Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
HYBRIDISM
Online EncyclopediaEncyclopedia Home :: HOR-I25
del.icio.us it!
|
HYBRIDISM . The Latin word hybrida, hibrida or ibrida has been assumed to be derived from the See also:Greek u(3pis, an insult or See also:outrage, and a hybrid or See also:mongrel has been supposed to be an outrage on nature, an unnatural product. As a See also:general See also:rule animals and See also:plants belonging to distinct See also:species do not produce offspring when crossed with each other, and the See also:term hybrid has been employed for the result of a fertile See also:cross between individuals of different species, the word mongrel for the more See also:common result of the See also:crossing of distinct varieties. A closer See also:scrutiny of the facts, however, makes the term hybridism less isolated and more vague. The words species and genus, and still more subspecies and variety, do not correspond with clearly marked and sharply defined zoological categories, and no exact See also:line can be See also:drawn between the various kinds of crossings from those between individuals apparently identical to those belonging to genera universally recognized as distinct. Hybridism therefore grades into mongrelism, mongrelism into cross-breeding, and cross-breeding into normal pairing, and we can say little more than that the success of the See also:union is the more unlikely or more unnatural the further apart the parents are in natural See also:affinity.
The See also:interest in hybridism was for a See also:long See also:time chiefly of a See also:practical nature, and was due to the fact that hybrids are often found to See also:present characters somewhat different from those of either See also:parent. The leading facts have been known in the See also:case of the See also:horse and See also:ass from time immemorial. The earliest recorded observation of a hybrid plant is by J. G. See also:Gmelin towards the end of the 17th See also:century; the next is that of See also: In the second See also:quarter of the IQth century C. F. Gartner made and published the results of a number of experiments that had not been equalled by any earlier worker. Next came See also: 3. The degree of both kinds of infertility varies in the case of different species, and in that of their hybrid progeny, from See also:absolute sterility up to See also:complete fertility. Thus, to take the case of plants, " when See also:pollen from a plant of one See also:family is placed on the stigma of a plant of a distinct family, it exerts no more See also:influence than so much inorganic dust. From this absolute zero of fertility, the pollen of different species, applied to the stigma of some 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; 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 See also: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 See also:flower of the hybrid to See also:wither earlier than it otherwise would have done; and the See also: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 up to perfect fertility." 4. Although there is, as a rule, a certain See also:parallelism, there is no fixed relation between the degree of sterility manifested by the parent species when crossed and that which is manifested by their hybrid progeny. There are many cases in which two pure species can be crossed with unusual facility, while the resulting hybrids are remarkably sterile; and, contrariwise, there are species which can only be crossed with extreme difficulty, though the hybrids, when produced, are very fertile. Even within the limits of the same genus, these two opposite cases may occur. 5. When two species are reciprocally crossed, i.e. male A with See also:female B, and male B with female A, the degree of sterility often differs greatly in the two cases. The sterility of the resulting hybrids may differ likewise. 6. The degree of sterility of first crosses and of hybrids runs, to a certain extent, parallel with the systematic affinity of the forms which are See also:united. " For species belonging to distinct genera can rarely, and those belonging to distinct families can never, be crossed. The parallelism, however, is far from complete; for a multitude of closely allied species will not unite, or unite with extreme difficulty, whilst other species, widely different from each other, can be crossed with perfect facility. Nor does the difficulty depend on See also:ordinary constitutional See also:differences; for See also:annual and perennial plants, See also:deciduous and See also:evergreen trees, plants flowering at different seasons, inhabiting different stations, and naturally living under the most opposite climates, can often be crossed with ease. The difficulty or facility apparently depends exclusively on the sexual constitution of the species which are crossed, or on their sexual elective affinity." There are many new records as to the production of hybrids. Horticulturists have been extremely active and successful in their attempts to produce new See also:flowers or new varieties of vegetables by seminal or See also:graft-hybrids, and any florist's See also:catalogue or the See also:account of any See also:special plant, such as is to be found in See also:Foster-Melliar's See also:Book of the See also:Rose, is in See also:great part a See also:history of successful hybridization. Much special experimental See also:work has been done by botanists, notably by de Vries, to the results of whose experiments we shall recur. Experiments show clearly that the obtaining of hybrids is