houses at his command of his many important statements I will hear give only a single one as an example namely that quote every ovule in a pod of crinum capense fertilized by crinum revolutum produced a plant which I never saw to occur in a case of its natural fecundation unquote so that here we have perfect or even more than commonly perfect fertility in a cross between two distinct species this case of the crinum leads me to refer to a singular fact namely that individual plants of certain species of lobelia, verbascum and paciflora can easily be fertilized by the pollen from a distinct species but not by pollen from the same plant though this pollen can be proved to be perfectly sound by fertilizing other plants or species in the genus Hipeastrum in Corridalis as shown by Professor Hildebrand in various orchids as shown by Mr. Scott and Fritz Muller all the individuals are in this peculiar condition so that with some species certain abnormal individuals and in other species all the individuals can actually be hybridized much more readily than they can be fertilized by pollen from the same individual plant to give one instance a bulb of Hipeastrum orlicum produced four flowers three were fertilized by Herbert with their own pollen and the fourth was subsequently fertilized by the pollen of a compound hybrid descended from three distinct species the result was that quote the ovaries of the three first flowers soon ceased to grow and after a few days perished entirely whereas the pod impregnated by the pollen of the hybrid made vigorous growth and rapid progress to maturity and bore good seed which vegetated freely unquote Mr. Herbert tried similar experiments during many years and always with the same result these cases serve to show on what slight and mysterious causes the lesser or greater fertility of a species sometimes depends the practical experiments of horticulturists though not made with scientific precision deserve some notice it is notorious in how complicated a manner the species of pelagornium, fuchsia calciolaria, petunia rhododendron etc have been crossed yet many of these hybrids seed freely for instance Herbert asserts that a hybrid from calciolaria integrifolia and plantagenia species most widely dissimilar in general habit quote reproduces itself as perfectly as if it had been a natural species from the mountains of Chile unquote I have taken some pains to ascertain the degree of fertility of some of the complex crosses of rhododendrons and I'm assured that many of them are perfectly fertile Mr. C. Noble for instance informs me that he raises stocks for grafting from a hybrid between rhododendron ponticum and caturbience and that this hybrid quote seeds as freely as it is possible to imagine unquote had hybrids when fairly treated always gone on decreasing infertility in each successive generation as Gartner believed to be the case the fact would have been notorious to nursery men horticulturists raise large beds of the same hybrid and such alone are fairly treated by insect agency the several individuals are allowed to cross freely with each other and the injurious influence of close interbreeding is thus prevented anyone may readily convince himself of the efficiency of insect agency by examining the flowers of the more sterile kinds of hybrid rhododendrons which produce no pollen for he will find on their stigmas plenty of pollen brought from other flowers in regard to animals much fewer experiments have been carefully tried than with plants if our systematic arrangements can be trusted that is if the genera of animals are as distinct from each other as are the genera of plants then we may infer that animals more widely distinct in the scale of nature can be crossed more easily than in the case of plants and plants themselves are I think more sterile it should however be borne in mind that owing to few animals breeding freely under confinement few experiments have been fairly tried for instance the canary bird has been crossed with nine distinct species of finches but as not one of these breeds freely in confinement we have no right to expect that the first crosses between the canary or that their hybrids should be perfectly fertile again with respect to the fertility in successive generations of the more fertile hybrid animals I hardly know of an instance in which two families of the same hybrid have been raised at the same time from different parents so as to avoid the ill effects of close interbreeding on the contrary brothers and sisters have usually been crossed in each successive generation in opposition to the constantly repeated admonition of every breeder and in this case it is not at all surprising that the inherent sterility in the hybrids should have gone on increasing although I know of hardly any thoroughly well authenticated cases of perfectly fertile hybrid animals I have reason to believe that the hybrids from Servulus vaginalis and Revesii and from Facianus colchicus with Facianus torcatus are perfectly fertile Monsieur Catrefage states that the hybrids from two moths Bombix synthia and arrindia were proved in Paris to be fertile inter-say for eight generations it has lately been asserted that two such distinct species as the hare and rabbit when they can be got to breed together produce offspring which are highly fertile when crossed with one of the parent species the hybrids from the common and Chinese geese a signoides species which are so different that they are generally ranked in distinct genera have often bred in this country with either pure parent and in one single instance they have bred inter-say which was effected by Mr Aiton who raised two hybrids from the same parents but from different hatches