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Darwiniana

T >> Thomas Henry Huxley >> Darwiniana




In science no man can safely speculate about the nature and relation of
things with which he is unacquainted at first hand, and the acquirement of
an intimate and practical knowledge of the process of species-making and of
all the uncertainties which underlie the boundaries between species and
varieties, drawn by even the most careful and conscientious systematists
[Footnote: "After describing a set of forms as distinct species, tearing up
my MS., and making them one species, tearing that up and making them
separate, and then making them one again (which has happened to me), I have
gnashed my teeth, cursed species, and asked what sin I had committed to be
so punished." (II. p. 40.) Is there any naturalist provided with a logical
sense and a large suite of specimens, who has not undergone pangs of the
sort described in this vigorous paragraph, which might, with advantage, be
printed on the title-page of every systematic monograph as a warning to the
uninitiated?] were of no less importance to the author of the "Origin of
Species" than was the bearing of the Cirripede work upon "the principles of
a natural classification." (I. p. 81.) No one, as Darwin justly observes,
has a "right to examine the question of species who has not minutely
described many." (II. p. 39.)

In September, 1854, the Cirripede work was finished, "ten thousand
barnacles" had been sent "out of the house, all over the world," and Darwin
had the satisfaction of being free to turn again to his "old notes on
species." In 1855, he began to breed pigeons, and to make observations on
the effects of use and disuse, experiments on seeds, and so on, while
resuming his industrious collection of facts, with a view "to see how far
they favour or are opposed to the notion that wild species are mutable or
immutable. I mean with my utmost power to give all arguments and facts on
both sides. I have a _number_ of people helping me every way, and
giving me most valuable assistance; but I often doubt whether the subject
will not quite overpower me." (II. p. 49.)

Early in 1856, on Lyell's advice, Darwin began to write out his views on
the origin of species on a scale three or four times as extensive as that
of the work published in 1859. In July of the same year he gave a brief
sketch of his theory in a letter to Asa Gray; and, in the year 1857, his
letters to his correspondents show him to be busily engaged on what he
calls his "big book." (II. pp. 85, 94.) In May, 1857, Darwin writes to
Wallace: "I am now preparing my work [on the question how and in what way
do species and varieties differ from each other] for publication, but I
find the subject so very large, that, though I have written many chapters,
I do not suppose I shall go to press for two years." (II. p. 95.) In
December, 1857, he writes, in the course of a long letter to the same
correspondent, "I am extremely glad to hear that you are attending to
distribution in accordance with theoretical ideas. I am a firm believer
that without speculation there is no good and original observation." (II.
p. 108.) [Footnote: The last remark contains a pregnant truth, but it must
be confessed it hardly squares with the declaration in the
_Autobiography_, (I. p. 83), that he worked on "true Baconian
principles."] In June, 1858, he received from Mr. Wallace, then in the
Malay Archipelago, an "Essay on the tendency of varieties to depart
indefinitely from the original type," of which Darwin says, "If Wallace had
my MS. sketch written out in 1842 he could not have made a better short
abstract! Even his terms stand now as heads of my chapters. Please return
me the MS., which he does not say he wishes me to publish, but I shall, of
course, at once write and offer to send it to any journal. So all my
originality, whatever it may amount to, will be smashed, though my book, if
ever it will have any value, will not be deteriorated; as all the labour
consists in the application of the theory." (II. p. 116.)

Thus, Darwin's first impulse was to publish Wallace's essay without note or
comment of his own. But, on consultation with Lyell and Hooker, the latter
of whom had read the sketch of 1844, they suggested, as an undoubtedly more
equitable course, that extracts from the MS. of 1844 and from the letter to
Dr. Asa Gray should be communicated to the Linnean Society along with
Wallace's essay. The joint communication was read on July 1, 1858, and
published under the title "On the Tendency of Species to form Varieties;
and on the Perpetuation of Varieties and Species by Natural Means of
Selection." This was followed, on Darwin's part, by the composition of a
summary account of the conclusions to which his twenty years' work on the
species question had led him. It occupied him for thirteen months, and
appeared in November, 1859, under the title "On the Origin of Species by
means of Natural Selection or the Preservation of Favoured Races in the
Struggle of Life."

