plant separately sown, a new surprise awaited the observer. The
progeny of the recessives remained pure recessive; and in any number
of subsequent generations never produced the dominant type again, that
is, never reverted to the original parent, whose qualities had failed
to appear in the second generation. When the seeds obtained by
self-fertilizing the plants with the dominant characteristics were
sown, it was found by the test of progeny that the dominants were not
all of like nature, but consisted of two classes--first, some which
gave rise to pure dominants; and secondly, others which gave a mixed
offspring, composed partly of recessives, partly of dominants. Once
more, however, the ratio of heredity asserted itself and it was found
that the average numerical proportions were constant--those with pure
dominant {212} offspring being to those with mixed offspring as one to
two. Hence, it was seen that the seventy-five per cent of dominants
are not really of identical constitution, but consist of twenty-five
per cent which are pure dominants and fifty per cent which are really
crossbreds, though like most of the crossbreds raised by crossing the
two original varieties, they exhibit the dominant character only.
These fifty crossbreds have mixed offspring; these offspring again in
their numerical proportion follow the same law, namely, three
dominants to one recessive. The recessives are pure like those of the
last generation, but the dominants can, by further self-fertilization
and cultivation of the seeds produced, be again shown to be made up of
pure dominant and crossbreds in the same proportion of one dominant to
two crossbreds.
The process of breaking up into the parent forms is thus continued in
each successive generation, the same numerical laws being followed so
far as observation has gone. As Mendel's observations have now been
confirmed by workers in many parts of the world, investigating many
different kinds of plants, it would seem that this law which he
discovered has a basis in the nature of things and is to furnish the
foundation for a new and scientific theory of heredity, while at the
same time affording scope for the collection of observations of the
most valuable character with a definite purpose and without any
theoretic bias.
{213}
The task of the practical breeder who seeks to establish or fix a new
variety produced by cross-breeding in a case involving two variable
characters is simply the isolation and propagation of that one in each
sixteen of the second generation offspring which will be pure as
regards the desired combination of characters. Mendel's discovery, by
putting the breeder in possession of this information enables him to
attack this problem systematically with confidence in the outcome,
whereas hitherto his work, important and fascinating as it is, has
consisted largely of groping for a treasure in the dark. The greater
the number of separately variable characters involved in a cross, the
greater will be the number of new combinations obtainable; the greater
too will be the number of individuals which it will be necessary to
raise in order to secure all the possible combinations; and the
greater again will be the difficulty of isolating the pure, that is,
the stable forms in such as are similar to them in appearance, but
still hybrid in one or more characters.
The law of Mendel reduces to an exact science the art of breeding in
the case most carefully studied by him, that of entire dominance. It
gives to the breeder a new conception of "purity." No animal or plant
is "pure," simply because it is descended from a long line of
ancestors, possessing a desired combination of characters; but any
animal or plant is pure if it produces _gametes_--that is, particles
for conjugation of only one sort--even though its grandparents may
among {214} themselves have possessed opposite characters. The
existence of purity can be established with certainty only by suitable
breeding tests, especially by crossing with recessives; but it may be
safely assumed for any animal or plant, descended from parents which
were like each other and had been shown by breeding tests to be pure.
This naturally leads us to what some biologists have considered to be
the most important part of his work--the theory which he elaborated to
explain his results, the principle which he considers to be the basis
of the laws he discovered. Mendel suggests as following logically from
the results of his experiments and observations a certain theory of
the constitution of germinal particles. He has put this important
matter so clearly himself and with such little waste of words that it
seems better to quote the translation of the passage as given by
Professor Bateson, [Footnote 18] than to attempt to explain it in
other words. Mendel says:--
[Footnote 18: Bateson: _Mendel's Principles of Heredity_.
Cambridge. The University Press. 1902.]
The results of the previously described experiments induced further
experiments, the results of which appear fitted to afford some
conclusions as regards the composition of the egg and pollen-cells
of hybrids. An important matter for consideration is afforded in
peas (_pisum_) by the circumstance that among the progeny of the
hybrids constant forms appear, and that this occurs, too, in all
combinations of the associated characters. So far as experience
goes, we find it in every {215} case confirmed that constant progeny
can only be formed when the egg-cells and the fertilizing pollen are
of like character, so that both are provided with the material for
creating quite similar individuals, as is the case with the normal
fertilization of pure species.
