CHOOSING THE LINES OF LEAST RESISTANCE
"Why do not plants hybridize in a state of nature?" a visitor asked me. "You seem to get most of your new varieties by hybridizing old ones. Why does not Nature take a leaf from your note-book and produce new species in the same way?" And I was able to inform my facetious questioner, much to his surprise, that the method he suggested was one that Nature had practiced from the beginning, and is constantly practicing all about us. We were standing near the gateway of my Sebastopol place. I pointed out across the road. "Why, just over by the roadside," I said, "you may see for yourself precisely such an experiment in hybridizing as I have made in the case of thousands of plants. Do you see those tarweeds there? Doubtless you are familiar with them. "There are two common species growing there together. One of them has large showy flowers, the other small and inconspicuous ones. The botanist calls the large flowered species Madiaelegans, and the other M. saliva. The two species do not look much alike, and some botanists even classify them in different genera. "If you look at all closely you will see that there is a third form of plant, bearing some resemblance to each of them, growing among the others, and I can assure you that this is a natural hybrid between the two. "If you examine this hybrid, you will find that its branches are less spreading than those of its large-flowered parent, although not upright like those of the other parent; and that the stem is stouter than that of either parent. As to foliage, the hybrid plants have larger and thicker leaves than those of the large flowered tarweed, more closely resembling the other species in this respect, but the ray-flowers are intermediate in size and shape as well as color, the reddish-brown that characterizes the flower of the more conspicuous parent being reduced in the hybrid to a spot just in the top of the tube. "So here you are probably witnessing the creation of a new species in nature. You, of course, are an evolutionist and therefore are aware that all species of plants as well as animals have been evolved in past ages by a development from earlier forms, but you very probably supposed that this creative process has now come to a standstill. Let me assure you, then, that this process is going on today very actively, in all probability quite as actively as at any time in the past. "Species of plants in a state of nature are constantly hybridizing, and new species are being developed under our eyes. "There is nothing anomalous about the case of the tarweeds, although they afford a very interesting illustration of the development of which I speak. The same thing may be observed in the case of certain genera of the mint family. Here in some cases the hybrids thrive almost to the entire exclusion of the parent species. In other cases they gradually disappear, being too unstable to establish themselves by seed. "Everything, of course, depends upon the qualities of the hybrid. If it is well adapted to the environment it survives. If better adapted than its parents, it probably runs them out altogether. But, on the other hand, if the hybrid is less well adapted than the parent forms to make its way in the world, it is of course weeded out by natural selection." In response to a further query, I named for my visitor, among plants that often hybridize in a state of nature, the various species of the genus Rubus, including the blackberry, raspberry, the tribe of wild roses and crabapples; the California lilac, the various members of the oak tribe, the willow, the strawberry and the huckleberry; nor are these all-the list might be almost indefinitely extended. Indeed, it is my firm conviction that hybridizing between natural species is a phenomenon of almost universal occurrence. I believe that no other equally plausible explanation has been given of the appearance of seeming spontaneous varieties or mutations that furnish the material for the operation of natural selection, and are thus the basis of organic evolution. It is true that such a suggestion as this would have seemed heretical not very long ago; but vast numbers of experiments in hybridizing different species, and even representatives of different genera, in my orchards and gardens have afforded a mass of evidence that no one can ignore. So today it is coming to be recognized more and more generally that the hybridizing of wild species is Nature's conventional method of producing variability, and, as it were, testing out the environment by supplying new forms that come in competition with the ones already developed.
