[3] The first half of this paper is reprinted with additions and
modifications from the _Journal of the Royal Horticultural Society_,
1900, vol. XXV., parts 1 and 2. Written almost immediately after the
rediscovery of Mendel, it will be seen to be already in some measure
out of date, but it may thus serve to show the relation of the new
conceptions to the old.
An exact determination of the laws of heredity will probably work more
change in man’s outlook on the world, and in his power over nature,
than any other advance in natural knowledge that can be clearly
foreseen.
There is no doubt whatever that these laws can be determined. In
comparison with the labour that has been needed for other great
discoveries we may even expect that the necessary effort will be small.
It is rather remarkable that while in other branches of physiology such
great progress has of late been made, our knowledge of the phenomena
of heredity has increased but little; though that these phenomena
constitute the basis of all evolutionary science and the very central
problem of natural history is admitted by all. Nor is this due to the
special difficulty of such inquiries so much as to general neglect of
the subject.
It is in the hope of inducing others to follow these lines of
investigation that I take the problems of heredity as the subject of
this lecture to the Royal Horticultural Society.
No one has better opportunities of pursuing such work than
horticulturists and stock breeders. They are daily witnesses of
the phenomena of heredity. Their success also depends largely on a
knowledge of its laws, and obviously every increase in that knowledge
is of direct and special importance to them.
The want of systematic study of heredity is due chiefly to
misapprehension. It is supposed that such work requires a lifetime. But
though for adequate study of the complex phenomena of inheritance long
periods of time must be necessary, yet in our present state of deep
ignorance almost of the outline of the facts, observations carefully
planned and faithfully carried out for even a few years may produce
results of great value. In fact, by far the most appreciable and
definite additions to our knowledge of these matters have been thus
obtained.
There is besides some misapprehension as to the kind of knowledge which
is especially wanted at this time, and as to the modes by which we may
expect to obtain it. The present paper is written in the hope that it
may in some degree help to clear the ground of these difficulties by a
preliminary consideration of the question, How far have we got towards
an exact knowledge of heredity, and how can we get further?
Now this is pre-eminently a subject in which we must distinguish what
we _can_ do from what we want to do. We _want_ to know the whole
truth of the matter; we want to know the physical basis, the inward
and essential nature, “the causes,” as they are sometimes called,
of heredity: but we want also to know the laws which the outward and
visible phenomena obey.
Let us recognise from the outset that as to the essential nature of
these phenomena we still know absolutely nothing. We have no glimmering
of an idea as to what constitutes the essential process by which the
likeness of the parent is transmitted to the offspring. We can study
the processes of fertilisation and development in the finest detail
which the microscope manifests to us, and we may fairly say that we
have now a considerable grasp of the visible phenomena; but of the
nature of the physical basis of heredity we have no conception at all.
No one has yet any suggestion, working hypothesis, or mental picture
that has thus far helped in the slightest degree to penetrate beyond
what we see. The process is as utterly mysterious to us as a flash of
lightning is to a savage. We do not know what is the essential agent
in the transmission of parental characters, not even whether it is a
material agent or not. Not only is our ignorance complete, but no one
has the remotest idea how to set to work on that part of the problem.
We are in the state in which the students of physical science were,
in the period when it was open to anyone to believe that heat was a
material substance or not, as he chose.
But apart from any conception of the essential modes of transmission of
characters, we _can_ study the outward facts of the transmission. Here,
if our knowledge is still very vague, we are at least beginning to see
how we ought to go to work. Formerly naturalists were content with
the collection of numbers of isolated instances of transmission--more
especially, striking and peculiar cases--the sudden appearance of
highly prepotent forms, and the like. We are now passing out of that
stage. It is not that the interest of particular cases has in any way
diminished--for such records will always have their value--but it
has become likely that general expressions will be found capable of
sufficiently wide application to be justly called “laws” of heredity.
That this is so was till recently due almost entirely to the work of Mr
F. Galton, to whom we are indebted for the first systematic attempt to
enuntiate such a law.
All laws of heredity so far propounded are of a statistical character
and have been obtained by statistical methods. If we consider for a
moment what is actually meant by a “law of heredity” we shall see at
once why these investigations must follow statistical methods. For
a “law” of heredity is simply an attempt to declare the course of
heredity under given conditions. But if we attempt to predicate the
course of heredity we have to deal with conditions and groups of causes
wholly unknown to us, whose presence we cannot recognize, and whose
magnitude we cannot estimate in any particular case. The course of
heredity in particular cases therefore cannot be foreseen.
Of the many factors which determine the degree to which a given
character shall be present in a given individual only one is usually
known to us, namely, the degree to which that character is present
in the parents. It is common knowledge that there is not that close
correspondence between parent and offspring which would result were
this factor the only one operating; but that, on the contrary, the
resemblance between the two is only an uncertain one.
