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Species and Varieties, Their Origin by Mutation

H >> Hugo DeVries >> Species and Varieties, Their Origin by Mutation




Nature ordinarily prefers the sexual way. Asexual multiplications,
although very common with perennial plants, appear not to offer
important material for selection. Hence it follows that in comparing the
work of nature with that of man, the results of selection followed by
vegetative propagation should always be carefully excluded. Our large
bulb-flowers and delicious fruits have nothing in common with natural
products, and do not yield a standard by which to judge nature's work.

It is very difficult for a botanist to give a survey of what practice
has attained by the asexual multiplication of extremes. Nearly all of
the large and more palatable fruits are due to such efforts. Some
flowers and garden-plants afford further instances. By far the greatest
majority of improved asexual varieties, however, are not the result of
pure intra-specific selection. They are due largely to the choice of the
best existing elementary species, and to some extent to crosses between
them, or between distinct systematic species. In practice selection and
hybridization go hand in hand and it is often difficult to ascertain
what part of [752] the result is due to the one, and what to the other
factor.

The scientist, on the contrary, has nothing to do with the industrial
product. His task is the analysis of the methods, in order to reach a
clear appreciation of the influence of all the competing factors. This
study of the working causes leads to a better understanding of the
practical processes, and may become the basis of improvement in methods.

Starting from these considerations, we will now give some illustrative
examples, and for the first, choose one in which hybridization is almost
completely excluded.

Sugar-canes have long been considered to be plants without seed. Their
numerous varieties are propagated only in a vegetative way. The stems
are cut into pieces, each bearing one or two or more nodes with their
buds. An entire variety, though it may be cultivated in large districts
and even in various countries, behaves with respect to variability as a
single individual. Its individual fluctuability has been limited to the
earliest period of its life, when it arose from an unknown seed. The
personal characters that have been stamped on this one seed, partly by
its descent, and partly in the development of its germ during the period
of ripening, have become the indelible characters [753] of the variety,
and only the partial fluctuability, due to the effect of later
influences, can now be studied statistically.

This study has for its main object the production of sugar in the stems,
and the curves, which indicate the percentage of this important
substance in different stems of the same variety, comply with Quetelet's
law. Each variety has its own average, and around this the data of the
majority of the stems are densely crowded, while deviations on both
sides are rare and become the rarer the wider they are. The "Cheribon"
cane is the richest variety cultivated in Java, and has an average of
19% sugar, while it fluctuates between 11% and 28%. "Chunnic" averages
14%, "Black Manilla" 13% and "White Manilla" 10%; their highest and
lowest extremes diverge in the same manner, being for the last named
variety 1% and 15%.

This partial variability is of high practical interest, because on it a
selection may be founded. According to the conceptions described in a
previous lecture, fluctuating variability is the result of those outward
factors that determine the strength of development of the plant or the
organ. The inconstancy of the degree of sensibility, combined with the
ever-varying weather conditions preclude any close proportionality, but
apart from this difficulty there is, in the [754] main, a distinct
relation between organic strength and the development of single
qualities. This correlation has not escaped observation in the case of
the sugar-cane, and it is known that the best grown stocks are generally
the richest in sugar. Now it is evident that the best grown and richest
stems will have the greater chance of transmitting these qualities to
the lateral-buds. This at once gives, a basis for vegetative selection,
upon which it is not necessary to choose a small number of very
excellent stems, but simply to avoid the planting of all those that are
below the average. By this means the yield of the cultures has often
noticeably been enhanced.

As far as experience goes, this sort of selection, however profitable,
does not conduce to the production of improved races. Only temporary
ameliorations are obtained, and the selection must be made in the same
manner every year. Moreover the improvement is very limited and does not
give any promise of further increase. In order to reach this, one has to
recur to the individual fluctuability, and therefore to seed.

Nearly half a century ago, Parris discovered, on the island of Barbados,
that seeds might occasionally be gathered from the canes. These,
however, yielded only grass-like plants of no real value. The same
observation was made [755] shortly afterwards in Java and in other sugar
producing countries. In the year 1885, Soltwedel, the director of one of
the experiment stations for the culture of sugar-cane in Java, conceived
the idea of making use of seedlings for the production of improved
races. This idea is a very practical one, precisely because of the
possibility of vegetative propagation. If individuals would show the
same range as that of partial fluctuability, then the choice of the
extremes would at once bring the average up to the richness of the best
stocks. Once attained, this average would be fixed, without further
efforts.