in many cases merely a See also:matter of taking sufficient trouble, and the successful crossing of genera is not infrequent. Focke, for instance, cites cases where hybrids were obtained between Brassica and Raphanus, Galium and Asperula, See also:Campanula and Phyteuma, Verbascum and Celsia. Among animals, new records and new experiments are almost equally numerous. Boveri has crossed See also:Echinus microtuberculatus with Sphaerechinus granularis. Thomas See also:Hunt See also:Morgan even obtained hybrids between Asterias, a See also:starfish, and Arbacia, a See also:sea-urchin, a cross as remote as would be that between a See also:fish and a mammal. See also:Vernon got many hybrids by fertilizing the eggs of Strongylocentrotus lividus with the sperm of Sphaerechinus granularis. Standfuss has carried on an enormous See also:series of experiments with Lepidopterous See also:insects, and has obtained a very large series of hybrids, of which he has kept careful See also:record. Lepidopterists generally begin to suspect that many curious forms offered by dealers as new species are products got by crossing known species. Apello has succeeded with Teleostean fish; Gebhardt and others with See also:Amphibia. Elliot and Suchetet have studied carefully the question of hybridization occurring normally among birds, and have got together a very large See also:body of See also:evidence. Among the cases cited by Elliot the most striking are that of the hybrid between Cola ptes cafer and C. auratus, which occurs over a very wide See also:area of See also:North See also:America and is known as C. hybridus, and the hybrid between Euplocamus lineatus and E. horsfieldi, which appears to be common in See also:Assam. St M. Podmore has produced successful crosses between the See also:wood-See also:pigeon (See also:Columba palumbus) and a domesticated variety of the See also:rock pigeon (C. livia). Among mammals noteworthy results have been obtained by See also:Professor Cossar See also:Ewart, who has bred nine See also:zebra hybrids by crossing mares of various sizes with a zebra stallion, and who has studied in addition three hybrids out of zebra mares, one sired by a donkey, the others by ponies. Crosses have been made between the common See also:rabbit (Lepus cuniculus) and the See also:guinea-See also:pig (Cavia cobaya), and examples of the results have been exhibited in the Zoological Gardens of See also:Sydney, New See also:South See also:Wales. The See also:Carnivora generally are very easy to hybridize, and many successful experiments have been made with animals in captivity. Karl See also:Hagenbeck of See also:Hamburg has produced crosses between the See also:lion (Felis See also:leo) and the See also:tiger (F. See also:tigris). What was probably a " tri-hybrid " in which lion, See also:leopard and See also:jaguar were mingled was exhibited by a See also:London show-See also:man in 1908. Crosses between various species of the smaller See also:cats have been fertile on many occasions. The See also:black See also:bear (Ursus americanus) and the See also:European See also: H. M. Vernon, working with sea-urchins, found that the obtaining of hybrids depended on the relative maturity of the sexual products. The difficulties in crossing apparently may ex-tend to the chemiotaxic processes of the actual sexual cells. Thus when the spermatozoa of an urchin were placed in a drop of sea-water containing ripe eggs of an urchin and of a starfish, the former eggs became surrounded by clusters of the male cells, while the latter appeared to exert little attraction for the See also:alien germ-cells. Finally, when the actual impregnation of the See also:egg is possible naturally, or has been secured by artificial means, the development of the hybrid may stop at an early See also:stage. Thus hybrids between the urchin and the starfish, animals belonging to different classes, reached only the stage of the pluteus larva. A. D. Apello, experimenting with Teleostean fish, found that very often impregnation and segmentation occurred, but that the development See also:broke down immediately afterwards. W. Gebhardt, crossing Rana esculenta with R. arvalis, found that the cleavage of the ovum was normal, but that abnormality began with the gastrula, and that development soon stopped. In a very general See also:fashion there appears to be a parallel between the zoological affinity and the extent to which the incomplete development of the hybrid proceeds. As to the sterility of hybrids inter se, or with either of the parent forms, See also:information is still wanted. Delage, summing up the evidence in a general way, states that mongrels are more fertile and stronger than their parents, while hybrids are at least equally See also:hardy but less fertile. While many of the hybrid products of horticulturists are certainly infertile, others appear to be indefinitely fertile. Focke, it is true, states that the hybrids between Primula See also:auricula and P. hirsuta are fertile for many generations, but not indefinitely so; but, while this may be true for the particular case, there seems no reason to doubt that many plant hybrids are quite fertile. In the case of animals the evidence is rather against fertility. Standfuss, who has made experiments lasting over many years, and who has dealt with many genera of See also:Lepidoptera, obtained no fertile hybrid See also:females, although he found that hybrid See also:males paired readily and successfully with pure-bred females of the parent races. Elliot, dealing with birds, concluded that no hybrids were