and from these two birds he raised no less than eight hybrids grandchildren of the pure geese from one nest in India however these cross-bred geese must be far more fertile for I am assured by two eminently capable judges namely Mr Blythe and Captain Hutton that whole flocks of these crossed geese are kept in various parts of the country and as they are kept for profit where neither pure parent species exists they must certainly be highly or perfectly fertile with our domesticated animals the various races when crossed together are quite fertile yet in many cases they are descended from two or more wild species from this fact we must conclude either that the Aboriginal parent species at first produced perfectly fertile hybrids or that the hybrids subsequently reared under domestication became quite fertile this latter alternative which was first propounded by palace seems by far the most probable and can indeed hardly be doubted it is for instance almost certain that our dogs are descended from several wild stocks yet with perhaps the exception of certain indigenous domestic dogs of South America all are quite fertile together but analogy makes me greatly doubt whether the several Aboriginal species would at first have freely bred together and have produced quite fertile hybrids so again I have lately acquired decisive evidence that the cross-bred offspring from the Indian humped and common cattle are in to say perfectly fertile and from the observations by rutimaya on their important osteological differences as well as from those by Mr. Blythe on their differences in habits voice, constitution etc these two forms must be regarded as good and distinct species the same remarks may be extended to the two chief races of the pig we must therefore either give up the belief of the universal sterility of species when crossed or we must look at this sterility in animals not as an indelible characteristic but as one capable of being removed by domestication finally considering all the ascertained facts on the intercrossing of plants and animals it may be concluded that some degree of sterility both in first crosses and in hybrids is an extremely general result but that it cannot under our present state of knowledge be considered as absolutely universal laws governing the sterility of first crosses and hybrids we will now consider a little more in detail the laws governing the sterility of first crosses and of hybrids our chief object will be to see whether or not these laws indicate that species have been specially endowed with this quality in order to prevent their crossing and blending together in utter confusion the following conclusions are drawn up chiefly from Gartner's admirable work on the hybridization of plants I have taken much pains to ascertain how far they apply to animals and considering how scanty our knowledge is in regard to hybrid animals I have been surprised to find how generally the same rules apply to both kingdoms it has been already remarked that the degree of fertility both of first crosses and of hybrids graduates from zero to perfect fertility it is surprising in how many curious ways this gradation can be shown but only the barest outline of the facts can here be given the pollen from a plant of one family is placed on the stigma of a plant of a distinct family it exerts no more influence than so much inorganic dust from this absolute zero of fertility the pollen of different species applied to the stigma of one species of the same genus yields a perfect gradation in the number of seeds produced up to nearly complete or even quite complete fertility and as we have seen in certain abnormal cases even to an excess of fertility beyond that which the plant's own pollen produces so in hybrids themselves there are some which never have produced and probably never would produce even with the pollen of the pure parents a single fertile seed but in some of these cases a first trace of fertility may be detected by the pollen of one of the pure parent species causing the flower of the hybrid to wither earlier than it otherwise would have done and the early withering of the flower is well known to be a sign of incipient fertilization from this extreme degree of sterility we have self fertilized hybrids producing a greater and greater number of seeds and a perfect fertility the hybrids raised from two species which are very difficult to cross and which rarely produce any offspring are generally very sterile but the parallelism between the difficulty of making a first cross and the sterility of the hybrids thus produced two classes of facts which are generally confounded together is by no means strict any cases in which two pure species as in the genus verbascum can be united with unusual facility and produce numerous hybrid offspring yet these hybrids are remarkably sterile on the other hand there are species which can be crossed very rarely or with extreme difficulty but the hybrids when at last produced are very fertile even within the limits of the same genus for instance in Dianthus these two opposite cases occur the fertility both of first crosses and of hybrids is more easily affected by unfavorable conditions than is that of pure species but the fertility of first crosses is likewise innately variable for it is not always the same in degree when the same two species are crossed under the same circumstances it depends in part upon the constitution of the individuals which happened to have been chosen for the experiment so it is with hybrids for their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the same capsule and exposed to the same conditions by the term systematic affinity is meant the general resemblance between species