Unfortunately there is one great drawback. This is the infertility of
the best variety, that of the "Cheribon" cane. It flowers abundantly in
some years, but it has never been known to produce ripe seeds. For this
reason Soltwedel had to start from the second best sort, and chose the
"Hawaii" cane. This variety usually yields about 14% sugar, and
Soltwedel found among his seedlings one that showed 15%. This fact was
quite unexpected at that time, and excited widespread interest in the
new method, and since then it has been applied to numerous varieties,
and many thousands of seedlings have been raised and tested as to their
sugar-production.

[756] From a scientific point of view the results are quite striking.
From the practical standpoint, however, the question is, whether the
"Hawaii" and other fertile varieties are adequate to yield seedlings,
which will surpass the infertile "Cheribon" cane. Now "Hawaii" averages
14% and "Cheribon" 19%, and it is easily understood that a "Hawaii"
seedling with more than 19% can be expected only from very large
sowings. Hundreds of thousands of seedlings must be cultivated, and
their juice tested, before this improvement can be reached. Even then,
it may have no significance for practical purposes. Next to the amount
of sugar comes the resistance to the disease called "Sereh," and the new
race requires to be ameliorated in this important direction, too. Other
qualities must also be considered, and any casual deterioration in other
characters would make all progress illusory. For these reasons much time
is required to attain distinct improvements.

These great difficulties in the way of selecting extremes for vegetative
propagation are of course met with everywhere. They impede the work of
the breeder to such a degree, that but few men are able to surmount
them. Breeding new varieties necessitates the bending of every effort to
this purpose, and a clear conception of [757] the manifold aspects of
this intricate problem. These fall under two heads, the exigencies of
practice, and the physiologic laws of variability. Of course, only the
latter heading comes within the limits of our discussion which includes
two main points. First comes the general law of fluctuation that, though
slight deviations from the average may be found by thousands, or rather
in nearly every individual, larger and therefore important deviations
are very rare. Thousands of seedlings must be examined carefully in
order to find one or two from which it might be profitable to start a
new race. This point is the same for practical and for scientific
investigation. In the second place however, a digression is met with.
The practical man must take into consideration all the varying qualities
of his improved strains. Some of them must be increased and others be
decreased, and their common dependency on external conditions often
makes it very difficult to discover the desired combinations. It is
obvious, however, that the neglect of one quality may make all
improvement of other characters wholly useless. No augmentation of
sugar-percentage, of size and flavor of fruits can counterbalance an
increase in sensitiveness to disease, and so it is with other qualities
also.

[758] Improved races for scientific investigation can be kept free from
infection, and protected against numerous other injuries. In the
experimental garden they may find conditions which cannot be realized
elsewhere. They may show a luxuriant growth, and prove to be excellent
material for research, but have features which, having been overlooked
at the period of selection, would at once condemn them if left to
ordinary conditions, or to the competition of other species.

Considering all these obstacles, it is only natural that breeders should
use every means to reach their goal. Only in very rare instances do they
follow methods analogous to scientific processes, which tend to simplify
the questions as much as possible. As a rule, the practical way is the
combination of as many causes of variability as possible. Now the three
great sources of variability are, as has been pointed out on several
occasions, the original multiformity of the species, fluctuating
variability, and hybridization. Hence, in practical experiments, all
three are combined. Together they yield results of the highest value,
and Burbank's improved fruits and flowers give testimony to the
practical significance of this combination.

From a scientific point of view however, it is [759] ordinarily
difficult, if not impossible, to discern the part which each of the
three great branches of variability has taken in the origination of the
product. A full analysis is rarely possible, and the treatment of one of
the three factors must necessarily remain incomplete.