fertile with one another beyord the second See also:generation, but thought that they were fertile with members of the parent races. See also:Wallace, on the other See also:hand, cites from Quatrefages See also:tile case of hybrids between the moths Bombyx cynthia and B. arrindia, which were stated to be fertile inter se for eight generations. He also states that hybrids between the See also:sheep and See also:goat have a limited fertility inter se. Charles Darwin, however, had evidence that some hybrid pheasants were completely fertile, and he himself interbred the progeny of crosses between the common and See also:Chinese geese, whilst there appears to be no doubt as to the complete fertility of the crosses between many species of ducks, J. L. Bonhote having interbred in various crosses for several generations the mallard (Anas boschas), the See also:Indian spot-See also:bill See also:duck (A. poecilorhyncha), the New See also:Zealand See also:grey duck (A. superciliosa) and the See also:pin-tail (Dafila acuta). Podmore's pigeon hybrids were fertile inter se, a specimen having been exhibited at the London Zoological Gardens. The hybrids between the brown and polar bears bred at Halle proved to be fertile, both with one of the parent species and with one another. Cornevin and Lesbre See also:state that in 1873 an Arab See also:mule was fertilized in See also:Africa by a stallion, and gave See also:birth to female offspring which she suckled. All three were brought to the Jardin d'Acclimatation in See also:Paris, and there the mule had a second female See also:colt to the same See also:father, and subsequently two male colts in See also:succession to an ass and to a stallion. The female progeny were fertilized, but their offspring were feeble and died at birth. Cossar Ewart gives an account of a See also:recent Indian case in which a female mule gave birth to a male colt. He points out, however, that many mistakes have been made about the breeding of hybrids, and is not altogether inclined to accept this supposed case. Very little has been published with regard to the most important question, as to the actual See also:condition of the sexual See also:organs and cells in hybrids. There does not appear to be gross anatomical defect to account for the infertility of hybrids, but microscopical examination in a large number of cases is wanted. Cossar Ewart, to whom indeed much of the most interesting recent work on hybrids is due, states that in male zebra-hybrids the sexual cells were immature, the tails of the spermatozoa being much shorter than those of the similar cells in stallions and zebras. He adds, however, that the male hybrids he examined were See also:young, and might not have been sexually mature. He examined microscopically the ovary of a female zebra-hybrid and found one large and several small Graafian follicles, in all respects similar to those in a normal See also:mare or female zebra. A careful study of the sexual organs in animal and plant hybrids is very much to be desired, but it may be said that so far as our present knowledge goes there is not to be expected any obvious microscopical cause of the relative infertility of hybrids. The relative variability of hybrids has received considerable attention from many writers. Horticulturists, as See also:Bateson has written, are " aware of the great and striking See also:variations which occur in so many orders of plants when hybridization is effected." The phrase has been used " breaking the constitution of a plant " to indicate the effect produced in the offspring of a hybrid union, and the See also:device is frequently used by those who are seeking for novelties to introduce on the See also:market. It may be said generally that hybrids are variable, and that the products of hybrids are still more variable. J. L. Bonhote found extreme variations amongst his hybrid ducks. Y. Delage states that in reciprocal crosses there is always a marked tendency for the offspring to resemble the male parents; he quotes from See also:Huxley that the mule, whose male parent is an ass, is more like the ass, and that the hinny, whose male parent is a horse, is more like the horse. Standfuss found among Lepidoptera that males were produced much more often than females, and that these males paired readily. The freshly hatched larvae closely resembled the larvae of the female parent, but in the course of growth the resemblance to the male increased, the extent of the final approximation to the male depending on the relative phylogenetic See also:age of the two parents, the parent of the older species being prepotent. In reciprocal pairing, he found that the male was able to transmit the characters of the parents in a higher degree. Cossar Ewart, in relation to zebra hybrids, has discussed the matter of resemblance to parents in very great detail, and See also:fuller information must be sought in his writings. He shows that the See also:wild parent is not necessarily prepotent, although many writers have urged that view. He described three hybrids bred out of a zebra mare by different horses, and found in all cases that the resemblance to- the male or horse parent was more profound. Similarly, zebra-donkey hybrids out of zebra mares bred in See also:France and in See also:Australia were in characters and disposition far more like the donkey parents. The results which he obtained in the hybrids which he bredfrom a zebra stallion and different mothers were more variable, but there was rather a See also:balance in favour of zebra disposition and against zebra shape and marking.
