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the first place, to be extremely cautious in pretending to decide what struc-
tures now are, or have formerly been of use to each species. In the second
place, it should always be borne in mind that when one part is modified,
so will be other parts, through certain dimly seen causes, such as an in-
creased or diminished flow of nutriment to a part, mutual pressure, an
early developed part affecting one subsequently developed, and so forth —
as well as through other causes which lead to the many mysterious cases
of correlation, which we do not in the least understand. These agencies may
be all grouped together, for the sake of brevity, under the expression of the
laws of growth. In the third place, we have to allow for the direct and
definite action of changed conditions of life, and for so-called spontaneous
variations, in which the nature of the conditions apparently plays a quite
subordinate part. Bud variations, such as the appearance of a moss-rose on
a common rose, or of a nectarine on a peach-tree, offer good instances of
spontaneous variations; but even in these cases, if we bear in mind the
power of a minute drop of poison in producing complex galls, we ought not
to feel too sure that the above variations are not the eff'ect of some local
change in the nature of the sap, due to some change in the conditions.
There must be some efficient cause for each slight individual difference, as
well as for more strongly marked variations which occasionally arise; and
if the unknown cause were to act persistently, it is almost certain that all
the individuals of the species would be similarly modified.

In the earlier editions of this work I underrated, as it now seems prob-
able, the frequency and importance of modifications due to spontaneous
variability. But it is impossible to attribute to this cause the innumerable
structures which are so well adapted to the habits of life of each species. I
can no more believe in this than that the well-adapted form of a race-
horse or greyhound, which before the principle of selection by man was


well understood, excited so much surprise in the minds of the older natural-
ists, can thus be explained.

It may be worth while to illustrate some of the foregoing remarks. With
respect to the assumed inutility of various parts and organs, it is hardly
necessary to observe that even in the higher and best-known animals many
structures exist, which are so highly developed that no one doubts that
they are of importance, yet their use has not been, or has only recently
been, ascertained. As Bronn gives the length of the ears and tail in the
several species of mice as instances, though trifling ones, of differences in
structure which can be of no special use, I may mention that, according to
Dr. Schobl, the external ears of the common mouse are supplied in an
extraordinary manner with nerves, so that they no doubt serve as tactile
organs; hence the length of the ears can hardly be quite unimportant. We
shall, also, presently see that the tail is a highly useful prehensile organ to
some of the species; and its use would be much influenced by its length.

With respect to plants, to which on account of Nageli's essay I shall con-
fine myself in the following remarks, it will be admitted that the flowers of
the orchids present a multitude of curious structures, which a few years ago
would have been considered as mere morphological differences without any
special function; but they are now known to be of the highest importance
for the fertilization of the species through the aid of insects, and have
probably been gained through natural selection. No one until lately would
have imagined that in dimorphic and trimorphic plants the different
lengths of the stamens and pistils, and their arrangement, could have been
of any service, but now we know this to be the case.

In certain whole groups of plants the ovules stand erect, and in others
they are suspended; and within the same ovarium of some few plants, one
ovule holds the former and a second ovule the latter position. These posi-
tions seem at first purely morphological, or of no physiological signification;
but Dr. Hooker informs me that within the same ovarium, the upper ovules
alone in some cases, and in others the lower ones alone are fertilized; and
he suggests that this probably depends on the direction in which the pollen-
tubes enter the ovarium. If so, the position of the ovules, even when one is
erect and the other suspended within the same ovarium, would follow the
selection of any slight deviations in position which favored their fertiliza-^
tion, and the production of seed.

Several plants belonging to distinct orders habitually produce flowers of
two kinds — the one open, of the ordinary structure, the other closed and
imperfect. These two kinds of flowers sometimes differ wonderfully in struc-
ture, yet may be seen to graduate into each other on the same plant. The
ordinary and open flowers can be intercrossed; and the benefits which
certainly are derived from this process are thus secured. The closed and
imperfect flowers are, however, manifestly of high importance, as they yield
with the utmost safety a large stock of seed, with the expenditure of won-
derfully little pollen. The two kinds of flowers often differ much, as just
stated, in structure. The petals in the imperfect flowers almost always con-


sist of mere rudiments, and the pollen-grains are reduced in diameter. In
Ononis columnse five of the alternate stamens are rudimentary; and in
some species of Viola three stamens are in this state, two retaining their
proper function, but being of very small size. In six out of thirty of the
closed flowers in an Indian violet (name unknown, for the plants have'
never produced with me perfect flowers), the sepals are reduced from the
normal number of five to three. In one section of the Malpighiacese the
closed flowers, according to A. de Jussieu, are still further modified, for the
five stamens which stand opposite to the sepals are all aborted, a sixth'
stamen standing opposite to a petal being alone developed; and this stamen
is not present in the ordinary flowers of this species; the style is aborted;
and the ovaria are reduced from three to two. Now although natural selec-
tion may well have had the power to prevent some of the flowers from
expanding, and to reduce the amount of pollen, when rendered by the
closure of the flowers superfluous, yet hardly any of the above special
modifications can have been thus determined, but must have followed from
the laws of growth, including the functional inactivity of parts, during the
progress of the reduction of the pollen and the closure of the flowers.

