One of the propositions of Darwin's theory of the struggle for life
maintains that the largest amount of life can be supported on any area,
by great diversification or divergence in the structure and constitution
of its inhabitants. Every meadow and every forest affords a proof of
this thesis. The numerical proportion of the different species of the
flora is always changing according to external influences. Thus, in a
given meadow, some species will flower abundantly in one year and then
almost disappear, until, after a series of years, circumstances allow
them again to multiply rapidly. Other species, which have taken their
places, will then become rare. It follows from this principle, that
notwithstanding the constantly changing conditions, a suitable
selection from the constituents of a meadow will ensure a continued
high production. But, although the principle is quite clear, artificial
selection has, as yet, done very little towards reaching a really high
standard.
The same holds good for cereals. In ordinary circumstances a field
will give a greater yield, if the crop grown consists of a number of
sufficiently differing types. Hence it happens that almost all older
varieties of wheat are mixtures of more or less diverging forms. In the
same variety the numerical composition will vary from year to year, and
in oats this may, in bad years, go so far as to destroy more than half
of the harvest, the wind-oats (Avena fatua), which scatter their grain
to the winds as soon as it ripens, increasing so rapidly that they
assume the dominant place. A severe winter, a cold spring and other
extreme conditions of life will destroy one form more completely
than another, and it is evident that great changes in the numerical
composition of the mixture may thus be brought about.
This mixed condition of the common varieties of cereals was well known
to Darwin. For him it constituted one of the many types of variability.
It is of that peculiar nature to which, in describing other groups,
he applies the term polymorphy. It does not imply that the single
constituents of the varieties are at present really changing their
characters. On the other hand, it does not exclude the possibility of
such changes. It simply states that observation shows the existence of
different forms; how these have originated is a question which it
does not deal with. In his well-known discussion of the variability of
cereals, Darwin is mainly concerned with the question, whether under
cultivation they have undergone great changes or only small ones.
The decision ultimately depends on the question, how many forms have
originally been taken into cultivation. Assuming five or six initial
species, the variability must be assumed to have been very large, but
on the assumption that there were between ten and fifteen types, the
necessary range of variability is obviously much smaller. But in regard
to this point, we are of course entirely without historical data.
Few of the varieties of wheat show conspicuous differences, although
their number is great. If we compare the differentiating characters of
the smaller types of cereals with those of ordinary wild species, even
within the same genus or family, they are obviously much less marked.
All these small characters, however, are strictly inherited, and this
fact makes it very probable that the less obvious constituents of the
mixtures in ordinary fields must be constant and pure as long as they do
not intercross. Natural crossing is in most cereals a phenomenon of rare
occurrence, common enough to admit of the production of all possible
hybrid combinations, but requiring the lapse of a long series of years
to reach its full effect.
Darwin laid great stress on this high amount of variability in the
plants of the same variety, and illustrated it by the experience of
Colonel Le Couteur ("On the Varieties, Properties, and Classification of
Wheat", Jersey, 1837.) on his farm on the isle of Jersey, who cultivated
upwards of 150 varieties of wheat, which he claimed were as pure as
those of any other agriculturalist. But Professor La Gasca of Madrid,
who visited him, drew attention to aberrant ears, and pointed out, that
some of them might be better yielders than the majority of plants in the
crop, whilst others might be poor types. Thence he concluded that the
isolation of the better ones might be a means of increasing his crops.
Le Couteur seems to have considered the constancy of such smaller types
after isolation as absolutely probable, since he did not even discuss
the possibility of their being variable or of their yielding a
changeable or mixed progeny. This curious fact proves that he considered
the types, discovered in his fields by La Gasca to be of the same kind
as his other varieties, which until that time he had relied upon as
being pure and uniform. Thus we see, that for him, the variability of
cereals was what we now call polymorphy. He looked through his fields
for useful aberrations, and collected twenty-three new types of wheat.
He was, moreover, clear about one point, which, on being rediscovered
after half a century, has become the starting-point for the new Swedish
principle of selecting agricultural plants. It was the principle of
single-ear sowing, instead of mixing the grains of all the selected ears
together. By sowing each ear on a separate plot he intended not only
to multiply them, but also to compare their value. This comparison
ultimately led him to the choice of some few valuable sorts, one of
which, the "Bellevue de Talavera," still holds its place among the
prominent sorts of wheat cultivated in France. This variety seems to be
really a uniform type, a quality very useful under favourable conditions
of cultivation, but which seems to have destroyed its capacity for
further improvement by selection.