LIMITS OF HYBRIDIZING
But why then, you will perhaps ask, does not the production of new forms between natural species take place so universally as to disturb the entire scheme of organic nature. In point of fact, the zoologist and the botanist are able to describe vast numbers of species, each of which has certain fairly well-defined characteristics and differs in certain definite regards from other forms. It is true that the more closely the matter is studied the more commonly varieties are found that manifest characteristics intermediate between those of the supposedly fixed species. But even when these are taken into consideration, it still remains true that the word "species" as applied to a vast number of familiar forms of vegetable and animal life, has a pretty definite and tangible meaning. How is this possible, if the interbreeding of species is a universal phenomenon? The answer is found in the facts (1) that the hybrid forms produced when species in nature are crossed, for the most part quickly disappear because they are not an improvement, from the standpoint of adaptation to their environment, on the parent forms; and (2) that limits are imposed by the relative lack of affinity of one species for another. As to the first point, it must be recalled that each existing species has been produced only after long generations of struggling against adverse conditions. Constantly there is a tendency to variation within certain limits even in the case of the most fixed species. Such variations collstitute tests of the fitness of the species to live in the environment in which it finds itself. Favorable variations are preserved by natural selection, simply because they have the capacity to outgrow the original form, or outlast it in times of drought or other hardship. And so every existing wild species proves by the very fact of its existence that it has a large measure of adaptability to the existing environment. It is always improbable, then, in the nature of the case, that any new intermediate form such as would arise from hybridizing two allied species, will be better adapted to survive than the parent form. Such cases do arise, else we should have no new species, but in general the rule holds. So we may fairly count it exceptional if a hybrid between natural species survives beyond the first or second generation. The struggle for existence is always keen, and the individual organism that lacks ever so little of equaling its fellows in vitality and responsiveness to its environment must inevitably perish. Nevertheless, the experiment of producing new forms through the hybridizing of old ones is perpetually being made, and must continue to he made, if existing forms are to remain plastic, ready to take advantage of the changed conditions of environment; ready, that is to say, to evolve in future as they have evolved in the past. But there are limits beyond which this perpetual experimentation with new nascent species could not advantageously be carried, and so nature puts a sharp limitation upon the extent to which the experiment may be undertaken.
AFFINITY FOUNDED ON COUSINSHIP
And this is done by the simple procedure of making it increasingly difficult for species to interbreed in proportion as the species become divergent in character. Tarweeds, for example, may interbreed among themselves, and various species of mint may similarly interbreed, but no species of tarweed could hybridize with a species of mint. One member of the rose family may hybridize with another-blackberry with raspberry, let us say, or quince with apple; and in the same way different species of oak may interbreed; but the hybridizing of apple or blackberry with any species of oak is almost unthinkable. Similarly, in my experiments I have been able to hybridize peach with almond, and almond with plum, and plum with apricot; also apple with quince, and quince with pear. Stone fruit with stone fruit, that is to say, and seed fruit with seed fruit-but never stone fruit with seed fruit. In a word, the possibility of cross-fertilization between species is conditioned on a certain closeness of relationship, which we speak of as affinity. This, as the evolutionist teaches us, is a matter of actual genetic relationship. All members of the rose family, for example, have branched from the primal ancestral stem at a period much more recent than that at which the common ancestor of the present-day apple and rose and blackberry branched from the primal stock of, let us say, the oaks. In the broadest view, there is a cousinship between all species of plants; just as there is relationship between all the twigs of an actual tree. But the species of an existing genus may be likened to twigs on a single branch; other genera representing different branches which may diverge in opposite directions, and only come together at the trunk. Then, too, there is a time element involved. Species that are closely similar in appearance are those that have branched from the ancestral stem in relatively recent epochs; species more distinct trace their cousinship through remoter lines; and forms so widely diverse as to be placed in different orders have been separated for still longer periods. And we must suppose that in each generation the new forms have taken on a modicum of new traits, and have tended to fix the divergence of earlier traits through which they attained specific difference. In due course, then, it comes to pass, that a given form has branched so widely from its cousins that the harmony of purpose, so to speak, that once obtained between them no longer obtains. The racial memory as to their common ancestry has become blurred, if the phrase be permitted, and each species has become so fixed in its own manner of life that no compromise between them would be possible. And so we find, in point of fact, that it becomes increasingly difficult to hybridize species that are obviously widely divergent in form of stem and foliage and flower, and that in a vast number of instances any attempt to hybridize these forms is altogether futile. It must be understood, however, that it is by no means always possible to predicate, from observation of a given pair of more or less distantly related species, whether or not the two would be mutually sterile. Sometimes the experiment results in a surprise, and we are able to produce offspring when the cross seemed altogether improbable. Such was the case, it will be recalled, with my experiments in hybridizing the dewberry with the pollen of the apple and pear and rose and mountain-ash. Such was the case also with the cross which resulted in producing the sunberry, and with that which developed the plumcot. In each of these cases, to be sure, the pistil of one plant accepted the pollen of the other, as it were, unwillingly. But persistent effort effected the desired result, and in the three instances last mentioned fertile offspring were produced. Possibly these might not have survived in the state of nature, but under the conditions of artificial selection they provided the foundation for the development of what may fairly be considered new species.
