(5) _of a Compound character_, borne by one gamete, transmitted
entire as a single character so long as fertilisation only occurs
between like gametes, or is, in other words, “symmetrical,” but if
fertilisation take place with a dissimilar gamete (or possibly by
other causes), resolved into integral constituent characters, each
separately transmissible.
Next, as, by the union of the gametes bearing the various
hypallelomorphs with other such gametes, or with gametes bearing
simple allelomorphs, in fertilisation, a number of new zygotes will
be formed, such as may not have been seen before in the breed: these
will inevitably be spoken of as _varieties_; and it is difficult not to
extend the idea of variation to them. To distinguish these from other
variations--which there must surely be--we may call them
(6) _Analytical_ variations in contradistinction to
(7) _Synthetical_ variations, occurring not by the separation of
pre-existing constituent-characters but by the addition of new
characters.
Lastly, it is impossible to be presented with the fact that in
Mendelian cases the cross-bred produces on an average _equal_ numbers
of gametes of each kind, that is to say, a symmetrical result, without
suspecting that this fact must correspond with some symmetrical figure
of distribution of those gametes in the cell-divisions by which they
are produced.
* * * * *
At the present time these are the main conceptions--though by no means
all--arising directly from Mendel’s work. The first six are all more
or less clearly embodied by him, though not in every case developed
in accordance with modern knowledge. The seventh is not a Mendelian
conception, but the facts before us justify its inclusion in the above
list though for the present it is little more than a mere surmise.
* * * * *
In Mendelian cases it will now be perceived that all the zygotes
composing the population consist of a limited number of possible types,
each of definite constitution, bearing gametes also of a limited and
definite number of types, and definite constitution in respect of
pre-existing characters. It is now evident that in such cases each
several progenitor need not be brought to account in reckoning the
probable characters of each descendant; for the gametes of cross-breds
are differentiated at each successive generation, some parental
(Mendelian) characters being left out in the composition of each gamete
produced by a zygote arising by the union of bearers of opposite
allelomorphs.
When from these considerations we return to the phenomena comprised in
the Law of Ancestral Heredity, what certainty have we that the same
conceptions are not applicable there also?
It has now been shown that the question whether in the cross-bred
zygotes in general the characters blend or are mutually exclusive is an
entirely subordinate one, and distinctions with regard to the essential
nature of heredity based on these circumstances become irrelevant.
In the case of a population presenting continuous variation in
regard to say, stature, it is easy to see how purity of the gametes
in respect of any intensities of that character might not in
ordinary circumstances be capable of detection. There are doubtless
more than two pure gametic forms of this character, but there may
quite conceivably be six or eight. When it is remembered that each
heterozygous combination of any two may have its own appropriate
stature, and that such a character is distinctly dependent on external
conditions, the mere fact that the observed curves of stature give
“chance distributions” is not surprising and may still be compatible
with purity of gametes in respect of certain pure types. In peas (_P.
sativum_), for example, from Mendel’s work we know that the tall forms
and the extreme dwarf forms exhibit gametic purity. I have seen at
Messrs Sutton’s strong evidence of the same nature in the case of
the tall Sweet Pea (_Lathyrus odoratus_) and the dwarf or procumbent
“Cupid” form.
But in the case of the Sweet Pea we know at least one pure form of
definitely intermediate height, and in the case of _P. sativum_ there
are many. When the _extreme_ types breed together it will be remembered
the heterozygote commonly exceeds the taller in height. In the next
generation, since there is, in the case of extremes, so much margin
between the types of the two pure forms, the return of the offspring
to the three forms of which two are homozygous and one heterozygous is
clearly perceptible.
If however instead of pure extreme varieties we were to take a pair of
varieties differing normally by only a foot or two, we might, owing
to the masking effects of conditions, &c., have great difficulty in
distinguishing the three forms in the second generation. There would
besides be twice as many heterozygous individuals as homozygous
individuals of each kind, giving a symmetrical distribution of
heights, and who might not--in pre-Mendelian days--have accepted such
evidence--made still less clear by influence of conditions--as proof of
Continuous Variation both of zygotes and gametes?
Suppose, then, that instead of two pure types, we had six or eight
breeding together, each pair forming their own heterozygote, there
would be a very remote chance of such purity or fixity of type whether
of gamete or zygote being detected.
