particularly in its most important and complicated parts, so that we
may say that there is no actively functional organ that has not
undergone a process of adaptation relative to its function and the
requirements of the organism. Not only is every gland structurally
adapted, down to the very minutest histological details, to its
function, but the function is equally minutely adapted to the needs of
the body. Every cell in the mucous lining of the intestine is exactly
regulated in its relation to the different nutritive substances, and
behaves in quite a different way towards the fats, and towards
nitrogenous substances, or peptones.
I have elsewhere called attention to the many adaptations of the whale
to the surrounding medium, and have pointed out--what has long been
known, but is not universally admitted, even now--that in it a great
number of important organs have been transformed in adaptation to the
peculiar conditions of aquatic life, although the ancestors of the
whale must have lived, like other hair-covered mammals, on land. I
cited a number of these transformations--the fish-like form of the
body, the hairlessness of the skin, the transformation of the
fore-limbs to fins, the disappearance of the hind-limbs and the
development of a tail fin, the layer of blubber under the skin, which
affords the protection from cold necessary to a warm-blooded animal,
the disappearance of the ear-muscles and the auditory passages, the
displacement of the external nares to the forehead for the greater
security of the breathing-hole during the brief appearance at the
surface, and certain remarkable changes in the respiratory and
circulatory organs which enable the animal to remain for a long time
under water. I might have added many more, for the list of adaptations
in the whale to aquatic life is by no means exhausted; they are found
in the histological structure and in the minutest combinations in the
nervous system. For it is obvious that a tail-fin must be used in
quite a different way from a tail, which serves as a fly-brush in
hoofed animals, or as an aid to springing in the kangaroo or as a
climbing organ; it will require quite different reflex-mechanisms and
nerve combinations in the motor centres.
I used this example in order to show how unnecessary it is to assume a
special internal evolutionary power for the phylogenesis of species,
for this whole order of whales is, so to speak, _made up of
adaptations_; it deviates in many essential respects from the usual
mammalian type, and all the deviations are adaptations to aquatic
life. But if precisely the most essential features of the organisation
thus depend upon adaptation, what is left for a phyletic force to do,
since it is these essential features of the structure it would have to
determine? There are few people now who believe in a phyletic
evolutionary power, which is not made up of the forces known to
us--adaptation and heredity--but the conviction that _every_ part of
an organism depends upon adaptation has not yet gained a firm footing.
Nevertheless, I must continue to regard this conception as the correct
one, as I have long done.
I may be permitted one more example. The feather of a bird is a
marvellous structure, and no one will deny that as a whole it depends
upon adaptation. But what part of it _does not_ depend upon
adaptation? The hollow quill, the shaft with its hard, thin, light
cortex, and the spongy substance within it, its square section
compared with the round section of the quill, the flat barbs, their
short, hooked barbules which, in the flight-feathers, hook into one
another with just sufficient firmness to resist the pressure of the
air at each wing-beat, the lightness and firmness of the whole
apparatus, the elasticity of the vane, and so on. And yet all this
belongs to an organ which is only passively functional, and therefore
can have nothing to do with the _Lamarckian principle_. Nor can the
feather have arisen through some magical effect of temperature,
moisture, electricity, or specific nutrition, and thus selection is
again our only anchor of safety.
But--it will be objected--the substance of which the feather consists,
this peculiar kind of horny substance, did not first arise through
selection in the course of the evolution of the birds, for it formed
the covering of the scales of their reptilian ancestors. It is quite
true that a similar substance covered the scales of the Reptiles, but
why should it not have arisen among them through selection? Or in what
other way could it have arisen, since scales are also passively useful
parts? It is true that if we are only to call adaptation what has been
acquired by the species we happen to be considering, there would
remain a great deal that could not be referred to selection; but we
are postulating an evolution which has stretched back through aeons,
and in the course of which innumerable adaptations took place, which
had not merely ephemeral persistence in a genus, a family or a class,
but which was continued into whole Phyla of animals, with continual
fresh adaptations to the special conditions of each species, family,
or class, yet with persistence of the fundamental elements. Thus the
feather, once acquired, persisted in all birds, and the vertebral
column, once gained by adaptation in the lowest forms, has persisted
in all the Vertebrates from Amphioxus upwards, although with constant
readaptation to the conditions of each particular group. Thus
everything we can see in animals is adaptation, whether of to-day, or
of yesterday, or of ages long gone by; every kind of cell, whether
glandular, muscular, nervous, epidermic, or skeletal, is adapted to
absolutely definite and specific functions, and every organ which is
composed of these different kinds of cells contains them in the proper
proportions, and in the particular arrangement which best serves the
function of the organ; it is thus adapted to its function.