in structure and constitution now the fertility of first crosses and of the hybrids produced from them is largely governed by their systematic affinity this is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families and on the other hand by very closely allied species generally uniting with facility but the correspondence between systematic affinity and the facility of crossing is by no means strict a multitude of cases could be given of very closely allied species which will not unite or only with extreme difficulty and on the other hand of very distinct species which unite with the utmost facility in the same family there may be a genus as Dianthus in which very many species can most readily be crossed and another genus as Silene in which the most persevering efforts have failed to produce between extremely close species a single hybrid even within the limits of the same genus we meet with the same difference for instance the many species Nicotiana have been more largely crossed than the species of almost any other genus but Gardner found that Nicotiana accuminata which is not a particularly distinct species obstinately failed to fertilize or to be fertilized by no less than 8 other species of Nicotiana many analogous facts could be given no one has been able to point out what kind or what amount of difference in any recognizable character is sufficient to prevent two species crossing it can be shown that plants most widely different in habit and general appearance and having strongly marked differences in every part of the flower even in the pollen in the fruit and in the cotyledons can be crossed annual and perennial plants deciduous and evergreen trees plants inhabiting different stations and fitted for extremely different climates can often be crossed with ease by a reciprocal cross between two species I mean the case for instance of a female ass being first crossed by a stallion and then a mare by a male ass these two species may then be said to have been reciprocally crossed there is often the widest possible difference in the facility of making reciprocal crosses such cases are highly important for they prove that the capacity in any two species to cross is often completely independent of their systematic affinity that is of any difference in their structure or constitution accepting in their reproductive systems the diversity of the result in reciprocal crosses between the same two species was longer go observed by Colroyta to give an instance Mirabilis Jalapa can easily be fertilized by the pollen of Mirabilis Longiflora and the hybrids thus produced are sufficiently fertile but Colroyta tried more than 200 times during eight following years to fertilize reciprocally Mirabilis Longiflora the pollen of Mirabilis Jalapa and utterly failed several other equally striking cases could be given Thuray has observed the same fact with certain seaweeds or fukai Gardner moreover found that this difference of facility in making reciprocal crosses is extremely common in a lesser degree he has observed it even between closely related forms as Matheola Anua and Glabra which many botanists rank only as varieties it is also a remarkable fact that hybrids raised from reciprocal crosses though of course compounded of the very same two species the one species having first been used as the father and then as the mother though they rarely differ in external characters yet generally differ in fertility in a small and occasionally in a high degree several other singular rules could be given from Gardner for instance some species have a remarkable power of crossing with other species other species of the same genus have a remarkable power of impressing their likeness on their hybrid offspring but these two powers do not at all necessarily go together there are certain hybrids which instead of having as is usual an intermediate character between their two parents always closely resemble one of them and such hybrids though externally so like one of their pure parent species are with rare exceptions extremely sterile so again among hybrids which are usually intermediate in structure between their parents exceptional and abnormal individuals sometimes are born which closely resemble one of their pure parents and these hybrids are almost always utterly sterile even when the other hybrids raised from seed from the same capsule have a considerable degree of fertility these facts show how completely the fertility of a hybrid may be independent of its external resemblance to either pure parent considering the several rules now given which govern the fertility of first crosses and of hybrids we see that when forms which must be considered as good and distinct species are united their fertility graduates from zero to perfect fertility or even to fertility under certain conditions in excess that their fertility besides being eminently susceptible to favourable and unfavourable conditions is innately variable that it is by no means always the same in degree in the first cross and in the hybrids produced from this cross that the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent and lastly that the facility of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance to each other this latter statement is clearly proved by the difference in the result of reciprocal crosses between the same two species for according as the one species or the other is used as the father or the mother there is generally some difference and occasionally the widest possible difference in the facility of effecting a union the hybrids moreover produced from reciprocal crosses often differ in fertility now do these complex and singular rules indicate that species have been endowed with