The characteristics that make it impossible to hybridize two species that have varied beyond certain limits are sometimes physical. Thus it may chance that the two species have developed the habit of blooming at different times. If the flowers of a given species are altogether out of bloom before the flowers of another species open, it is obvious that, in a state of nature, hybridizing between these species will never occur, however close their affinity. Similarly there are two closely related species of evening primrose that are not hybridized under natural conditions because the flower of one opens only for a brief period at midday and that of the other only during the night. Again it occasionally happens that the physical structure of the style which carries the pollen tube to the ovules is such as to prevent the carrying out of this essential process. In the case of a large pollen grain and an exceptionally slender style, it is probable that the fructifying substance of the pollen is debarred from finding its way to the ovule. Such cases are probably exceptional, however, and the usual barrier erected by Nature between species is not so much physical as chemical. That is to say, the antagonism is inherent in the plants themselves. Allied species are of such chemical constitution that the protoplasm from one mingles readily with protoplasm from the other. In the case of more widely divergent species, it may come to pass that the juices of one plant are actually poisonous to another. In such a case it is futile for the pollen grain and pistil to meet, because no fertilizing influence will be transmitted. Even if the degree of chemical antagonism developed has not reached a stage that makes fertilization wholly impossible, it may be sufficient to prevent the development of a thrifty offspring. Or, as is quite usual, it may result in the sterility of the hybrid progeny, and thus put a barrier upon farther advance along that line. If proof were needed that such a chemical antagonism prevents the cross-fertilization of species separated too widely, further evidence may be found in the negative results that attend the attempt to graft a branch of one of these species upon the stock of the other. Generally speaking, it will be found that species that cannot be cross-fertilized, also cannot be cross-grafted. In exceptional cases, it is possible to effect the graft where efforts at hybridization have proved futile. Such was the case, for example, with my grafted tomato and potato vine. But in general, the plant that refuses to mate with another plant refuses also to accept its steni as a companion organism when grafted or budded. However carefully the grafting experiment may be performed in such a case, the uncongeniality between stock and cion is soon made manifest. The surfaces do not unite; or if union takes place there is but slight tendency to grow; or the cion does not thrive, and is presently blighted. There are all gradations-from actual poisoning in which there is no tendency whatever to unite, to a partial or even temporarily complete union, followed by separation even after years of growth-according to the degree of antagonism. This chemical antagonism between the tissues of the plants themselves affords the surest evidence of the long periods of time during which the two species have lived under more or less divergent conditions, and have been occupied, each in its own way, in the development of new characteristics. Yet that such intimate differences of constitution should obtain between species that show many outward points of resemblance must always be matter for surprise to the plant lover whose attention is called to it for the first time. That this intimate record of grades of cousinship should be permanently graven in the protoplasm of the plant itself is one of the most mystifying and thought-compelling of biological revelations. If any one were to doubt that the intimate chemical structure of plant protoplasm and plant juices may thus be depended on to reveal genetic relationships, and to mark nice shades of distinction between allied forms, evidence from a quite alien field might be cited that would set the matter at rest.