It is doubtful if any single book, except the "Principia," ever worked so
great and so rapid a revolution in science, or made so deep an impression
on the general mind. It aroused a tempest of opposition and met with
equally vehement support, and it must be added that no book has been more
widely and persistently misunderstood by both friends and foes. In 1861,
Darwin remarks to a correspondent, "You understand my book perfectly, and
that I find a very rare event with my critics." (I. p. 313.) The immense
popularity which the "Origin" at once acquired was no doubt largely due to
its many points of contact with philosophical and theological questions in
which every intelligent man feels a profound interest; but a good deal must
be assigned to a somewhat delusive simplicity of style, which tends to
disguise the complexity and difficulty of the subject, and much to the
wealth of information on all sorts of curious problems of natural history,
which is made accessible to the most unlearned reader. But long occupation
with the work has led the present writer to believe that the "Origin of
Species" is one of the hardest of books to master; [Footnote: He is
comforted to find that probably the best qualified judge among all the
readers of the _Origin_ in 1859 was of the same opinion. Sir J. Hooker
writes, "It is the very hardest book to read, to full profit, that I ever
tried." (II. p. 242.)] and he is justified in this conviction by observing
that although the "Origin" has been close on thirty years before the world,
the strangest misconceptions of the essential nature of the theory therein
advocated are still put forth by serious writers.

Although, then, the present occasion is not suitable for any detailed
criticism of the theory, or of the objections which have been brought
against it, it may not be out of place to endeavour to separate the
substance of the theory from its accidents; and to show that a variety not
only of hostile comments, but of friendly would-be improvements lose their
_raison d'être_ to the careful student. Observation proves the
existence among all living beings of phenomena of three kinds, denoted by
the terms heredity, variation, and multiplication. Progeny tend to resemble
their parents; nevertheless all their organs and functions are susceptible
of departing more or less from the average parental character; and their
number is in excess of that of their parents. Severe competition for the
means of living, or the struggle for existence, is a necessary consequence
of unlimited multiplication; while selection, or the preservation of
favourable variations and the extinction of others, is a necessary
consequence of severe competition. "Favourable variations" are those which
are better adapted to surrounding conditions. It follows, therefore, that
every variety which is selected into a species is so favoured and preserved
in consequence of being, in some one or more respects, better adapted to
its surroundings than its rivals. In other words, every species which
exists, exists in virtue of adaptation, and whatever accounts for that
adaptation accounts for the existence of the species.

To say that Darwin has put forward a theory of the adaptation of species,
but not of their origin, is therefore to misunderstand the first principles
of the theory. For, as has been pointed out, it is a necessary consequence
of the theory of selection that every species must have some one or more
structural or functional peculiarities, in virtue of the advantage
conferred by which, it has fought through the crowd of its competitors and
achieved a certain duration. In this sense, it is true that every species
has been "originated" by selection.

There is another sense, however, in which it is equally true that selection
originates nothing. "Unless profitable variations ... occur natural
selection can do nothing" ("Origin," Ed. I. p. 82). "Nothing can be
effected unless favourable variations occur" (_ibid_., p. 108). "What
applies to one animal will apply throughout time to all animals--that is,
if they vary--for otherwise natural selection can do nothing. So it will be
with plants" (_ibid_., p. 113). Strictly speaking, therefore, the
origin of species in general lies in variation; while the origin of any
particular species lies, firstly, in the occurrence, and secondly, in the
selection and preservation of a particular variation. Clearness on this
head will relieve one from the necessity of attending to the fallacious
assertion that natural selection is a _deus ex machinâ_, or occult
agency.

Those, again, who confuse the operation of the natural causes which bring
about variation and selection with what they are pleased to call "chance"
can hardly have read the opening paragraph of the fifth chapter of the
"Origin" (Ed. I, p. 131): "I have sometimes spoken as if the variations ...
had been due to chance. This is of course a wholly incorrect expression,
but it seems to acknowledge plainly our ignorance of the cause of each
particular variation."