All parts of the organism are tuned to one another, that is, _they are
adapted to one another_, and in the same way _the organism as a whole
is adapted to the conditions of its life, and it is so at every stage
of its evolution._
But all adaptations _can_ be referred to selection; the only point
that remains doubtful is whether they all _must_ be referred to it.
However that may be, whether the _Lamarckian principle_ is a factor
that has cooeperated with selection in evolution, or whether it is
altogether fallacious, the fact remains, that selection is the cause
of a great part of the phyletic evolution of organisms on our earth.
Those who agree with me in rejecting the _Lamarckian principle_ will
regard selection as the only _guiding_ factor in evolution, which
creates what is new out of the transmissible variations, by ordering
and arranging these, selecting them in relation to their number and
size, as the architect does his building-stones so that a particular
style must result.[55] But the building-stones themselves, the
variations, have their basis in the influences which cause variation
in those vital units which are handed on from one generation to
another, whether, taken together they form the _whole_ organism, as in
Bacteria and other low forms of life, or only a germ-substance, as in
unicellular and multicellular organisms.
FOOTNOTES:
[Footnote 33: _Vortraege ueber Descendenztheorie_, Jena, 1904, II. 269.
Eng. Transl. London, 1904, II. p. 317.]
[Footnote 34: See Poulton, _Essays on Evolution_, Oxford, 1908. pp.
xix-xxii.]
[Footnote 35: _Origin of Species_ (6th edit), pp. 176 _et seq._]
[Footnote 36: Chun, _Reise der Valdivia_, Leipzig, 1904.]
[Footnote 37: Plate, _Selektionsprinzip u. Probleme der Artbildung_
(3rd edit.), Leipzig, 1908.]
[Footnote 38: _Studien zur Descendenz-Theorie_ II., "Die Enstehung der
Zeichnung bei den Schmetterlings-raupen," Leipzig, 1876.]
[Footnote 39: _Origin of Species_ (6th edit.), p. 232.]
[Footnote 40: _Origin of Species_, p. 233; see also edit. 1, p. 242.]
[Footnote 41: _Ibid._ p. 230.]
[Footnote 42: _The Effect of External Influences upon Development_,
Romanes Lecture, Oxford, 1894.]
[Footnote 43: See Poulton, _Essays on Evolution_, 1908, pp. 316, 317.]
[Footnote 44: _The Evolution Theory_, London, 1904, I. p. 219.]
[Footnote 45: _Report of the British Association_ (Bristol, 1898),
London, 1899, pp. 906-909.]
[Footnote 46: _Proc. Ent. Soc._, London, May 6, 1903.]
[Footnote 47: _Essays on Evolution_, 1889-1907, Oxford, 1908,
_passim_, e.g. p. 269.]
[Footnote 48: The expression does not refer to all the enemies of this
butterfly; against ichneumon-flies, for instance, their unpleasant
smell usually gives no protection.]
[Footnote 49: Professor Poulton has corrected some wrong descriptions
which I had unfortunately overlooked in the Plates of my book
_Vortraege ueber Descendenztheorie_, and which refer to _Papilio
dardanus_ (_merope_). These mistakes are of no importance as far as an
understanding of the mimicry-theory is concerned, but I hope shortly
to be able to correct them in a later edition.]
[Footnote 50: _Journ. Linn. Soc. London_ (_Zool._), Vol. xxvi. 1898,
pp. 598-602.]