" Of the nine zebra-horse hybrids I have bred," he says, " only two in their make and disposition take decidedly after the wild parent. As explained fully below, all the hybrids differ profoundly in the See also:plan of their markings from the zebra, while in their ground See also:colour they take after their respective dams or the ancestors of their dams far more than after the zebra—the hybrid out of the yellow and See also: The same subject has of course been discussed at length by Darwin, in relation to the cross-breeding of varieties of pigeons; but the See also:modern experimentalists who are following the work of Mendel interpret reversion differently (see See also:MENDELISM). Graft-Hybridism.—It is well known that, when two varieties or allied species are grafted together, each retains its distinctive characters. But to this general, if not universal, rule there are on record several alleged exceptions, in which either the See also:scion is said to have partaken of the qualities of the stock, the stock of the scion, or each to have affected the other. Supposing any of these influences to have been exerted, the resulting product would deserve to be called a graft-hybrid. It is clearly a matter of great interest to ascertain whether such formation of hybrids by grafting is really possible; for, if even one instance of such formation could be unequivocally proved, it would show that sexual and asexual See also:reproduction are essentially identical. The cases of alleged graft-hybridism are exceedingly few, considering the enormous number of grafts that are made every See also:year by horticulturists, and have been so made for centuries. Of these cases the most celebrated are those of See also:Adam's See also:laburnum (Cytisus Adami) and the bizzarria See also:orange. Adam's laburnum is now flourishing in numerous places throughout See also:Europe, all the trees having been raised as cuttings from the See also:original graft, which was made by inserting a bud- of the See also:purple laburnum into a stock of the yellow. M. Adam, who made the graft, has See also:left on record that from it there sprang the existing hybrid. There can be no question as to the truly hybrid See also:character of the latter—all the peculiarities of both parent species being often blended in the same raceme, flower or even petal; but until the experiment shall have been successfully repeated there must always remain a strong suspicion that, notwithstanding the assertion and doubt-less the belief of M. Adam, the hybrid arose as a cross in the ordinary way of seminal reproduction. Similarly, the bizzarria orange, which is unquestionably a hybrid between the See also:bitter orange and the citron—since it presents the remarkable spectacle of these two different fruits blended into one—is stated by the gardener who first succeeded in producing it to have arisen as a graft-hybrid; but here again a similar doubt, similarly due to the need of corroboration, attaches to the statement. And the same remark applies to the still more wonderful case of the so-called trifacial orange, which blends three distinct kinds of fruit in one, and which is said to have been produced by artificially splitting and uniting the seeds taken from the three distinct species, the fruits of which now occur blended in the triple hybrid. The other instances of alleged graft-hybridism are too numerous to be here noticed in detail; they refer to jessamine, ash, See also:hazel, See also:vine, See also:hyacinth, See also:potato, See also:beet and rose. Of these the cases of the vine, beet and rose are the strongest as evidence of See also:graf t-hybridization, from the fact that some of them were produced as the result of careful experiments made by very competent a new species, so also a portion separated by infertility with the others would tend to See also:form a new species. According to Romanes, therefore, mutual infertility was the starting-point, not the result, of specific modification. Romanes, however, did not See also:associate his interesting theory with a sufficient number of facts, and it has left little See also:mark on the history of the subject. A. R. Wallace, on the other hand, has argued that sterility between incipient species may have been increased by natural selection in the same fashion as other favourable variations are supposed to have been accumulated. He thought that " some slight degree of infertility was a not infrequent See also:accompaniment of the See also:external differences which always arise in ,a state of nature between varieties and incipient species." See also:Weismann concluded, from an examination of a series of plant hybrids, that from the same cross hybrids of different character may be obtained, but that the characters are determined at the moment of fertilization; for he found that all the flowers on the same hybrid plant resembled one another in the minutest details of colour and pattern. Darwin already had pointed to the See also:act of fertilization as the determining point, and it is in this direction that the theory of hybridism has made the greatest advance. The starting-point of the modern views comes from the experiments and conclusions on plant hybrids made by Gregor Mendel and published in 1865. It is uncertain if Darwin had paid attention to this work; Romanes, See also:writing in the 9th edition of this Encyclopaedia, cited