The principle of single-ear sowing, with a view to obtain pure and
uniform strains without further selection, has, until a few years ago,
been almost entirely lost sight of. Only a very few agriculturists have
applied it: among these are Patrick Shirreff ("Die Verbesserung der
Getreide-Arten", translated by R. Hesse, Halle, 1880.) in Scotland
and Willet M. Hays ("Wheat, varieties, breeding, cultivation", Univ.
Minnesota, Agricultural Experimental Station, Bull. no. 62, 1899.) in
Minnesota. Patrick Shirreff observed the fact, that in large fields of
cereals, single plants may from time to time be found with larger ears,
which justify the expectation of a far greater yield. In the course of
about twenty-five years he isolated in this way two varieties of wheat
and two of oats. He simply multiplied them as fast as possible, without
any selection, and put them on the market.
Hays was struck by the fact that the yield of wheat in Minnesota was far
beneath that in the neighbouring States. The local varieties were Fife
and Blue Stem. They gave him, on inspection, some better specimens,
"phenomenal yielders" as he called them. These were simply isolated and
propagated, and, after comparison with the parent-variety and with some
other selected strains of less value, were judged to be of sufficient
importance to be tested by cultivation all over the State of Minnesota.
They have since almost supplanted the original types, at least in most
parts of the State, with the result that the total yield of wheat in
Minnesota is said to have been increased by about a million dollars
yearly.
Definite progress in the method of single-ear sowing has, however, been
made only recently. It had been foreshadowed by Patrick Shirreff, who
after the production of the four varieties already mentioned, tried
to carry out his work on a larger scale, by including numerous minor
deviations from the main type. He found by doing so that the chances
of obtaining a better form were sufficiently increased to justify
the trial. But it was Nilsson who discovered the almost inexhaustible
polymorphy of cereals and other agricultural crops and made it the
starting-point for a new and entirely trustworthy method of the highest
utility. By this means he has produced during the last fifteen years a
number of new and valuable races, which have already supplanted the old
types on numerous farms in Sweden and which are now being introduced on
a large scale into Germany and other European countries.
It is now twenty years since the station at Svalof was founded. During
the first period of its work, embracing about five years, selection was
practised on the principle which was then generally used in Germany. In
order to improve a race a sample of the best ears was carefully selected
from the best fields of the variety. These ears were considered as
representatives of the type under cultivation, and it was assumed that
by sowing their grains on a small plot a family could be obtained, which
could afterwards be improved by a continuous selection. Differences
between the collected ears were either not observed or disregarded. At
Svalof this method of selection was practised on a far larger scale than
on any German farm, and the result was, broadly speaking, the same.
This may be stated in the following words: improvement in a few cases,
failure in all the others. Some few varieties could be improved and
yielded excellent new types, some of which have since been introduced
into Swedish agriculture and are now prominent races in the southern
and middle parts of the country. But the station had definite aims, and
among them was the improvement of the Chevalier barley. This, in
Middle Sweden, is a fine brewer's barley, but liable to failure during
unfavourable summers on account of its slender stems. It was selected
with a view of giving it stiffer stems, but in spite of all the care and
work bestowed upon it no satisfactory result was obtained.
This experience, combined with a number of analogous failures, could
not fail to throw doubt upon the whole method. It was evident that good
results were only exceptions, and that in most cases the principle
was not one that could be relied upon. The exceptions might be due
to unknown causes, and not to the validity of the method; it became
therefore of much more interest to search for the causes than to
continue the work along these lines.
In the year 1892 a number of different varieties of cereals were
cultivated on a large scale and a selection was again made from
them. About two hundred samples of ears were chosen, each apparently
constituting a different type. Their seeds were sown on separate plots
and manured and treated as much as possible in the same manner. The
plots were small and arranged in rows so as to facilitate the comparison
of allied types. During the whole period of growth and during the
ripening of the ears the plots were carefully studied and compared: they
were harvested separately; ears and kernels were counted and weighed,
and notes were made concerning layering, rust and other cereal pests.
The result of this experiment was, in the main, no distinct improvement.