Another point of great importance to the right comprehension of the theory,
is, that while every species must needs have some adaptive advantageous
characters to which it owes its preservation by selection, it may possess
any number of others which are neither advantageous nor disadvantageous,
but indifferent, or even slightly disadvantageous. (_Ibid_., p. 81.)
For variations take place, not merely in one organ or function at a time,
but in many; and thus an advantageous variation, which gives rise to the
selection of a new race or species, may be accompanied by others which are
indifferent, but which are just as strongly hereditary as the advantageous
variations. The advantageous structure is but one product of a modified
general constitution which may manifest itself by several other products;
and the selective process carries the general constitution along with the
advantageous special peculiarity. A given species of plant may owe its
existence to the selective adaptation of its flowers to insect fertilisers;
but the character of its leaves may be the result of variations of an
indifferent character. It is the origin of variations of this kind to which
Darwin refers in his frequent reference to what he calls "laws of
correlation of growth" or "correlated variation."

These considerations lead us further to see the inappropriateness of the
objections raised to Darwin's theory on the ground that natural selection
does not account for the first commencements of useful organs. But it does
not pretend to do so. The source of such commencements is necessarily to be
sought in different variations, which remain unaffected by selection until
they have taken such a form as to become utilisable in the struggle for
existence.

It is not essential to Darwin's theory that anything more should be assumed
than the facts of heredity, variation, and unlimited multiplication; and
the validity of the deductive reasoning as to the effect of the last (that
is, of the struggle for existence which it involves) upon the varieties
resulting from the operation of the former. Nor is it essential that one
should take up any particular position in regard to the mode of variation,
whether, for example, it takes place _per saltum_ or gradually;
whether it is definite in character or indefinite. Still less are those who
accept the theory bound to any particular views as to the causes of
heredity or of variation.

That Darwin held strong opinions on some or all of these points may be
quite true; but, so far as the theory is concerned, they must be regarded
as _obiter dicta_. With respect to the causes of variation, Darwin's
opinions are, from first to last, put forward altogether tentatively. In
the first edition of the "Origin," he attributes the strongest influence to
changes in the conditions of life of parental organisms, which he appears
to think act on the germ through the intermediation of the sexual organs.
He points out, over and over again, that habit, use, disuse, and the direct
influence of conditions have some effect, but he does not think it great,
and he draws attention to the difficulty of distinguishing between effects
of these agencies and those of selection. There is, however, one class of
variations which he withdraws from the direct influence of selection,
namely, the variations in the fertility of the sexual union of more or less
closely allied forms. He regards less fertility, or more or less complete
sterility, as "incidental to other acquired differences." (_Ibid_., p.
245.)

Considering the difficulties which surround the question of the causes of
variation, it is not to be wondered at, that Darwin should have inclined,
sometimes, rather more to one and, sometimes, rather more to another of the
possible alternatives. There is little difference between the last edition
of the "Origin" (1872) and the first on this head. In 1876, however, he
writes to Moritz Wagner, "In my opinion, the greatest error which I have
committed has been not allowing sufficient weight to the direct action of
the environments, i.e., food, climate, &c., independently of natural
selection. ...When I wrote the 'Origin,' and for some years afterwards, I
could find little good evidence of the direct action of the environment;
now there is a large body of evidence, and your case of the Saturnia is one
of the most remarkable of which I have heard." (III, p. 159.) But there is
really nothing to prevent the most tenacious adherent to the theory of
natural selection from taking any view he pleases as to the importance of
the direct influence of conditions and the hereditary transmissibility of
the modifications which they produce. In fact, there is a good deal to be
said for the view that the so-called direct influence of conditions is
itself a case of selection. Whether the hypothesis of Pangenesis be
accepted or rejected, it can hardly be doubted that the struggle for
existence goes on not merely between distinct organisms, but between the
physiological units of which each organism is composed, and that changes in
external conditions favour some and hinder others.