EVIDENCE FROM THE ANIMAL WORLD
The evidence in question is furnished by an extraordinary series of blood tests through which Dr. G. H. F. Nuttall, the American Professor of Biology at Cambridge University, has traced the intimate relationship of large numbers of animals of different orders. By inoculating rabbits with the blood of different species of birds, reptiles, and mammals, Dr. Nuttall was able to develop an extraordinary serum with which the intimate constitution of the blood of other species of animals could be tested. He thus demonstrated, for example, that lizards and serpents are more closely related than turtles and crocodiles, but that all these reptiles are nearer to one another than they are to birds, and nearer to birds than to mammals. He showed that the dog carries in every drop of its blood chemical proof of closer relationship with wolves, and foxes, and jackals of every species than with any member whatever of the cat family. Similarly, all cats-tigers, lions, leopards, along with the domestic tabby-give proof, in the chemical constitution of their blood, of a common origin. And, bringing the comparison still nearer home, the blood of man is more like that of the chimpanzee, the gorilla, and the orang, than it is like that of any other creatures; and the monkey tribes of the Old World are more manlike in the constitution of their blood than are the monkeys of the New World. Dr. Nuttall's experiments comprised sixteen thousand individual tests, with a total of at least 586 species of mammals, birds, reptiles, batrachians, fishes, and crustaceans, coming from all parts of the globe. The biological implications of his experiments have been commented upon as follows: "Doubtless some hundreds of thousands of years have elapsed since the direct ancestors of men branched from a common stein with the direct ancestors of the gorilla. There has been no blending of blood in the intervening centuries. Cats have been cats and dogs dogs from geological epochs so remote that we hesitate to guess their span in terms of years. So the intimate chemical qualities that denote man or ape or cat or dog, each in contradistinction to all the others, must have been transmitted unmodified through countless thousands of generations. "It taxes credulity to believe that such intangible properties could be transmitted unmodified through the blood streams of such myriads of individuals; but the evidence of the test-tubes proves that this has been done. "What makes the marvel greater is the fact that the bodies of the animals have meantime been so modified as to develop utterly divergent species-for example, the lion, the tiger, the puma, the leopard, and the house cat; different types of dogs, wolves, foxes and their allies. But in each case some intangible quality of the blood remains unchanged to prove the common origin. Blood is indeed thicker than water." The bearing of these extraordinary experiments upon the case in hand will be obvious. If animals carry in their veins generation after generation, through untold thousands of years, these intimate chemical conditions, then the same thing may well be supposed to be true of plants. And so the affinity shown between species that can be hybridized, and the antagonism between species that refuse to hybridize, can be explained on the basis of a fundamental intrinsic quality of the protoplasm that is the foundation substance of life. This gives us a more profound and comprehensive appreciation of the word "affinity" as applied to various species of plants than we could otherwise have. It also makes it in a measure comprehensible that the traits of remote ancestors should be carried latent in the tissues of the germ-plasm, as we have seen that they are carried, for untold generations.
THE CONTINUITY OF THE GERM-PLASM
This germ-plasm, which is the connecting link between one generation and another, is passed only according to the prevalent idea, from parent to offspring, generation after generation, subject only to such modifications as may from time to time be imposed through environing influences. The physical mechanism that underlies this transfer we shall have occasion to examine in another connection when we discuss at some length the theories of heredity. For the moment it is enough to reflect that as the offspring in each successive generation spring from the substance of the parent, the germ-plasm may be thought of as a continuous stream uniting the remotest ancestor of any given strain with the most recent descendant. Every tree in the orchard, for example, carries within its tissues a portion of protoplasmic matter that has come down to it through an almost infinite series of growths and divisions in unbroken succession from the first tree that ever developed on the earth, or, for that matter, from a vast series of more primitive organisms that were the progenitors of the first tree. And while this stream of primordial protoplasm has been changed by an infinitesimal quantity in each successive era, it has retained even to the present the fundamental characteristics that it had from the outset. That such is the case seems little less than a miracle; that an almost microscopical speck of protoplasm which we term a pollen grain should contain the potentialities of thousands of generations of ancestors, and should be able to transmit them with such force that the seed growing from the ovule fertilized by that pollen grain will inevitably produce, let us say, an apple tree, not a pear tree or a plum, is beyond comprehension. Yet we know it to be true. And so the plant hybridizer who consciously merges two different protoplasmic streams when he brings the pollen of one flower to the pistil of another, participates in what must be considered the most wonderful of all experiments. He brings tokens out of an almost infinite past to blend with the divergent tokens of another ancestral stream no less ancient. And it is not strange if he feels a certain impulse of elation when he reflects that his conscious efforts have thus brought together ancestral tendencies that have long been separated and that now will appear in new combinations stimulating such interplay of life-forces as may bring into being plant forms that may be described, without violence to the use of words, as new creations.
-That the intimate record of cousinship, in all its grades, should be permanently graven in the protoplasm of every living thing, is a thought-compelling biological revelation.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 3 Chapter 2