kind was required. To learn the laws of Heredity and Variation there
is no other way than that which Darwin himself followed, the direct
examination of the phenomena. A beginning could be made by collecting
fortuitous observations of this class, which have often thrown a
suggestive light, but such evidence can be at best but superficial and
some more penetrating instrument of research is required. This can
only be provided by actual experiments in breeding.
The truth of these general considerations was becoming gradually clear
to many of us when in 1900 Mendel's work was rediscovered.
Segregation, a phenomenon of the utmost novelty, was thus revealed.
From that moment not only in the problem of the origin of species, but
in all the great problems of biology a new era began. So unexpected
was the discovery that many naturalists were convinced it was untrue,
and at once proclaimed Mendel's conclusions as either altogether
mistaken, or if true, of very limited application. Many fantastic
notions about the workings of Heredity had been asserted as general
principles before: this was probably only another fancy of the same
class.
Nevertheless those who had a preliminary acquaintance with the facts
of Variation were not wholly unprepared for some such revelation. The
essential deduction from the discovery of segregation was that the
characters of living things are dependent on the presence of definite
elements or factors, which are treated as units in the processes of
Heredity. These factors can thus be recombined in various ways. They
act sometimes separately, and sometimes they interact in conduction
with each other, producing their various effects. All this indicates a
definiteness and specific order in heredity, and therefore in
variation. This order cannot by the nature of the case be dependent on
Natural Selection for its existence, but must be a consequence of the
fundamental chemical and physical nature of living things. The study
of Variation had from the first shown that an orderliness of this kind
was present. The bodies and the properties of livings things are
cosmic, not chaotic. No matter how low in the scale we go, never do we
find the slightest hint of a diminution in that all-pervading
orderliness, nor can we conceive an organism existing for a moment in
any other state. Moreover not only does this order prevail in normal
forms, but again and again it is to be seen in newly-sprung varieties,
which by general consent cannot have been subjected to a prolonged
Selection. The discovery of Mendelian elements admirably coincided
with and at once gave a rationale of these facts. Genetic Variation is
then primarily the consequence of additions to, or omissions from, the
stock of elements which the species contains. The further
investigation of the species-problem must thus proceed by the
analytical method which breeding experiments provide.
In the nine years which have elapsed since Mendel's clue became
generally known, progress has been rapid. We now understand the
process by which a polymorphic race maintains its polymorphism. When a
family consists of dissimilar members, given the numerical proportions
in which these members are occurring, we can represent their
composition symbolically and state what types can be transmitted by
the various members. The difficulty of the "swamping effects of
inter-crossing" is practically at an end. Even the famous puzzle of
sex-limited inheritance is solved, at all events in its more regular
manifestations, and we know now how it is brought about that the
normal sisters of a colour-blind man can transmit the colour-blindness
while his normal brothers cannot transmit it.
We are still only on the fringe of the inquiry. It can be seen
extending and ramifying in many directions. To enumerate these here
would be impossible. A whole new range of possibilities is being
brought into view by study of the inter-relations between the simple
factors. By following up the evidence as to segregation, indications
have been obtained which can only be interpreted as meaning that when
many factors are being simultaneously redistributed among the
germ-cells, certain of them exert what must be described as a
repulsion upon other factors. We cannot surmise whither this discovery
may lead.
In the new light all the old problems wear a fresh aspect. Upon the
question of the nature of Sex, for example, the bearing of Mendelian
evidence is close. Elsewhere I have shown that from several sets of
parallel experiments the conclusion is almost forced upon us that, in
the types investigated, of the two sexes the female is to be regarded
as heterozygous in sex, containing one unpaired dominant element,
while the male is similarly homozygous in the absence of that
element.[68] It is not a little remarkable that on this point--which
is the only one where observations of the nuclear processes of
gameto-genesis have yet been brought into relation with the visible
characteristics of the organisms themselves--there should be
diametrical opposition between the results of breeding experiments and
those derived from cytology.
Those who have followed the researches of the American school will be
aware that, after it had been found in certain insects that the
spermatozoa were of two kinds according as they contained or did not
contain the accessory chromosome, E. B. Wilson succeeded in proving
that the sperms possessing this accessory body were destined to form
_females_ on fertilisation, while sperms without it form males, the
eggs being apparently indifferent. Perhaps the most striking of all
this series of observations is that lately made by T. H. Morgan,[69]
since confirmed by von Baehr, that in a Phylloxeran two kinds of
spermatids are formed, respectively with and without an accessory (in
this case, _double_) chromosome. Of these, only those possessing the
accessory body become functional spermatozoa, the others degenerating.