Notwithstanding these considerations, I will now give some examples in
order to show that fluctuating variability plays a prominent part in
these improvements. Of course it is the third in importance in the
series. First comes the choice of the material from the assemblage of
species, elementary species and varieties. Hybridization comes next in
importance. But even the hybrids of the best parents may be improved,
because they are no less subject to Quetelet's law than any other
strain. Any large number of hybrids of the same ancestry will prove
this, and often the excellency of a hybrid variety depends chiefly, or
at least definitely, on the selection of the best individuals. Being
propagated only in a vegetative way, they retain their original good
qualities through all further culture and multiplication.

As an illustrative example I will take the genus _Canna_. Originally
cultivated for its large and bright foliage only, it has since become a
flowering plant of value. Our garden strains have originated by the
crossing of [760] a number of introduced wild species, among which the
_Canna indica_ is the oldest, now giving its name to the whole group. It
has tall stems and spikes with rather inconspicuous flowers with narrow
petals. It has been crossed with _C. nepalensis_ and _C. warczewiczii_,
and the available historic evidence points to the year 1846 as that of
the first cross. This was made by Annee between the _indica_ and the
_nepalensis_; it took ten years to multiply them to the required degree
for introduction into commerce. These first hybrids had bright foliage
and were tall plants, but their flowers were by no means remarkable.

Once begun, hybridization was widely practiced. About the year 1889
Crozy exhibited at Paris the first beautifully flowering form, which he
named for his wife, "Madame Crozy." Since that time he and many others,
have improved the flowers in the shape and size, as well as in color and
its patterns. In the main, these ameliorations have been due to the
discovery and introduction of new wild species possessing the required
characters. This is illustrated by the following incident. In the year
1892 I visited Mr. Crozy at Lyons. He showed me his nursery and numerous
acquisitions, those of former years as well as those that were quite
new, and which were in the process of rapid [761] multiplication,
previous to being given to the trade. I wondered, and asked, why no pure
white variety was present. His answer was "Because no white species had
been found up to the present time, and there is no other means of
producing white varieties than by crossing the existing forms with a new
white type."

Comparing the varieties produced in successive periods, it is very easy
to appreciate their gradual improvement. On most points this is not
readily put into words, but the size of the petals can be measured, and
the figures may convey at least some idea of the real state of things.
Leaving aside the types with small flowers and cultivated exclusively
for their foliage, the oldest flowers of _Canna_ had petals of 45 mm.
length and 13 mm. breadth. The ordinary types at the time of my visit
had reached 61 by 21 mm., and the "Madame Crozy" showed 66 by 30 mm. It
had however, already been surpassed by a few commercial varieties, which
had the same length but a breadth of 35 mm. And the latest production,
which required some years of propagation before being put on the market,
measured 83 by 43 mm. Thus in the lapse of some thirty years the length
had been doubled and the breadth tripled, giving flowers with broad
corollas and with petals [762] joined all around, resembling the best
types of lilies and _Amaryllis_.

Striking as this result unquestionably is, it remains doubtful as to
what part of it is due to the discovery and introduction of new large
flowered species, and what to the selection of the extremes of
fluctuating variability. As far as I have been able to ascertain
however, and according to the evidence given to me by Mr. Crozy,
selection has had the largest part in regard to the size, while the
color-patterns are introduced qualities.

The scientific analysis of other intricate examples is still more
difficult. To the practical breeder they often seem very simple, but the
student of heredity, who wishes to discern the different factors, is
often quite puzzled by this apparent simplicity. So it is in the case of
the double lilacs, a large number of varieties of which have recently
been originated and introduced into commerce by Lemoine of Nancy. In the
main they owe their origin to the crossing and recrossing of a single
plant of the old double variety with the numerous existing
single-flowered sorts.

This double variety seems to be as old as the culture of the lilacs. It
was already known to Munting, who described it in the year 1671. Two
centuries afterwards, in 1870, a new description [763] was given by
Morren, and though more than one varietal name is recorded in his paper,
it appears from the facts given that even at that time only one variety
existed. It was commonly called _Syringa vulgaris azurea plena_, and
seems to have been very rare and without real ornamental value.