We must therefore regard it as essential that exactly similar
factors are at work also in the production of the constant forms in
the hybrid plants. Since the various constant forms are produced in
one plant, or even in one flower of a plant, the conclusion appears
logical that in the ovaries of the hybrids there are formed as many
sorts of egg-cells and in the anthers as many sorts of pollen-cells
as there are possible constant combination forms, and that these egg
and pollen-cells agree in their internal composition with those of
the separate forms.
In point of fact, it is possible to demonstrate theoretically that
this hypothesis would fully suffice to account for the development
of the hybrids in the separate generations, if we might at the same
time assume that the various kinds of egg and pollen-cells were
formed in the hybrids on the average in equal numbers.
Bateson says in a note on this passage that this last and the
preceding paragraph contain the essence of the Mendelian principles of
heredity. Mendel himself, after stating this hypothesis, gives the
details of a series of experiments by which he was able to decide that
the theoretic considerations suggested were founded in the nature of
plants and their germinal cells.
It will, of course, be interesting to realize what the bearing of
Mendel's discoveries is on the question of the stability of species as
well as on the origin of species. Professor Morgan, in his {216}
article on Darwinism in the "Light of Modern Criticism," already
quoted, says the important fact (with regard to Mendel's Law) from the
point of view of the theory of evolution is that "the new species have
sprung fully armed from the old ones, like Minerva from the head of
Jove." "From de Vries's results," he adds, "we understand better how
it is that we do not see new forms arising, because they appear, as it
were, fully equipped over night. Old species are not slowly changed
into new ones, but a shaking up of the old organization takes place
and the egg brings forth a new species. It is like the turning of the
kaleidoscope, a slight shift and the new figure suddenly appears. It
needs no great penetration to see that this point of view is entirely
different from the conception of the formation of new species by
accumulating individual variations, until they are carried so far that
the new form may be called a new species."
With regard to this question of the transformation of one species into
another, Mendel himself, in the concluding paragraphs of his article
on hybridization, seems to agree with the expressions of Morgan. He
quotes Gaertner's opinion with apparent approval: "Gaertner, by the
results of these transformation experiments was led to oppose the
opinion of those naturalists who dispute the stability of plant
species and believe in a continuous evolution of vegetation. He
perceives in the complete transformation of one species into another
an indubitable proof that {217} species are fixed within limits beyond
which they cannot change." "Although this opinion," adds Mendel,
"cannot be unconditionally accepted, we find, on the other hand, in
Gaertner's experiments a noteworthy confirmation of that supposition
regarding the variability of cultivated plants which has already been
expressed." This expression of opinion is not very definite, and
Bateson, in what Professor Wilson of Columbia calls his "recent
admirable little book on Mendel's principles," adds the following note
that may prove of service in elucidating Mendel's meaning, as few men
have entered so fully into the understanding of Mendel's work as
Bateson, who introduced him to the English-speaking scientific public,
"The argument of this paragraph appears to be that though the general
mutability of natural species might be doubtful, yet among cultivated
plants the transference of characters may be accomplished and may
occur by _integral steps_ [italics ours], until one species is
definitely 'transformed' into the other."
Needless to say, this is quite different from the gradual
transformation of species that Darwinism or Lamarckism assumes to take
place. One species becomes another _per saltum_ in virtue of some
special energy infused into it, some original tendency of its
intrinsic nature, not because of gradual modification by forces
outside of the organisms, nor because of the combination of influences
they are subjected to from without and within, because of tendency to
evolute plus {218} environmental forces. This throws biology back to
the permanency of species in themselves, though successive generations
may be of different species, and does away with the idea of missing
links, since there are no gradual connecting gradations.
A very interesting phase of Mendel's discoveries is concerned with the
relative value of the egg-cell and the pollen-cell, as regards their
effect upon future generations. It is an old and oft-discussed problem
as to which of these germinal particles is the more important in its
influence upon the transmission of parental qualities. Mendel's
observations would seem to decide definitely that, in plants and, by
implication, in animals, since the germinal process is biogenetically
similar, the value of both germinal particles is exactly equal.