We have thus an elucidation of the puzzling fact that in these forms
fertilisation results in the formation of _females_ only. How the
males are formed--for of course males are eventually produced by the
parthenogenetic females--we do not know.
If the accessory body is really to be regarded as bearing the factor
for femaleness, then in Mendelian terms female is DD and male is DR.
The eggs are indifferent and the spermatozoa are each male, _or_
female. But according to the evidence derived from a study of the
sex-limited descent of certain features in other animals the
conclusion seems equally clear that in them female must be regarded as
DR and male as RR. The eggs are thus each either male or female and
the spermatozoa are indifferent. How this contradictory evidence is to
be reconciled we do not yet know. The breeding work concerns fowls,
canaries, and the Currant moth (_Abraxas grossulariata_). The
accessory chromosome has been now observed in most of the great
divisions of insects,[70] except, as it happens, Lepidoptera. At first
sight it seems difficult to suppose that a feature apparently so
fundamental as sex should be differently constituted in different
animals, but that seems at present the least improbable inference. I
mention these two groups of facts as illustrating the nature and
methods of modern genetic work. We must proceed by minute and specific
analytical investigation. Wherever we look we find traces of the
operation of precise and specific rules.
In the light of present knowledge it is evident that before we can
attack the Species-problem with any hope of success there are vast
arrears to be made up. He would be a bold man who would now assert
that there was no sense in which the term Species might not have a
strict and concrete meaning in contradistinction to the term Variety.
We have been taught to regard the difference between species and
variety as one of degree. I think it unlikely that this conclusion
will bear the test of further research. To Darwin the question, What
is a variation? presented no difficulties. Any difference between
parent and offspring was a variation. Now we have to be more precise.
First we must, as de Vries has shown, distinguish real, genetic,
variation from _fluctuational_ variations, due to environmental and
other accidents, which cannot be transmitted. Having excluded these
sources of error the variations observed must be expressed in terms of
the factors to which they are due before their significance can be
understood. For example, numbers of the variations seen under
domestication, and not a few witnessed in nature, are simply the
consequence of some ingredient being in an unknown way omitted from
the composition of the varying individual. The variation may on the
contrary be due to the addition of some new element, but to prove that
it is so is by no means an easy matter. Casual observation is useless,
for though these latter variations will always be dominants, yet many
dominant characteristics may arise from another cause, namely the
meeting of complementary factors, and special study of each case in
two generations at least is needed before these two phenomena can be
distinguished.
When such considerations are fully appreciated it will be realised
that medleys of most dissimilar occurrences are all confused together
under the term Variation. One of the first objects of genetic analysis
is to disentangle this mass of confusion.
To those who have made no study of heredity it sometimes appears that
the question of the effect of conditions in causing variation is one
which we should immediately investigate, but a little thought will
show that before any critical inquiry into such possibilities can be
attempted, a knowledge of the working of heredity under conditions as
far as possible uniform must be obtained. At the time when Darwin was
writing, if a plant brought into cultivation gave off an albino
variety, such an event was without hesitation ascribed to the change
of life. Now we see that albino _gametes_, germs, that is to say,
which are destitute of the pigment-forming factor, may have been
originally produced by individuals standing an indefinite number of
generations back in the ancestry of the actual albino, and it is
indeed almost certain that the variation to which the appearance of
the albino is due cannot have taken place in a generation later than
that of the grandparents. It is true that when a new _dominant_
appears we should feel greater confidence that we were witnessing the
original variation, but such events are of extreme rarity, and no such
case has come under the notice of an experimenter in modern times, as
far as I am aware. That they must have appeared is clear enough.
Nothing corresponding to the Brown-breasted Game fowl is known wild,
yet that colour is a most definite dominant, and at some moment since
_Gallus bankiva_ was domesticated, the element on which that special
colour depends must have at least once been formed in the germ-cell of
a fowl; but we need harder evidence than any which has yet been
produced before we can declare that this novelty came through
over-feeding, or change of climate, or any other disturbance
consequent on domestication. When we reflect on the intricacies of
genetic problems as we must now conceive them there come moments when
we feel almost thankful that the Mendelian principles were unknown to
Darwin. The time called for a bold pronouncement, and he made it, to
our lasting profit and delight. With fuller knowledge we pass once
more into a period of cautious expectation and reserve.
In every arduous enterprise it is pleasanter to look back at
difficulties overcome than forward to those which still seem
insurmountable, but in the next stage there is nothing to be stained
by disguising the fact that the attributes of living things are not
what we used to suppose. If they are more complex in the sense that
the properties they display are throughout so regular[71] that the
Selection of minute random variations is an unacceptable account of
the origin of their diversity, yet by virtue of that very regularity
the problem is limited in scope and thus simplified.