Lemoine, however, conceived the desirability of a combination of the
doubling with the bright colors and large flower-racemes of other
lilacs, and performed a series of crosses. The "_azurea plena_" has no
stamens, and therefore must be used in all crosses as the pistil-parent;
its ovary is narrowly inclosed in the tube of the flower, and difficult
to fertilize. On the other hand, new crosses could be made every year,
and the total number of hybrids with different pollen-parents was
rapidly increased. After five years the hybrids began to flower and
could be used for new crosses, yielding a series of compound hybrids,
which however, were not kept separate from the products of the first
crosses.

Gradually the number of the flowering specimens increased, and the
character of doubling was observed to be variable to a high degree.
Sometimes only one supernumerary petal was produced, sometimes a whole
new typical corolla was extruded from within the first. In the same
[764] way the color and the number of the flowers on each raceme were
seen to vary. Thousands of hybrids were produced, and only those which
exhibited real advantages were selected for trade. These were multiplied
by grafting, and each variety at present consists only of the buds of
one original individual and their products. No constancy from seed is
assumed, many varieties are even quite sterile.

Of course, no description was given of the rejected forms. It is only
stated that many of them bore either single or poorly filled flowers, or
were objectionable in some other way. The range of variability, from
which the choices were made, is obscure and only the fact of the
selection is prominent. What part is due to the combination of the
parental features and what to the individual fluctuation of the hybrid
itself cannot be ascertained.

So it is in numerous other instances. The dahlias have been derived from
three or more original species, and been subjected to cultivation and
hybridization in an ever-increasing scale for a century. The best
varieties are only propagated in the vegetative way, by the roots and
buds, or by grafting and cutting. Each of them is, with regard to its
hereditary qualities, only one individual, and the individual characters
were selected at the same time with the [765] varietal and hybrid
characters. Most of them are very inconstant from seed and as a rule,
only mixtures are offered for sale in seed-lists. Which of their
ornamental features are due to fluctuating deviation from an average is
of course unknown. _Amaryllis_ and _Gladiolus_ are surrounded with the
same scientific uncertainties. Eight or ten, or even more, species have
been combined into one large and multiform strain, each bringing its
peculiar qualities into the mixed mass. Every hybrid variety is one
individual, being propagated by bulbs only. Colors and color-patterns,
shape of petals and other marks, have been derived from the wild
ancestors, but the large size of many of the best varieties is probably
due to the selection of the extremes of fluctuating variability. So it
is with the begonias of our gardens, which are also composite hybrids,
but are usually sown on a very large scale. Flowers of 15 cm. diameter
are very showy, but there can be no doubt about the manner in which they
are produced, as the wild species fall far short of this size.

Among vegetables the potatoes afford another instance. Originally quite
a number of good species were in culture, most of them having small
tubers. Our present varieties are due to hybridization and selection,
each of them being propagated only in the vegetative way.

[766] Selection is founded upon different qualities, according to the
use to be made of the new sort. Potatoes for the factory have even been
selected for their amount of starch, and in this case at least,
fluctuating variability has played a very important part in the
improvement of the race.

Vegetative propagation has the great advantage of exempting the
varieties from regression to mediocrity, which always follows
multiplication by seeds. It affords the possibility of keeping the
extremes constant, and this is not its only advantage. Another, likewise
highly interesting, side of the question is the uniformity of the whole
strain. This is especially important in the case of fruits, though
ordinarily it is regarded as a matter of course, but there are some
exceptions which give proof of the real importance of the usual
condition. For example, the walnut-tree. Thousands of acres of
walnut-orchards consist of seedling trees grown from nuts of unknown
parentage. The result is a great diversity in the types of trees and in
the size and shape of the nuts, and this diversity is an obvious
disadvantage to the industry. The cause lies in the enormous
difficulties attached to grafting or budding of these trees, which make
this method very expensive and to a high degree uncertain and
unsatisfactory.