[Footnote 51: In _Kosmos_, 1879, p. 100.]
[Footnote 52: _Habit and Instinct_, London. 1896.]
[Footnote 53: This has been discussed in many of my earlier works. See
for instance _The All-Sufficiency of Natural Selection, a reply to
Herbert Spencer_, London, 1893.]
[Footnote 54: _The Evolution Theory_, London, 1904, p. 144.]
[Footnote 55: _Variation under Domestication_, 1875, II. pp. 426,
427.]
III
HEREDITY AND VARIATION IN MODERN LIGHTS
BY W. BATESON, M.A., F.R.S.
_Professor of Biology in the University of Cambridge_
Darwin's work has the property of greatness in that it may be admired
from more aspects than one. For some the perception of the principle
of Natural Selection stands out as his most wonderful achievement to
which all the rest is subordinate. Others, among whom I would range
myself, look up to him rather as the first who plainly distinguished,
collected, and comprehensively studied that new class of evidence from
which hereafter a true understanding of the process of Evolution may
be developed. We each prefer our own standpoint of admiration; but I
think that it will be in their wider aspect that his labours will most
command the veneration of posterity.
A treatise written to advance knowledge may be read in two moods. The
reader may keep his mind passive, willing merely to receive the
impress of the writer's thought; or he may read with his attention
strained and alert, asking at every instant how the new knowledge can
be used in a further advance, watching continually for fresh footholds
by which to climb higher still. Of Shelley it has been said that he
was a poet for poets: so Darwin was a naturalist for naturalists. It
is when his writings are used in the critical and more exacting spirit
with which we test the outfit for our own enterprise that we learn
their full value and strength. Whether we glance back and compare his
performance with the efforts of his predecessors, or look forward
along the course which modern research is disclosing, we shall honour
most in him not the rounded merit of finite accomplishment, but the
creative power by which he inaugurated a line of discovery endless in
variety and extension. Let us attempt thus to see his work in true
perspective between the past from which it grew, and the present which
is its consequence. Darwin attacked the problem of Evolution by
reference to facts of three classes: Variation; Heredity; Natural
Selection. His work was not as the laity suppose, a sudden and
unheralded revelation, but the first fruit of a long and hitherto
barren controversy. The occurrence of variation from type, and the
hereditary transmission of such variation had of course been long
familiar to practical men, and inferences as to the possible bearing
of those phenomena on the nature of specific difference had been from
time to time drawn by naturalists. Maupertuis, for example, wrote: "Ce
qui nous reste a examiner, c'est comment d'un seul individu, il a pu
naitre tant d'especes si differentes." And again: "La Nature contient
le fonds de toutes ces varietes: mais le hasard ou l'art les mettent
en oeuvre. C'est ainsi que ceux dont l'industrie s'applique a
satisfaire le gout des curieux, sont, pour ainsi dire, createurs
d'especes nouvelles."[56]
Such passages, of which many (though few so emphatic) can be found in
eighteenth century writers, indicate a true perception of the mode of
Evolution. The speculations hinted at by Buffon,[57] developed by
Erasmus Darwin, and independently proclaimed above all by Lamarck,
gave to the doctrine of descent a wide renown. The uniformitarian
teaching which Lyell deduced from geological observation had gained
acceptance. The facts of geographical distribution[58] had been shown
to be obviously inconsistent with the Mosaic legend. Prichard, and
Lawrence, following the example of Blumenbach, had successfully
demonstrated that the races of Man could be regarded as different
forms of one species, contrary to the opinion up till then received.
These treatises all begin, it is true, with a profound obeisance to
the sons of Noah, but that performed, they continue on strictly modern
lines. The question of the mutability of species was thus prominently
raised.
Those who rate Lamarck no higher than did Huxley in his contemptuous
phrase "_buccinator tantum_," will scarcely deny that the sound of the
trumpet had carried far, or that its note was clear. If then there
were few who had already turned to evolution with positive conviction,
all scientific men must at least have known that such views had been
promulgated; and many must, as Huxley says, have taken up his own
position of "critical expectancy."[59]
Why, then, was it, that Darwin succeeded where the rest had failed?