sterility simply to prevent their becoming confounded in nature I think not for why should the sterility be so extremely different in degree when various species are crossed all of which we must suppose it would be equally important to keep from blending together why should the degree of sterility be innately variable in the individuals of the same species why should some species cross with facility and yet produce very sterile hybrids and other species cross with extreme difficulty and yet produce fairly fertile hybrids why should there often be so greater difference in the result of a reciprocal cross between the same two species why it may be asked has the production of hybrids been permitted to grant to species the same power of producing hybrids and then to stop their further propagation by different degrees of sterility not strictly related to the facility of the first union between their parents seems a strange arrangement the foregoing rules and facts on the other hand appear to me clearly to indicate that the sterility both of first crosses and of hybrids is simply incidental or dependent on unknown differences in their reproductive systems the difference is being of so peculiar and limited a nature that in reciprocal crosses the same two species the male sexual element of the one will often freely act on the female sexual element of the other but not in a reverse direction it will be advisable to explain a little more fully by an example what I mean by sterility being incidental on other differences and not especially endowed quality as the capacity of one plant to be grafted or budded on another is unimportant for their welfare in a state of nature I presume that no one will suppose that this capacity is a specially endowed quality but will admit that it is incidental on differences in the laws of growth of the two plants we can sometimes see the reason why one tree will not take on another from differences in their rate of growth in the hardness of their wood in the period of the flow or nature of their sap etc but in a multitude of cases we can assign no reason whatsoever great diversity in the size of two plants or one being woody and the other herbaceous one being evergreen and the other deciduous and adaptation to widely different climates does not always prevent the two grafting together as in hybridisation so with grafting the capacity is limited by systematic affinity for no one has been able to graft together trees belonging to quite distinct families and on the other hand closely allied species and varieties of the same species can usually but not invariably be grafted with ease but this capacity as in hybridisation is by no means absolutely governed by systematic affinity although many distinct genera within the same family have been grafted together in other cases species of the same genus will not take on each other a pair can be grafted far more readily on the quints which is ranked as a distinct genus than on the apple which is a member of the same genus even different varieties of the pair take with different degrees of facility on the quints so do different varieties of the apricot and peach on certain varieties of the plum as Gartner found that there was sometimes an innate difference in different individuals of the same two species in crossing so Sagare believes this to be the case with different individuals of the same two species in being grafted together as in reciprocal crosses the facility of effecting a union is often very far from equal so it sometimes is in grafting the common gooseberry for instance cannot be grafted on the current whereas the current will take though with difficulty on the gooseberry we have seen that the sterility of hybrids which have their reproductive organs in an imperfect condition is a different case from the difficulty of uniting two pure species which have their reproductive organs perfect yet these two distinct classes of cases run to a large extent parallel something analogous occurs in grafting for Thuin found that three species of robinia which seeded freely on their own roots and which could be grafted with no great difficulty on a fourth species when thus grafted were rendered barren on the other hand certain species of sorbus when grafted on other species yielded twice as much fruit as when on their own roots we are reminded by this latter fact of the extraordinary cases of hippiastrum paciflora etc which seed much more freely when fertilized by the pollen of a distinct species than when fertilized with pollen from the same plant we thus see that although there is a clear and great difference between the mere adhesion of grafted stocks and the union of the male and female elements in the act of reproduction yet that there is a rude degree of parallelism in the results of grafting and of crossing distinct species and as we must look at the curious and complex laws governing the facility with which trees can be grafted on each other as incidental on unknown differences in their vegetative systems so I believe that the still more complex laws governing the facility of first crosses are incidental on unknown differences in their reproductive systems these differences in both cases follow to a certain extent as might have been expected systematic affinity by which term every kind of resemblance and dissimilarity between organic beings is attempted to be expressed the fact by no means seem to indicate that the greater or lesser difficulty of either grafting or crossing various species has been a special endowment although in the case of crossing the difficulty is as important for the endurance and stability of specific forms as in the case of grafting it is unimportant for their welfare