To begin with, we must relegate Selection to its proper place.
Selection permits the viable to continue and decides that the
non-viable shall perish; just as the temperature of our atmosphere
decides that no liquid carbon shall be found on the face of the earth:
but we do not suppose that the form of the diamond has been gradually
achieved by a process of Selection. So again, as the course of descent
branches in the successive generations, Selection determines along
which branch Evolution shall proceed, but it does not decide what
novelties that branch shall bring forth. "_La Nature contient le fonds
de toutes ces varietes, mais le hazard ou l'art les mettent en
oeuvre_," as Maupertuis most truly said.
Not till knowledge of the genetic properties of organisms has attained
to far greater completeness can evolutionary speculations have more
than a suggestive value. By genetic experiment, cytology and
physiological chemistry aiding, we may hope to acquire such knowledge.
In 1872 Nathusius wrote:[72] "Das Gesetz der Vererbung ist noch nicht
erkannt; der Apfel ist noch nicht vom Baum der Erkenntniss gefallen,
welcher, der Sage nach, Newton auf den rechten Weg zur Ergruendung der
Gravitationsgesetze fuehrte." We cannot pretend that the words are not
still true, but in Mendelian analysis the seeds of that apple-tree at
last are sown.
If we were asked what discovery would do most to forward our inquiry,
what one bit of knowledge would more than any other illuminate the
problem, I think we may give the answer without hesitation. The
greatest advance that we can foresee will be made when it is found
possible to connect the geometrical phenomena of development with the
chemical. The geometrical symmetry of living things is the key to a
knowledge of their regularity, and the forces which cause it. In the
symmetry of the dividing cell the basis of that resemblance we call
Heredity is contained. To imitate the morphological phenomena of life
we have to devise a system which can divide. It must be able to
divide, and to segment as--grossly--a vibrating plate or rod does, or
as an icicle can do as it becomes ribbed in a continuous stream of
water; but with this distinction, that the distribution of chemical
differences and properties must simultaneously be decided and disposed
in orderly relation to the pattern of the segmentation. Even if a
model which would do this could be constructed it might prove to be a
useful beginning.
This may be looking too far ahead. If we had to choose some one piece
of more proximate knowledge which we would more especially like to
acquire, I suppose we should ask for the secret of interracial
sterility. Nothing has yet been discovered to remove the grave
difficulty, by which Huxley in particular was so much oppressed, that
among the many varieties produced under domestication--which we all
regard as analogous to the species seen in nature--no clear case of
interracial sterility has been demonstrated. The phenomenon is
probably the only one to which the domesticated products seem to
afford no parallel. No solution of the difficulty can be offered which
has positive value, but it is perhaps worth considering the facts in
the light of modern ideas. It should be observed that we are not
discussing incompatibility of two species to produce offspring (a
totally distinct phenomenon), but the sterility of the offspring which
many of them do produce.
When two species, both perfectly fertile severally, produce on crossing a
sterile progeny, there is a presumption that the sterility is due to the
development in the hybrid of some substance which can only be formed by the
meeting of two complementary factors. That some such account is correct in
essence may be inferred from the well-known observation that if the hybrid
is not totally sterile but only partially so, and thus is able to form some
good germ-cells which develop into new individuals, the sterility of these
daughter-individuals is sensibly reduced or may be entirely absent. The
fertility once re-established, the sterility does not return in the later
progeny, a fact strongly suggestive of segregation. Now if the sterility of
the cross-bred be really the consequence of the meeting of two
complementary factors, we see that the phenomenon could only be produced
among the divergent offspring of one species by the acquisition of at least
_two_ new factors; for if the acquisition of a single factor caused
sterility the line would then end. Moreover each factor must be separately
acquired by distinct individuals, for if both were present together, the
possessors would by hypothesis be sterile. And in order to imitate the case
of species each of these factors must be acquired by distinct breeds. The
factors need not, and probably would not, produce any other perceptible
effects; they might, like the colour-factors present in white flowers, make
no difference in the form or other characters. Not till the cross was
actually made between the two complementary individuals would either factor
come into play, and the effects even then might be unobserved until an
attempt was made to breed from the cross-bred.