After a short stay in Cambridge, Darwin resided in London for the first
five years which followed his return to England; and for three years, he
held the post of Secretary to the Geological Society, though he shared to
the full his friend Lyell's objection to entanglement in such engagements.
In fact, he used to say in later life, more than half in earnest, that he
gave up hoping for work from men who accepted official duties and,
especially, Government appointments. Happily for him, he was exempted from
the necessity of making any sacrifice of this kind, but an even heavier
burden was laid upon him. During the earlier half of his voyage Darwin
retained the vigorous health of his boyhood, and indeed proved himself to
be exceptionally capable of enduring fatigue and privation. An anomalous
but severe disorder, which laid him up for several weeks at Valparaiso in
1834, however, seems to have left its mark on his constitution; and, in the
later years of his London life, attacks of illness, usually accompanied by
severe vomiting and great prostration of strength, became frequent. As he
grew older, a considerable part of every day, even at his best times, was
spent in misery; while, not unfrequently, months of suffering rendered work
of any kind impossible. Even Darwin's remarkable tenacity of purpose and
methodical utilisation of every particle of available energy could not have
enabled him to achieve a fraction of the vast amount of labour he got
through, in the course of the following forty years, had not the wisest and
the most loving care unceasingly surrounded him from the time of his
marriage in 1839. As early as 1842, the failure of health was so marked
that removal from London became imperatively necessary; and Darwin
purchased a house and grounds at Down, a solitary hamlet in Kent, which was
his home for the rest of his life. Under the strictly regulated conditions
of a valetudinarian existence, the intellectual activity of the invalid
might have put to shame most healthy men; and, so long as he could hold his
head up, there was no limit to the genial kindness of thought and action
for all about him. Those friends who were privileged to share the intimate
life of the household at Down have an abiding memory of the cheerful
restfulness which pervaded and characterised it.

After mentioning his settlement at Down, Darwin writes in his
Autobiography:--

"My chief enjoyment and sole employment throughout life has been scientific
work; and the excitement from such work makes me, for the time, forget, or
drives quite away, my daily discomfort. I have, therefore, nothing to
record during the rest of my life, except the publication of my several
books." (I, p. 79.)

Of such works published subsequently to 1859, several are monographic
discussions of topics briefly dealt with in the "Origin," which, it must
always be recollected, was considered by the author to be merely an
abstract of an _opus majus_.

The earliest of the books which may be placed in this category, "On the
Various Contrivances by which Orchids are Fertilised by Insects," was
published in 1862, and whether we regard its theoretical significance, the
excellence of the observations and the ingenuity of the reasonings which it
records, or the prodigious mass of subsequent investigation of which it has
been the parent, it has no superior in point of importance. The conviction
that no theory of the origin of species could be satisfactory which failed
to offer an explanation of the way in which mechanisms involving
adaptations of structure and function to the performance of certain
operations are brought about, was, from the first, dominant in Darwin's
mind. As has been seen, he rejected Lamarck's views because of their
obvious incapacity to furnish such an explanation in the case of the great
majority of animal mechanisms, and in that of all those presented by the
vegetable world.

So far back as 1793, the wonderful work of Sprengel had established, beyond
any reasonable doubt, the fact that, in a large number of cases, a flower
is a piece of mechanism the object of which is to convert insect visitors
into agents of fertilisation. Sprengel's observations had been most
undeservedly neglected and well-nigh forgotten; but Robert Brown having
directed Darwin's attention to them in 1841, he was attracted towards the
subject, and verified many of Sprengel's statements. (III, p. 258.) It may
be doubted whether there was a living botanical specialist, except perhaps
Brown, who had done as much. If, however, adaptations of this kind were to
be explained by natural selection, it was necessary to show that the plants
which were provided with mechanisms for ensuring the aid of insects as
fertilisers, were by so much the better fitted to compete with their
rivals. This Sprengel had not done. Darwin had been attending to cross
fertilisation in plants so far back as 1839, from having arrived, in the
course of his speculations on the origin of species, at the conviction
"that crossing played an important part in keeping specific forms constant"
(I, p. 90). The further development of his views on the importance of cross
fertilisation appears to have taken place between this time and 1857, when
he published his first papers on the fertilisation of flowers in the
"Gardener's Chronicle." If the conclusion at which he ultimately arrived,
that cross fertilisation is favourable to the fertility of the parent and
to the vigour of the offspring, is correct, then it follows that all those
mechanisms which hinder self-fertilisation and favour crossing must be
advantageous in the struggle for existence; and, the more perfect the
action of the mechanism, the greater the advantage. Thus the way lay open
for the operation of natural selection in gradually perfecting the flower
as a fertilisation-trap. Analogous reasoning applies to the fertilising
insect. The better its structure is adapted to that of the trap, the more
will it be able to profit by the bait, whether of honey or of pollen, to
the exclusion of its competitors. Thus, by a sort of action and reaction, a
two-fold series of adaptive modifications will be brought about.