_Dominance_, as we have seen, is merely a phenomenon incidental to
specific cases, between which no other common property has yet been
perceived. In the phenomena of _blended_ inheritance we clearly have no
dominance. In the cases of _alternative_ inheritance studied by Galton
and Pearson there is evidently no _universal_ dominance. From the
tables of Basset hound pedigrees there is clearly no definite dominance
of either of the coat-colours. In the case of eye-colour the published
tables do not, so far as I have discovered, furnish the material for a
decision, though it is scarcely possible the phenomenon, even if only
occasional, could have been overlooked. We must take it, then, there is
no sensible dominance in these cases; but whether there is or is not
sensible gametic purity is an altogether different question, which,
so far as I can judge, is as yet untouched. It may perfectly well be
that we shall be compelled to recognize that in many cases there is no
such purity, and that the characters may be carried by the gametes
in any proportion from zero to totality, just as some substances may
be carried in a solution in any proportion from zero to saturation
without discontinuous change of properties. That this will be found
true in _some_ cases is, on any hypothesis, certain; but to prove the
fact for any given case will be an exceedingly difficult operation, and
I scarcely think it has been yet carried through in such a way as to
leave no room for doubt.
Conversely, the _absolute_ and _universal_ purity of the gametes has
certainly not yet been determined for any case; not even in those
cases where it looks most likely that such universal purity exists.
Impairment of such purity we may conceive either to occur in the form
of mosaic gametes, or of gametes with blended properties. On analogy
and from direct evidence we have every right to believe that gametes
of both these classes may occur in rare and exceptional cases, of as
yet unexplored nature[21], but such a phenomenon will not diminish the
significance of observed purity.
[21] It will be understood from what follows, that the existence of
mosaic zygotes is no _proof_ that either component gamete was mosaic.
* * * * *
We have now seen the essential nature of the Mendelian principles and
are able to appreciate the exact relation in which they stand to the
group of cases included in the Law of Ancestral Heredity. In seeking
any general indication as to the common properties of the phenomena
which are already known to obey Mendelian principles we can as yet
point to none, and whether some such common features exist or not is
unknown.
* * * * *
There is however one group of cases, definite though as yet not
numerous, where we know that the Mendelian principles do not apply.
These are the phenomena upon which Mendel touches in his brief paper
on _Hieracium_. As he there states, the hybrids, if they are fertile
at all, produce offspring like themselves, not like their parents. In
further illustration of this phenomenon he cites Wichura’s _Salix_
hybrids. Perhaps some dozen other such illustrations could be given
which rest on good evidence. To these cases the Mendelian principle
will in nowise apply, nor is it easy to conceive any modification of
the law of ancestral heredity which can express them. There the matter
at present rests. Among these cases, however, we perceive several more
or less common features. They are often, though not always, hybrids
between forms differing in many characters. The first cross frequently
is not the exact intermediate between the two parental types, but
may as in the few _Hieracium_ cases be irregular in this respect.
There is often some degree of sterility. In the absence of fuller and
statistical knowledge of such cases further discussion is impossible.
* * * * *
Another class of cases, untouched by any hypothesis of heredity yet
propounded, is that of the false hybrids of Millardet, where we
have fertilisation without transmission of one or several parental
characters. In these not only does the first cross show, in some
respect, the character or characters of _one parent only_, but in
its posterity _no reappearance of the lost character or characters
is observed_. The nature of such cases is still quite obscure, but
we have to suppose that the allelomorph of one gamete only developes
after fertilisation to the exclusion of the corresponding allelomorph
of the other gamete, much--if the crudity of the comparison may be
pardoned--as occurs on the female side in parthenogenesis without
fertilisation at all.
To these as yet altogether unconformable cases we can scarcely doubt
that further experiment will add many more. Indeed we already have
tolerably clear evidence that many phenomena of inheritance are of a
much higher order of complexity. When the paper on _Pisum_ was written
Mendel apparently inclined to the view that with modifications his
law might be found to include all the phenomena of hybridisation, but
in the brief subsequent paper on _Hieracium_ he clearly recognized
the existence of cases of a different nature. Those who read that
contribution will be interested to see that he lays down a principle
which may be extended from hybridisation to heredity in general, that
the laws of each new case must be determined by separate experiment.
* * * * *
As regards the Mendelian principles, which it is the chief aim of
this introduction to present clearly before the reader, a professed
student of variation will easily be able to fill in the outline now
indicated, and to illustrate the various conceptions from phenomena
already familiar. To do this is beyond the scope of this short sketch.
But enough perhaps has now been said to show that by the application of
those principles we are enabled to reach and deal in a comprehensive
manner with phenomena of a fundamental nature, lying at the very root
of all conceptions not merely of the physiology of reproduction and
heredity, but even of the essential nature of living organisms; and I
think that I used no extravagant words when, in introducing Mendel’s
work to the notice of readers of the Royal Horticultural Society’s
Journal, I ventured to declare that his experiments are worthy to rank
with those which laid the foundation of the Atomic laws of Chemistry.
As some biographical particulars of this remarkable investigator will
be welcome, I give the following brief notice, first published by Dr
Correns on the authority of Dr von Schanz: Gregor Johann Mendel was
born on July 22, 1822, at Heinzendorf bei Odrau, in Austrian Silesia.