Next, if the factors responsible for sterility were acquired, they
would in all probability be peculiar to certain individuals and would
not readily be distributed to the whole breed. Any member of the breed
also into which _both_ the factors were introduced would drop out of
the pedigree by virtue of its sterility. Hence the evidence that the
various domesticated breeds say of dogs or fowls can when mated
together produce fertile offspring, is beside the mark. The real
question is, Do they ever produce sterile offspring? I think the
evidence is clearly that sometimes they do, oftener perhaps than is
commonly supposed. These suggestions are quite amenable to
experimental tests. The most obvious way to begin is to get a pair of
parents which are known to have had any sterile offspring, and to find
the proportions in which these steriles were produced. If, as I
anticipate, these proportions are found to be definite, the rest is
simple.
In passing, certain other considerations may be referred to. First,
that there are observations favouring the view that the production of
totally sterile cross-breds is seldom a universal property of two
species, and that it may be a matter of individuals, which is just
what on the view here proposed would be expected. Moreover, as we all
know now, though incompatibility may be dependent to some extent on
the degree to which the species are dissimilar, no such principle can
be demonstrated to determine sterility or fertility in general. For
example, though all our Finches can breed together, the hybrids are
all sterile. Of Ducks some species can breed together without
producing the slightest sterility; others have totally sterile
offspring, and so on. The hybrids between several _genera_ of Orchids
are perfectly fertile on the female side, and some on the male side
also, but the hybrids produced between the Turnip (_Brassica napus_)
and the Swede (_Brassica campestris_), which, according to our
estimates of affinity, should be nearly allied forms, are totally
sterile.[73] Lastly, it may be recalled that in sterility we are
almost certainly considering a meristic phenomenon. _Failure to
divide_ is, we may feel fairly sure, the immediate "cause" of the
sterility. Now, though we know very little about the heredity of
meristic differences, all that we do know points to the conclusion
that the less-divided is dominant to the more-divided, and we are thus
justified in supposing that there are factors which can arrest or
prevent cell-division. My conjecture therefore is that in the case of
sterility of cross-breds we see the effect produced by a complementary
pair of such factors. This and many similar problems are now open to
our analysis.
The question is sometimes asked, Do the new lights on Variation and
Heredity make the process of Evolution easier to understand? On the
whole the answer may be given that they do. There is some appearance
of loss of simplicity, but the gain is real. As was said above, the
time is not ripe for the discussion of the origin of species. With
faith in Evolution unshaken--if indeed the word faith can be used in
application to that which is certain--we look on the manner and
causation of adapted differentiation as still wholly mysterious. As
Samuel Butler so truly said: "To me it seems that the 'Origin of
Variation,' whatever it is, is the only true 'Origin of Species,'"[74]
and of that Origin not one of us knows anything. But given
Variation--and it is given: assuming further that the variations are
not guided into paths of adaptation--and both to the Darwinian and to
the modern school this hypothesis appears to be sound if unproven--an
evolution of species proceeding by definite steps is more, rather than
less, easy to imagine than an evolution proceeding by the accumulation
of indefinite and insensible steps. Those who have lost themselves in
contemplating the miracles of Adaptation (whether real or spurious)
have not unnaturally fixed their hopes rather on the indefinite than
on the definite changes. The reasons are obvious. By suggesting that
the steps through which an adaptative mechanism arose were indefinite
and insensible, all further trouble is spared. While it could be said
that species arise by an insensible and imperceptible process of
variation, there was clearly no use in tiring ourselves by trying to
perceive that process. This labour-saving counsel found great favour.
All that had to be done to develop evolution-theory was to discover
the good in everything, a task which, in the complete absence of any
control or test whereby to check the truth of the discovery, is not
very onerous. The doctrine "_que tout est au mieux_" was therefore
preached with fresh vigour, and examples of that illuminating
principle were discovered with a facility that Pangloss himself might
have envied, till at last even the spectators wearied of such dazzling
performances.
But in all seriousness, why should indefinite and unlimited variation
have been regarded as a more probable account of the origin of
Adaptation? Only, I think, because the obstacle was shifted one plane
back, and so looked rather less prominent. The abundance of
Adaptation, we all grant, is an immense, almost an unsurpassable
difficulty in all non-Lamarckian views of Evolution; but if the steps
by which that adaptation arose were fortuitious, to imagine them
insensible is assuredly no help. In one most important respect indeed,
as has often been observed, it is a multiplication of troubles. For
the smaller the steps, the less could Natural Selection act upon them.
Definite variations--and of the occurrence of definite variations in
abundance we have now the most convincing proof--have at least the
obvious merit that they can make and often do make a real difference
in the chances of life.
There is another aspect of the Adaptation problem to which I can
allude very briefly. May not our present ideas of the universality and
precision of Adaptation be greatly exaggerated? The fit of organism to
its environment is not after all so very close--a proposition
unwelcome perhaps, but one which could be illustrated by very copious
evidence. Natural Selection is stern, but she has her tolerant moods.