In a note, Mendel says:--
_In pisum_ (i. e. in peas), it is beyond doubt that, for the
formation of the new embryo, a perfect union of the elements of both
fertilizing cells must take place. How could we otherwise explain
that, among the offspring of the hybrids, both original types
reappear in equal numbers, and with all their peculiarities? If the
influence of the egg-cell upon the pollen-cell were only external,
if it fulfilled the role of a nurse only, then the result of each
artificial fertilization could be no other than that the developed
hybrid should exactly resemble the pollen parent, or, at any rate,
do so very closely. These experiments, so far, have in no wise been
confirmed. An evident proof of the complete union of the contents of
both cells is afforded by the {219} experience gained on all sides,
that it is immaterial as regards the form of the hybrid which of the
original species is the seed cell, or which the pollen parent!
This is the first actual demonstration of the equivalent value of both
germinal particles as regards their influence on transmission
inheritance in future generations.
It is only by simplifying the problem so that all disturbing factors
could be eliminated that Mendel succeeded in making this
demonstration. Too many qualities have hitherto been considered with
consequent confusion as to the results obtained.
It is of the genius of the man that he should have been able to
succeed in seeing the problem in simple terms while it is apparently
so complex, and thus obtain results that are as far-reaching as the
problem they solve is basic in its character.
Bateson, in his work Mendel's _Principles of Heredity_, says:--
It may seem surprising that a work of such importance should so long
have failed to find recognition and to become current in the world
of science. It is true that the Journal in which it appeared is
scarce, but this circumstance has seldom long delayed general
recognition. The cause is unquestionably to be found in that neglect
of the experimental study of the problem of species which supervened
on the general acceptance of the Darwinian doctrine. The problem of
species, as Koelreuter, Gaertner, Naudin, Wichura, and the hybridists
of the middle of the nineteenth century conceived it, attracted
thenceforth no workers.
{220}
The question, it was imagined, had been answered and the debate
ended. No one felt much interest in the matter. A host of other
lines of work was suddenly opened up, and in 1865 the more original
investigators naturally found these new methods of research more
attractive than the tedious observations of hybridizers, whose
inquiries were supposed, moreover, to have led to no definite
results.
In 1868 appeared the first edition of Darwin's _Animals and Plants_,
marking the very zenith of these studies with regard to hybrids and
the questions in heredity which they illustrate, and thenceforth the
decline in the experimental investigation of evolution and the
problem of species have been studied. With the rediscovery and
confirmation of Mendel's work by de Vries, Correns and Tschermak in
1900 a new era begins. Had Mendel's work come into the hands of
Darwin it is not too much to say that the history of the development
of evolutionary philosophy would have been very different from that
which we have witnessed.
That Mendel's work, appearing as it did at a moment when several
naturalists of the first rank were still occupied with these
problems, should have passed wholly unnoted, will always remain
inexplicable, the more so as the Bruenn society exchanged its
publication with most of the great academies of Europe, including
both the Royal and the Linnean societies of London.
The whole history of Mendel's work, its long period without effect
upon scientific thought, its thoroughly simple yet satisfactory
character, its basis in manifold observations of problems simplified
to the last degree, and its present complete acceptance illustrate
very well the chief defect of the last two generations of workers in
biology. {221} There has been entirely too much theorizing, too much
effort at observations for the purpose of bolstering up preconceived
ideas--preaccepted dogmas of science that have proved false in the
end--and too little straightforward observation and simple reporting
of the facts without trying to have them fit into any theory
prematurely, that is until their true place was found. This will be
the criterion by which the latter half of nineteenth century biology
will be judged; and because of failure here much of our supposed
progress will have no effect on the current of biological progress,
but will represent only an eddy in which there was no end of bustling
movement manifest but no real advance.
As stated very clearly by Professor Morgan at the beginning of this
paper, and Professor Bateson near the end, Darwin's doctrine of
natural selection as the main factor in evolution and its practically
universal premature acceptance by scientific workers in biology are
undoubtedly responsible for this. The present generation may well be
warned, then, not to surrender their judgment to taking theories, but
to wait in patience for the facts in the case, working, not
theorizing, while they wait.
End of Project Gutenberg's Catholic Churchmen in Science, by James J. Walsh