In 1865, the important bearing of this subject on his theory led Darwin to
commence a great series of laborious and difficult experiments on the
fertilisation of plants, which occupied him for eleven years, and furnished
him with the unexpectedly strong evidence in favour of the influence of
crossing which he published in 1876, under the title of "The Effects of
Cross and Self Fertilisation in the Vegetable Kingdom." Incidentally, as it
were, to this heavy piece of work, he made the remarkable series of
observations on the different arrangements by which crossing is favoured
and, in many cases, necessitated, which appeared in the work on "The
Different Forms of Flowers in Plants of the same Species" in 1877.

In the course of the twenty years during which Darwin was thus occupied in
opening up new regions of investigation to the botanist and showing the
profound physiological significance of the apparently meaningless
diversities of floral structure, his attention was keenly alive to any
other interesting phenomena of plant life which came in his way. In his
correspondence, he not unfrequently laughs at himself for his ignorance of
systematic botany; and his acquaintance with vegetable anatomy and
physiology was of the slenderest. Nevertheless, if any of the less common
features of plant life came under his notice, that imperious necessity of
seeking for causes which nature had laid upon him, impelled, and indeed
compelled, him to inquire the how and the why of the fact, and its bearing
on his general views. And as, happily, the atavic tendency to frame
hypotheses was accompanied by an equally strong need to test them by
well-devised experiments, and to acquire all possible information before
publishing his results, the effect was that he touched no topic without
elucidating it.

Thus the investigation of the operations of insectivorous plants, embodied
in the work on that topic published in 1875, was started fifteen years
before, by a passing observation made during one of Darwin's rare holidays.

"In the summer of 1860, I was idling and resting near Hartfield, where two
species of Drosera abound; and I noticed that numerous insects had been
entrapped by the leaves. I carried home some plants, and on giving them
some insects saw the movements of the tentacles, and this made me think it
possible that the insects were caught for some special purpose.
Fortunately, a crucial test occurred to me, that of placing a large number
of leaves in various nitrogenous and non-nitrogenous fluids of equal
density; and as soon as I found that the former alone excited energetic
movements, it was obvious that here was a fine new field for
investigation." (I, p. 95.)

The researches thus initiated led to the proof that plants are capable of
secreting a digestive fluid like that of animals, and of profiting by the
result of digestion; whereby the peculiar apparatuses of the insectivorous
plants were brought within the scope of natural selection. Moreover, these
inquiries widely enlarged our knowledge of the manner in which stimuli are
transmitted in plants, and opened up a prospect of drawing closer the
analogies between the motor processes of plants and those of animals.

So with respect to the books on "Climbing Plants" (1875), and on the "Power
of Movement in Plants" (1880), Darwin says;--

"I was led to take up this subject by reading a short paper by Asa Gray,
published in 1858. He sent me some seeds, and on raising some plants I was
so much fascinated and perplexed by the revolving movements of the tendrils
and stems, which movements are really very simple, though appearing at
first sight very complex, that I procured various other kinds of climbing
plants and studied the whole subject.... Some of the adaptations displayed
by climbing plants are as beautiful as those of orchids for ensuring
cross-fertilisation." (I, p. 93.)

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