Nilsson was especially struck by the fact that the plots, which should
represent distinct types, were far from uniform. Many of them were as
multiform as the fields from which the parent-ears were taken. Others
showed variability in a less degree, but in almost all of them it was
clear that a pure race had not been obtained. The experiment was a fair
one, inasmuch as it demonstrated the polymorphic variability of cereals
beyond all doubt and in a degree hitherto unsuspected; but from the
standpoint of the selectionist it was a failure. Fortunately there were,
however, one or two exceptions. A few lots showed a perfect uniformity
in regard to all the stalks and ears: these were small families. This
fact suggested the idea that each might have been derived from a single
ear. During the selection in the previous summer, Nilsson had tried to
find as many ears as possible of each new type which he recognised in
his fields. But the variability of his crops was so great, that he was
rarely able to include more than two or three ears in the same group,
and, in a few cases, he found only one representative of the supposed
type. It might, therefore, be possible that those small uniform plots
were the direct progeny of ears, the grains of which had not been mixed
with those from other ears before sowing. Exact records had, of course,
been kept of the chosen samples, and the number of ears had been noted
in each case. It was, therefore, possible to answer the question and it
was found that those plots alone were uniform on which the kernels of
one single ear only had been sown. Nilsson concluded that the mixture
of two or more ears in a single sowing might be the cause of the lack of
uniformity in the progeny. Apparently similar ears might be different in
their progeny.
Once discovered, this fact was elevated to the rank of a leading
principle and tested on as large a scale as possible. The fields were
again carefully investigated and every single ear, which showed a
distinct divergence from the main type in one character or another,
was selected. A thousand samples were chosen, but this time each sample
consisted of one ear only. Next year, the result corresponded to the
expectation. Uniformity prevailed almost everywhere; only a few
lots showed a discrepancy, which might be ascribed to the accidental
selection of hybrid ears. It was now clear that the progeny of single
ears was, as a rule, pure, whereas that of mixed ears was impure.
The single-ear selection or single-ear sowing, which had fallen into
discredit in Germany and elsewhere in Europe, was rediscovered. It
proved to be the only trustworthy principle of selection. Once isolated,
such single-parent races are constant from seed and remain true to their
type. No further selection is needed; they have simply to be multiplied
and their real value tested.
Patrick Shirreff, in his early experiments, Le Couteur, Hays and others
had observed the rare occurrence of exceptionally good yielders and the
value of their isolation to the agriculturist. The possibility of error
in the choice of such striking specimens and the necessity of judging
their value by their progeny were also known to these investigators, but
they had not the slightest idea of all the possibilities suggested by
their principle. Nilsson, who is a botanist as well as an agriculturist,
discovered that, besides these exceptionably good yielders, every
variety of a cereal consists of hundreds of different types, which find
the best conditions for success when grown together, but which, after
isolation, prove to be constant. Their preference for mixed growth is so
definite, that once isolated, their claims on manure and treatment
are found to be much higher than those of the original mixed variety.
Moreover, the greatest care is necessary to enable them to retain
their purity, and as soon as they are left to themselves they begin to
deteriorate through accidental crosses and admixtures and rapidly return
to the mixed condition.
Reverting now to Darwin's discussion of the variability of cereals, we
may conclude that subsequent investigation has proved it to be exactly
of the kind which he describes. The only difference is that in reality
it reaches a degree, quite unexpected by Darwin and his contemporaries.
But it is polymorphic variability in the strictest sense of the word.
How the single constituents of a variety originate we do not see. We
may assume, and there can hardly be a doubt about the truth of the
assumption, that a new character, once produced, will slowly but surely
be combined through accidental crosses with a large number of
previously existing types, and so will tend to double the number of the
constituents of the variety. But whether it first appears suddenly or
whether it is only slowly evolved we cannot determine. It would, of
course, be impossible to observe either process in such a mixture. Only
cultures of pure races, of single-parent races as we have called them,
can afford an opportunity for this kind of observation. In the fields of
Svalof new and unexpected qualities have recently been seen, from time
to time, to appear suddenly. These characters are as distinct as the
older ones and appear to be constant from the moment of their origin.
Darwin has repeatedly insisted that man does not cause variability. He
simply selects the variations given to him by the hand of nature. He may
repeat this process in order to accumulate different new characters
in the same family, thus producing varieties of a higher order. This
process of accumulation would, if continued for a longer time, lead to
the augmentation of the slight differences characteristic of varieties
into the greater differences characteristic of species and genera. It is
in this way that horticultural and agricultural experience contribute
to the problem of the conversion of varieties into species, and to the
explanation of the admirable adaptations of each organism to its complex
conditions of life. In the long run new forms, distinguished from their
allies by quite a number of new characters, would, by the extermination
of the older intermediates, become distinct species.
Thus we see that the theory of the origin of species by means of natural
selection is quite independent of the question, how the variations to
be selected arise. They may arise slowly, from simple fluctuations, or
suddenly, by mutations; in both cases natural selection will take hold
of them, will multiply them if they are beneficial, and in the course of
time accumulate them, so as to produce that great diversity of organic
life, which we so highly admire.
Darwin has left the decision of this difficult and obviously subordinate
point to his followers. But in his Pangenesis hypothesis he has given us
the clue for a close study and ultimate elucidation of the subject under
discussion.