In dealing with phenomena of this class the study of single instances
reveals no regularity. It is only by collection of facts in great
numbers, and by statistical treatment of the mass, that any order or
law can be perceived. In the case of a chemical reaction, for instance,
by suitable means the conditions can be accurately reproduced, so that
in every individual case we can predict with certainty that the same
result will occur. But with heredity it is somewhat as it is in the
case of the rainfall. No one can say how much rain will fall to-morrow
in a given place, but we can predict with moderate accuracy how much
will fall next year, and for a period of years a prediction can be made
which accords very closely with the truth.
Similar predictions can from statistical data be made as to the
duration of life and a great variety of events, the conditioning causes
of which are very imperfectly understood. It is predictions of this
kind that the study of heredity is beginning to make possible, and in
that sense laws of heredity can be perceived.
We are as far as ever from knowing _why_ some characters are
transmitted, while others are not; nor can anyone yet foretell which
individual parent will transmit characters to the offspring, and which
will not; nevertheless the progress made is distinct.
As yet investigations of this kind have been made in only a few
instances, the most notable being those of Galton on human stature, and
on the transmission of colours in Basset hounds. In each of these cases
he has shown that the expectation of inheritance is such that a simple
arithmetical rule is approximately followed. The rule thus arrived at
is that of the whole heritage of the offspring the two parents together
on an average contribute one half, the four grandparents one-quarter,
the eight great-grandparents one-eighth, and so on, the remainder
being contributed by the remoter ancestors.
Such a law is obviously of practical importance. In any case to which
it applies we ought thus to be able to predict the degree with which
the purity of a strain may be increased by selection in each successive
generation.
To take a perhaps impossibly crude example, if a seedling show any
particular character which it is desired to fix, on the assumption that
successive self-fertilisations are possible, according to Galton’s
law the expectation of purity should be in the first generation of
self-fertilisation 1 in 2, in the second generation 3 in 4, in the
third 7 in 8, and so on[4].
[4] See later. Galton gave a simple diagrammatic representation of
his law in _Nature_, 1898, vol. LVII. p. 293.
But already many cases are known to which the rule in any simple
form will not apply. Galton points out that it takes no account of
individual prepotencies. There are, besides, numerous cases in which
on crossing two varieties the character of one variety almost always
appears in each member of the first cross-bred generation. Examples of
these will be familiar to those who have experience in such matters.
The offspring of the Polled Angus cow and the Shorthorn bull is almost
invariably polled or with very small loose “scurs.” Seedlings raised
by crossing _Atropa belladonna_ with the yellow-fruited variety have
without exception the blackish-purple fruits of the type. In several
hairy species when a cross with a glabrous variety is made, the first
cross-bred generation is altogether hairy[5].
[5] These we now recognize as examples of Mendelian ‘dominance.’
Still more numerous are examples in which the characters of one variety
very largely, though not exclusively, predominate in the offspring.
These large classes of exceptions--to go no further--indicate that,
as we might in any case expect, the principle is not of universal
application, and will need various modifications if it is to be
extended to more complex cases of inheritance of varietal characters.
No more useful work can be imagined than a systematic determination of
the precise “law of heredity” in numbers of particular cases.
Until lately the work which Galton accomplished stood almost alone in
this field, but quite recently remarkable additions to our knowledge
of these questions have been made. In the year 1900 Professor de Vries
published a brief account[6] of experiments which he has for several
years been carrying on, giving results of the highest value.
[6] _Comptes Rendus_, March 26, 1900, and _Ber. d. Deutsch. Bot.
Ges._ xviii. 1900, p. 83.
The description is very short, and there are several points as to which
more precise information is necessary both as to details of procedure
and as to statement of results. Nevertheless it is impossible to doubt
that the work as a whole constitutes a marked step forward, and the
full publication which is promised will be awaited with great interest.
The work relates to the course of heredity in cases where definite
varieties differing from each other in some _one_ definite character
are crossed together. The cases are all examples of discontinuous
variation: that is to say, cases in which actual intermediates between
the parent forms are not usually produced on crossing[7]. It is shown
that the subsequent posterity obtained by self-fertilising these
cross-breds or hybrids, or by breeding them with each other, break up
into the original parent forms according to fixed numerical rule.
[7] This conception of discontinuity is of course pre-Mendelian.
Professor de Vries begins by reference to a remarkable memoir by Gregor
Mendel[8], giving the results of his experiments in crossing varieties
of _Pisum sativum_. These experiments of Mendel’s were carried out on
a large scale, his account of them is excellent and complete, and the
principles which he was able to deduce from them will certainly play a
conspicuous part in all future discussions of evolutionary problems.
It is not a little remarkable that Mendel’s work should have escaped
notice, and been so long forgotten.
[8] ‘Versuche üb. Pflanzenhybriden’ in the _Verh. d. Naturf. Ver.
Brünn_, iv. 1865.
For the purposes of his experiments Mendel selected seven pairs of
characters as follows:--