He was the son of well-to-do peasants. In 1843 he entered as a novice
the “Königinkloster,” an Augustinian foundation in Altbrünn. In 1847 he
was ordained priest. From 1851 to 1853 he studied physics and natural
science at Vienna. Thence he returned to his cloister and became a
teacher in the Realschule at Brünn. Subsequently he was made Abbot,
and died January 6, 1884. The experiments described in his papers were
carried out in the garden of his Cloister. Besides the two papers on
hybridisation, dealing respectively with _Pisum_ and _Hieracium_,
Mendel contributed two brief notes to the _Verh. Zool. bot. Verein_,
Wien, on _Scopolia margaritalis_ (1853, III., p. 116) and on _Bruchus
pisi_ (_ibid._ 1854, IV., p. 27). In these papers he speaks of himself
as a pupil of Kollar.
Mendel published in the Brünn journal statistical observations of a
meteorological character, but, so far as I am aware, no others relating
to natural history. Dr Correns tells me that in the latter part of his
life he engaged in the Ultramontane Controversy. He was for a time
President of the Brünn Society[22].
[22] A few additional particulars are given in Tschermak’s edition.
For the photograph of Mendel which forms the frontispiece to this work,
I am indebted to the Very Rev. Dr Janeischek, the present Abbot of
Brünn, who most kindly supplied it for this purpose.
So far as I have discovered there was, up to 1900, only one reference
to Mendel’s observations in scientific literature, namely that of
Focke, _Pflanzenmischlinge_, 1881, p. 109, where it is simply stated
that Mendel’s numerous experiments on _Pisum_ gave results similar to
those obtained by Knight, but that he believed he had found constant
numerical ratios among the types produced by hybridisation. In the same
work a similar brief reference is made to the paper on _Hieracium_.
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 doctrines. The problem of Species, as
Kölreuter, Gärtner, Naudin, Wichura, and the other hybridists of the
middle of the nineteenth century conceived it, attracted thenceforth
no workers. 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 were suddenly opened up, and in 1865 the more original
investigators naturally found those new methods of research more
attractive than the tedious observations of the hybridisers, whose
inquiries were supposed, moreover, to have led to no definite result.
Nevertheless the total neglect of such a discovery is not easy to
account for. Those who are acquainted with the literature of this
branch of inquiry will know that the French Academy offered a prize
in 1861 to be awarded in 1862 on the subject “_Étudier les Hybrides
végétaux au point de vue de leur fécondité et de la perpétuité de
leurs caractères_.” This subject was doubtless chosen with reference
to the experiments of Godron of Nancy and Naudin, then of Paris. Both
these naturalists competed, and the accounts of the work of Godron on
_Datura_ and of Naudin on a number of species were published in the
years 1864 and 1865 respectively. Both, especially the latter, are
works of high consequence in the history of the science of heredity.
In the latter paper Naudin clearly enuntiated what we shall henceforth
know as the Mendelian conception of the dissociation of characters of
cross-breds in the formation of the germ-cells, though apparently he
never developed this conception.
In the year 1864, George Bentham, then President of the Linnean
Society, took these treatises as the subject of his address to the
Anniversary meeting on the 24 May, Naudin’s work being known to him
from an abstract, the full paper having not yet appeared. Referring
to the hypothesis of dissociation which he fully described, he said
that it appeared to be new and well supported, but required much more
confirmation before it could be held as proven. (_J. Linn. Soc., Bot._,
VIII., _Proc._, p. XIV.)
In 1865, the year of Mendel’s communication to the Brünn Society,
appeared Wichura’s famous treatise on his experiments with _Salix_
to which Mendel refers. There are passages in this memoir which come
very near Mendel’s principles, but it is evident from the plan of his
experiments that Mendel had conceived the whole of his ideas before
that date.
In 1868 appeared the first edition of Darwin’s _Animals and Plants_,
marking the very zenith of these studies, and thenceforth the decline
in the experimental investigation of Evolution and the problem of
Species has been steady. With the rediscovery and confirmation of
Mendel’s work by de Vries, Correns and Tschermak in 1900 a new era
begins.
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 unnoticed, will always remain inexplicable,
the more so as the Brünn Society exchanged its publications with most
of the Academies of Europe, including both the Royal and Linnean
Societies.
Naudin’s views were well known to Darwin and are discussed in _Animals
and Plants_ (ed. 1885, II., p. 23); but, put forward as they were
without full proof, they could not command universal credence. Gärtner,
too, had adopted opposite views; and Wichura, working with cases of
another order, had proved the fact that some hybrids breed true.
Consequently it is not to be wondered at that Darwin was sceptical.
Moreover, the Mendelian idea of the “hybrid-character,” or heterozygous
form, was unknown to him, a conception without which the hypothesis of
dissociation of characters is quite imperfect.
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.