[767] After this hasty survey of the more reliable facts of the practice
of an asexual multiplication of the extremes of fluctuating variability,
we may now return to the previously mentioned theoretical
considerations. These are concerned with an estimation of the chances of
the occurrence of deviations, large enough to exhibit commercial value.
This chance may be calculated on the basis of Quetelet's law, whenever
the agreement of the fluctuation of the quality under consideration has
been empirically determined. In the discussion of the methods of
comparing two curves, we have pointed to the quartiles as the decisive
points, and to the necessity of drawing the curves so that these points
are made to overlie one another, on each side of the average. If now we
calculate the binomium of Newton for different values of the exponent,
the sum of the coefficients is doubled for each higher unit of the
exponent, and at the same time the extreme limit of the curve is
extended one step farther. Hence it is possible to calculate a relation
between the value of the extreme and the number of cases required. It
would take us too long to give this calculation in detail, but it is
easily seen that for each succeeding step the number of individuals must
be doubled, though the length of the steps, or the amount of increase of
the quality [768] remains the same. The result is that many thousands of
seedlings are required to go beyond the ordinary range of variations,
and that every further improvement requires the doubling of the whole
culture. If ten thousand do not give a profitable deviation, the next
step requires twenty thousand, the following forty thousand, and so on.
And all this work would be necessary for the improvement of a single
quality, while practice requires the examination and amelioration of
nearly all the variable characters of the strain.

Hence the rule that great results can only be obtained by the use of
large numbers, but it is of no avail to state this conclusion from a
scientific point of view. Scientific experimenters will rarely be able
to sacrifice fifty thousand plants to a single selection. The problem is
to introduce the principle into practice and to prove its direct
usefulness and reliability. It is to Luther Burbank that we owe this
great achievement. His principles are in full harmony with the teachings
of science. His methods are hybridization and selection in the broadest
sense and on the largest scale. One very illustrative example of his
methods must suffice to convey an idea of the work necessary to produce
a new race of superlative excellency. Forty thousand blackberry and
raspberry [769] hybrids were produced and grown until the fruit matured.
Then from the whole lot a single variety was chosen as the best. It is
now known under the name of "Paradox." All others were uprooted with
their crop of ripening berries, heaped up into a pile twelve feet wide,
fourteen feet high and twenty-two feet long, and burned. Nothing
remained of that expensive and lengthy experiment, except the one
parent-plant of the new variety. Similar selections and similar amount
of work have produced the famous plums, the brambles and the
blackberries, the Shasta daisy, the peach almond, the improved
blueberries, the hybrid lilies, and the many other valuable fruits and
garden-flowers that have made the fame of Burbank and the glory of
horticultural California.



[770]

LECTURE XXVII

INCONSTANCY OF IMPROVED RACES

The greater advantages of the asexual multiplication of extremes are of
course restricted to perennial and woody plants. Annual and biennial
species cannot as a rule, be propagated in this way, and even with some
perennials horticulturists prefer the sale of seeds to that of roots and
bulbs. In all these cases it is clear that the exclusion of the
individual variability, which was shown to be an important point in the
last lecture, must be sacrificed.

Seed-propagation is subject to individual as well as to fluctuating
variability. The first could perhaps be designated by another term,
embryonic variability, since it indicates the fluctuations occurring
during the period of development of the germ. This period begins with
the fusion of the male and female elements and is largely dependent upon
the vigor of these cells at the moment, and on the varying qualities
they may have acquired. It comprises in the main the time of the
ripening of the seed, and [771] might perhaps best be considered to end
with the beginning of the resting stage of the ripe seed. Hence it is
clear that the variability of seed-propagated annual races has a wider
range than that of perennials, shrubs and trees. At present it is
difficult to discern exactly the part each of these two main factors
plays in the process. Many indications are found however, that make it
probable that embryonic variability is wider, and perhaps of far greater
importance than the subsequent partial fluctuations. The high degree of
similarity between the single specimens of a vegetative variety, and the
large amount of variability in seed-races strongly supports this view.
The propagation and multiplication of the extremes of fluctuating
variability by means of seeds requires a close consideration of the
relation between seedling and parent. The easiest way to get a clear
conception of this relation is to make use of the ideas concerning the
dependency of variability upon nourishment. Assuming these to be correct
in the main, and leaving aside all minor questions, we may conclude that
the chosen extreme individual is one of the best nourished and
intrinsically most vigorous of the whole culture. On account of these
very qualities it is capable of nourishing all of its organs better and
also its seeds. In other words, the seeds [772] of the extreme
individuals have exceptional chances of becoming better nourished than
the average of the seeds of the race. Applying the same rule to them, it
is easily understood that they will vary, by reason of this better
nourishment, in a direction corresponding to that of their parent.

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