Popular Science Monthly/Volume 75/September 1909/Another Mode of Species Forming
|ANOTHER MODE OF SPECIES FORMING|
By LUTHER BURBANK
SANTA ROSA, CAL.
THE more usual concept of the formation of species is by slow variations so well known as the Darwinian theory, which though attacked from every point, still is and must always in the main be accepted, for without question it gives the fundamental principles of evolution as had never been done before. Yet the boundless amount of research along these lines during the last half century has developed strong new sidelights which illuminate, and in some cases compel a slightly different view of, some of the suggestions of the master, Darwin.
During the period of forty years that I have been experimenting with plant life both in bleak New England and in sunny California, extensively operating on much more than four thousand five hundred distinct species of plants, including all known economic and ornamental plant forms which are grown in the open air in temperate and semi-tropic climates, as well as many of those commonly grown in greenhouses and numerous absolutely new ones not before domesticated and on a scale never before attempted by any individual or body of individuals, numerous general principles have pressed themselves forward for discussion and observation. Only one of these can be discussed at this time, and this briefly, more as a text for further observations and experiments than as anything like a full view of this highly interesting mode of species formation.
In the first place, let me say that our so-called species are only tentative bundles of plants, no two individuals of which are exactly alike, but nearly all of which quite closely resemble each other in general outside appearances and in hereditary tendencies. Yet no one can tell just what the result will be when combinations of these inherent tendencies are crossed or subjected to any other disturbing factor or factors. Like the chemist who has new elements to work with, we may predict with some degree of accuracy what the general results will be, but any definite knowledge of the results of these combinations is far more difficult, even impossible, as the life forces of plants and animals act in infinitely more new directions than can any ordinary number of combinations of chemicals.
Only a few years ago, it was generally supposed that by crossing two somewhat different species or varieties a mongrel might be produced which might, or more likely might not, surpass its parents. The fact that crossing was only the first step and that selection from the numerous variations secured in the second and a few succeeding generations was the real work of new plant creation had never been appreciated; and to-day its significance is not fully understood either by breeders or even by many scientific investigators along these very lines. Old tailings are constantly being worked over at great expense of time and with small profit, while the mother lode is repudiated and neglected.
Plant breeding to be successful must be conducted like architecture. Definite plans must be carefully laid for the proposed creation; suitable materials selected with judgment, and these must be securely placed in their proper order and position. No occupation requires more accuracy, foresight and skill than does scientific plant or animal breeding.
As before noted, the first generation after a cross has been made is usually a more or less complete blend of all the characteristics of both parents; not only the visible characters, but an infinite number of invisible ones are inherent and will shape the future character and destiny of the descendants, often producing otherwise unaccountable so-called mutations, saltations or sports, the selection and perpetuation of which give to new plant creations their unique forms and often priceless values, like the Burbank potato produced thirty-six years ago and which is now grown on this western coast almost to the exclusion of all others (fourteen millions of bushels per annum, besides the vast amount grown in the-eastern United States and other countries), or the Bartlett pear, Baldwin apple and navel oranges, all of which are variations selected by some keen observer. Millions of others are forever buried in oblivion for the lack of such an observer.
But in this paper I wish to call attention to a not unusual result of crossing quite distinct wild species which deserves the most careful analysis, as it seems to promise a new text for scientific investigation, especially on biometric lines. The subject was most forcibly brought to my attention twenty years ago by the singular behavior of the second-generation seedlings of raspberry-blackberry hybrids. By crossing the Siberian raspberry (Rubus cratægifolius) with our native trailing blackberry (Rubus vitifolius), a thoroughly fixed new species was summarily produced. The seedlings of this composite Rubus (named Primus), though a most perfect blend of both parents but resembling neither, never reverted either way; all the seedlings coming much more exactly like the new type than do the seedlings of any ordinary wild rubus. Many thousand plants have been raised generation after generation, all repeating themselves after the new and unique type. No botanist on earth could do otherwise than classify it if found wild as a valid new species, which it truly is, though so summarily produced by crossing. Since the Primus species was originated, numerous similar cases have attracted attention, such as my now popular Phenomenal produced by crossing the Cuthbert raspberry with our native Pacific coast blackberry, and the Logan berry, both of which, though a complete blend of two such distinct species, yet reproduce from seed as truly as any wild rubus species.
I have had also growing on my grounds for some fifteen years or more hybrids of Rubus idæus and Rubus villosus, both red and yellow varieties. All are exactly intermediate between these two very widely different species, yet both always come true intermediates from seed, generation after generation, never reverting either way.
By crossing the great African "stubble berry" (Solanum guinense) with our Pacific coast "rabbit weed" (Solanum villosum) an absolutely new species has also been produced, the fruit of which resembles in almost every particular the common blueberry (Vaccinium Pennsylvanicum), and while the fruit of neither parent species is edible, the fruit of the newly created one is most delicious and most abundantly produced, and the seedlings, generation after generation though produced by the million, still, all come as true to the new type as do either parent species to their normal type.
Still another example of this mode may be found in my experiments with opuntias. By crossing O. tuna with O. vulgaris, thousands of seedlings have been produced, all of which, in the first, second and third generations, though a well-balanced blend of the two natural species, still come as true to the newly created species as do either parent species to their own natural types.
Not only does this new mode hold true under cultivation but species are also summarily produced in a wild state by natural crossing.
The western blackcap (Rubus occidentalis) and the eastern red raspberry (Rubus strigosus) when growing contiguous, as they very commonly do in Central British America, often cross, forming an intermediate new species which sometimes sorely crowds both of the parent species, and when brought under cultivation still firmly maintains its intermediate characters, no matter how often reproduced from seed. And still further, our common "tarweed" (Madia elegans) with its beautiful large blossoms often crosses with M. saliva with its insignificant pale yellow flowers, producing a complete intermediate. I have not yet determined whether the intermediate will reproduce true from seed, but confidently expect it to do so. Similar results among wild evergreens and deciduous trees and shrubs and herbaceous plants have been frequently and forcefully brought to my attention, leaving little doubt in my own mind that the evolution of species is by more modes than some are inclined to admit.
- Read at the annual meeting of the American Breeders' Association, at Columbia, Mo., January 5 to 8, 1909.