The cause of that success was twofold. First, and obviously, in the
principle of Natural Selection he had a suggestion which would work.
It might not go the whole way, but it was true as far as it went.
Evolution could thus in great measure be fairly represented as a
consequence of demonstrable processes. Darwin seldom endangers the
mechanism he devised by putting on it strains much greater than it can
bear. He at least was under no illusion as to the omnipotence of
Selection; and he introduces none of the forced pleading which in
recent years has threatened to discredit that principle.
For example, in the latest text of the _Origin_[60] we find him
saying:
"But as my conclusions have lately been much misrepresented,
and it has been stated that I attribute the modification of
species exclusively to natural selection, I may be permitted
to remark that in the first edition of this work, and
subsequently, I placed in a most conspicuous
position--namely, at the close of the Introduction--the
following words: 'I am convinced that natural selection has
been the main but not the exclusive means of modification.'"
But apart from the invention of this reasonable hypothesis, which may
well, as Huxley estimated, "be the guide of biological and
psychological speculation for the next three or four generations,"
Darwin made a more significant and imperishable contribution. Not for
a few generations, but through all ages he should be remembered as the
first who showed clearly that the problems of Heredity and Variation
are soluble by observation, and laid down the course by which we must
proceed to their solution.[61] The moment of inspiration did not come
with the reading of Malthus, but with the opening of the "first
note-book on Transmutation of Species."[62] Evolution is a process of
Variation and Heredity. The older writers, though they had some vague
idea that it must be so, did not study Variation and Heredity. Darwin
did, and so begat not a theory, but a science.
The extent to which this is true, the scientific world is only
beginning to realise. So little was the fact appreciated in Darwin's
own time that the success of his writings was followed by an almost
total cessation of work in that special field. Of the causes which led
to these remarkable consequences I have spoken elsewhere. They
proceeded from circumstances peculiar to the time; but whatever the
causes there is no doubt that this statement of the result is
historically exact, and those who make it their business to collect
facts elucidating the physiology of Heredity and Variation are well
aware that they will find little to reward their quest in the leading
scientific Journals of the Darwinian epoch.
In those thirty years the original stock of evidence current and in
circulation even underwent a process of attrition. As in the story of
the Eastern sage who first wrote the collected learning of the
universe for his sons in a thousand volumes and by successive
compression and burning reduced them to one and from this by further
burning distilled the single ejaculation of the Faith "There is no god
but God and Mohammed is the Prophet of God," which was all his maturer
wisdom deemed essential:--so in the books of that period do we find
the _corpus_ of genetic knowledge dwindle to a few prerogative
instances and these at last to the brief formula of an unquestioned
creed.
And yet in all else that concerns biological science this period was,
in very truth, our Golden Age, when the natural history of the earth
was explored as never before; morphology and embryology were
exhaustively ransacked; the physiology of plants and animals began to
rival chemistry and physics in precision of method and in the rapidity
of its advances; and the foundations of pathology were laid.
In contrast with this immense activity elsewhere the neglect which
befel the special physiology of Descent, or Genetics as we now call
it, is astonishing. This may of course be interpreted as meaning that
the favoured studies seemed to promise a quicker return for effort,
but it would be more true to say that those who chose these other
pursuits did so without making any such comparison; for the idea that
the physiology of Heredity and Variation was a coherent science,
offering possibilities of extraordinary discovery, was not present to
their minds at all. In a word, the existence of such a science was
well nigh forgotten. It is true that in ancillary periodicals, as for
example those that treat of entomology or horticulture, or in the
writings of the already isolated systematists,[63] observations with
this special bearing were from time to time related, but the class of
fact on which Darwin built his conceptions of Heredity and Variation
was not seen in the highways of biology. It formed no part of the
official curriculum of biological students, and found no place among
the subjects which their teachers were investigating.