it without comment. First H. de Vries, then W. Bateson and a series of observers returned to the work of Mendel (see MENDELISa1), and made it the foundation of much experimental work and still more theory. It is still too soon to decide if the confident predictions of the Mendelians are justified, but it seems clear that a See also:combination of Mendel's numerical results with Weismann's (see See also:HEREDITY) conception of the particulate character of the germ-plasm, or hereditary material, is at the See also:root of the phenomena of hybridism, and that Darwin was justified in supposing it to See also:lie outside the See also:sphere of natural selection and to be a fundamental fact of living matter. experimentalists. On the whole, the results of some of these experiments, although so few in number, must be regarded as making out a strong case in favour of the possibility of graft-hybridism. For it must always be remembered that, in experiments of this See also:kind, negative evidence, however great in amount, may be logically dissipated by a single See also:positive result. Theory of Hybridism.—Charles Darwin was interested in hybridism as an experimental See also:side of See also:biology, but still more from the bearing of the facts on the theory of the origin of species. It is obvious that although hybridism is occasionally possible as an exception to the general infertility of species inter se, the exception is still more minimized when it is re-membered that the hybrid progeny usually display some degree of sterility. The See also:main facts of hybridism appear to lend support to the old See also:doctrine that there are placed between all species the barriers of mutual sterility. The See also:argument for the fixity of species appears still stronger when the general infertility of species crossing is contrasted with the general fertility of the crossing of natural and artificial varieties. Darwin himself, and afterwards G. J. Romanes, showed, however, that the theory of natural selection did not require the possibility of the commingling of specific types, and that there was no reason to suppose that the mutation of species should depend upon their mutual crossing. There existed more than enough evidence, and this has been added to since, to show that infertility with other species is no criterion of a species, and that there is no exact parallel between the degree of affinity between forms and their readiness to cross. The problem of hybridism is no more than the explanation of the generally reduced fertility of remoter crosses as compared with the generally increased fertility of crosses between organisms slightly different. Darwin considered and rejected the view that the inter-sterility of species could have been the result of natural selection. " At one time it appeared to me probable," he wrote (Origin of Species, 6th ed. p. 247), " as it has to others, that the sterility of first crosses and of hybrids might have been slowly acquired through the natural selection of slightly lessened degrees of fertility, which, like any other variation, spontaneously appeared in certain individuals of one variety when crossed with those of another variety. For it would clearly be advantageous to two varieties or incipient species if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them See also:separate. In the first See also:place, it may be remarked that species inhabiting distinct regions are often sterile when crossed; now it could clearly have been of no See also:advantage to such separated species to have been rendered mutually sterile and, consequently, this could not have been effected through natural selection; but it may perhaps be argued that, if a species were rendered sterile with some one compatriot, sterility with other species would follow as a necessary contingency. In the second place, it is almost as much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male See also:element of one form should have been rendered utterly impotent on a second form, whilst at the same time the male element of this second form is enabled freely to fertilize the first form; for this peculiar state of the reproductive See also:system could hardly have been advantageous to either species." Darwin came to the conclusion that the sterility of crossed species must be due to some principle quite See also:independent of natural selection. In his See also:search for such a principle he brought together much evidence as to the instability of the reproductive system, pointing out in particular how frequently wild animals in captivity fail to breed, whereas some domesticated races have been so modified by confinement as to be fertile together although they are descended from species probably mutually infertile. He was disposed to regard the phenomena of See also:differential sterility as, so to speak, by-products of the See also:process of See also:evolution. G. J. Romanes afterwards See also:developed his theory of physiological selection, in which he supposed that the appearance of differential fertility within a species was the starting-point of new species; certain individuals by becoming fertile only inter se proceeded along lines of modification diverging from the lines followed by other members of the species. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] HYBLA |
[next] HYDANTOIN (glycolyl urea), C3H4N202 |