During this period nevertheless one distinct advance was made, that
with which Weismann's name is prominently connected. In Darwin's
genetic scheme the hereditary transmission of parental experience and
its consequences played a considerable role. Exactly how great that
role was supposed to be, he with his habitual caution refrained from
specifying, for the sufficient reason that he did not know.
Nevertheless much of the process of Evolution, especially that by
which organs have become degenerate and rudimentary, was certainly
attributed by Darwin to such inheritance, though since belief in the
inheritance of acquired characters fell into dispute, the fact has
been a good deal overlooked. The _Origin_ without "use and disuse"
would be a materially different book. A certain vacillation is
discernible in Darwin's utterances on this question, and the fact gave
to the astute Butler an opportunity for his most telling attack. The
discussion which best illustrates the genetic views of the period
arose in regard to the production of the rudimentary condition of the
wings of many beetles in the Madeira group of islands, and by
comparing passages from the _Origin_[64] Butler convicts Darwin of
saying first that this condition was in the main the result of
Selection, with disuse aiding, and in another place that the main
cause of degeneration was disuse, but that Selection had aided. To
Darwin however I think the point would have seemed one of dialetics
merely. To him the one paramount purpose was to show that somehow an
Evolution by means of Variation and Heredity might have brought about
the facts observed, and whether they had come to pass in the one way
or the other was a matter of subordinate concern.
To us moderns the question at issue has a diminished significance. For
over all such debates a change has been brought by Weismann's
challenge for evidence that use and disuse have any transmitted
effects at all. Hitherto the transmission of many acquired
characteristics had seemed to most naturalists so obvious as not to
call for demonstration.[65] Weismann's demand for facts in support of
the main proposition revealed at once that none having real cogency
could be produced. The time-honoured examples were easily shown to be
capable of different explanations. A few certainly remain which cannot
be so summarily dismissed, but--though it is manifestly impossible
here to do justice to such a subject--I think no one will dispute that
these residual and doubtful phenomena, whatever be their true nature,
are not of a kind to help us much in the interpretation of any of
those complex cases of adaptation which on the hypothesis of unguided
Natural Selection are especially difficult to understand. Use and
disuse were invoked expressly to help us over these hard places; but
whatever changes can be induced in offspring by direct treatment of
the parents, they are not of a kind to encourage hope of real
assistance from that quarter. It is not to be denied that through the
collapse of this second line of argument the Selection hypothesis has
had to take an increased and perilous burden. Various ways of meeting
the difficulty have been proposed, but these mostly resolve themselves
into improbable attempts to expand or magnify the powers of Natural
Selection.
Weismann's interpellation, though negative in purpose, has had a
lasting and beneficial effect, for through his thorough demolition of
the old loose and distracting notions of inherited experience, the
ground has been cleared for the construction of a true knowledge of
heredity based on experimental fact.
In another way he made a contribution of a more positive character,
for his elaborate speculations as to the genetic meaning of
cytological appearances have led to a minute investigation of the
visible phenomena occurring in those cell-divisions by which
germ-cells arise. Though the particular views he advocated have very
largely proved incompatible with the observed facts of heredity, yet
we must acknowledge that it was chiefly through the stimulus of
Weismann's ideas that those advances in cytology were made; and though
the doctrine of the continuity of germ-plasm cannot be maintained in
the form originally propounded, it is in the main true and
illuminating.[66] Nevertheless in the present state of knowledge we
are still as a rule quite unable to connect cytological appearances
with any genetic consequences and save in one respect (obviously of
extreme importance--to be spoken of later) the two sets of phenomena
might, for all we can see, be entirely distinct.
I cannot avoid attaching importance to this want of connection between
the nuclear phenomena and the features of bodily organisation. All
attempts to investigate Heredity by cytological means lie under the
disadvantage that it is the nuclear changes which can alone be
effectively observed. Important as they must surely be, I have never
been persuaded that the rest of the cell counts for nothing. What we
know of the behaviour and variability of chromosomes seems in my
opinion quite incompatible with the belief that they alone govern
form, and are the sole agents responsible in heredity.[67]