It is so necessary to appreciate the important eff'ects of the laws of
growth, that I will give some additional cases of another kind, namely of
differences in the same part or organ, due to diff"erences in relative position
on the same plant. In the Spanish chestnut, and in certain fir-trees, the
angles of divergence of the leaves differ, according to Schacht, in the nearly
horizontal and in the upright branches. In the common rue and some
other plants, one flower, usually the central or terminal one, opens first,
and has five sepals and petals, and five divisions to the ovarium; while all
the other flowers on the plant are tetramerous. In the British Adoxa the
uppermost flower generally has two calyx-lobes with the other organs
tetramerous, while the surrounding flowers generally have three calyx-lobes
with the other organs pentamerous. In many compositae and umbelliferae
(and in some other plants) the circumferential flowers have their corollas
much more developed than those of the centre; and this seems often con-
nected with the abortion of the reproductive organs. It is a more curious
fact, previously referred to, that the achenes or seeds of the circumference
and centre sometimes differ greatly in form, color, and other characters. In
Carthamus and some other compositae the central achenes alone are fur-
nished with a pappus; and in Hyoseris the same head yields achenes of
three different forms. In certain umbelliferae the exterior seeds, according
to Tausch, are orthospermous, and the central one ccelospermous, and this
is a character which was considered by De Candolle to be in other species
of the highest systematic importance. Professor Braun mentions a Fuma-
riaceous genus, in which the flowers in the lower part of the spike bear
oval, ribbed, one-seeded nutlets; and in the upper part of the spike,
lanceolate, two-valved, and two-seeded siliques. In these several cases, with
the exception of that of the well-developed ray-florets, which are of service
in making the flowers conspicuous to insects, natural selection cannot, as


far as we can judge, have come into play, or only in a quite subordinate
manner. All these modifications follow from the relative position and
interaction of the parts; and it can hardly be doubted that if all the flowers
and leaves on the same plant had been subjected to the same external and
internal condition, as are the flowers and leaves in certain positions, all
would have been modified in the same manner.

In numerous other cases we find modifications of structure, which are
considered by botanists to be generally of a highly important nature, affect-
ing only some of the flowers on the same plant, or occurring on distinct
plants, which grow close together under the same conditions. As these
variations seem of no special use to the plants, they cannot have been in-
fluenced by natural selection. Of their cause we are quite ignorant; we
cannot even attribute them, as in the last class of cases, to any proximate
agency, such as relative position. I will give only a few instances. It is so
common to observe on the same plant, flowers indifferently tetramerous,
pentamerous, etc., that I need not give examples; but as numerical varia-
tions are comparatively rare when the parts are few, I may mention that,
according to De Candolle, the flowers of Papaver bracteatum offer either
two sepals with four petals (which is the common type with poppies), or
three sepals with six petals. The manner in which the petals are folded in
the bud is, in most groups, a very constant morphological character; but
Professor Asa Gray states that with some species of Mimulus, the aestivation
is almost as frequently that of the Rhinanthidese as of the Antirrhinidese,
to which latter tribe the genus belongs. Aug. Saint-Hilaire gives the follow-
ing cases : the genus Zanthoxylon belongs to a division of the Rutaceae with
a single ovary, but in some species flowers may be found on the same plant,
and even in the same panicle, with either one or two ovaries. In Helian-
themum the capsule has been described as unilocular or trilocular; and
in H. mutabile, "Une lame plus ou moins large s'etend entre le pericarpe
et le placenta." In the flowers of Saponaria officinalis Dr. Masters has
observed instances of both marginal and free central placentation. Lastly,
Saint-Hilaire found toward the southern extreme of the range of Gomphia
oleaeformis two forms which he did not at first doubt were distinct species,
but he subsequently saw them growing on the same bush; and he then
adds, "Voila done dans un meme individu des loges et un style qui se
rattachent tantot a un axe vertical et tantot a un gynobase."

We thus see that with plants many morphological changes may be at-
tributed to the laws of growth and the interaction of parts, independently
of natural selection. But with respect to Nageli's doctrine of an innate
tendency toward perfection or progressive development, can it be said in
the case of these strongly pronounced variations, that the plants have been
caught in the act of progressing toward a higher state of development? On
the contrary, I should infer from the mere fact of the parts in question
diff"ering or varying greatly on the same plant, that such modifications were
of extremely small importance to the plants themselves, of whatever im-
portance they may generally be to us for our classifications. The acquisition


of a useless part can hardly be said to raise an organism in the natural
scale; and in the case of the imperfect, closed flowers, above described, if any
new principle has to be invoked, it must be one of retrogression rather than
of progression; and so it must be with many parasitic and degraded animals.
We are ignorant of the exciting cause of the above specified modifications;
but if the unknown cause were to act almost uniformly for a length of time,
we may infer that the result would be almost uniform; and in this case all
the individuals of the species would be modified in the same manner.

From the fact of the above characters being unimportant for the welfare
of the species, any slight variations which occurred in them would not have
been accumulated and augmented through natural selection. A structure
which has been developed through long-continued selection, when it ceases
to be of service to a species, generally becomes variable, as we see with
rudimentary organs; for it will no longer be regulated by this same power
of selection. But when, from the nature of the organism and of the condi-
tions, modifications have been induced which are unimportant for the wel-
fare of the species, they may be, and apparently often have been, trans-
mitted in nearly the same state to numerous, otherwise modified, de-
scendants. It cannot have been of much importance to the greater number
of mammals, birds, or reptiles, whether they were clothed with hair,
feathers, or scales; yet hair has been transmitted to almost all mammals,
feathers to all birds, and scales to all true reptiles. A structure, whatever it
may be, which is common to many allied forms, is ranked by us as of high
systematic importance, and consequently is often assumed to be of high
vital importance to the species. Thus, as I am inclined to believe, morpho-
logical differences, which we consider as important — such as the arrange-
ment of the leaves, the divisions of the flower or of the ovarium, the posi-
tion of the ovules, etc., first appeared in many cases as fluctuating varia-
tions, which sooner or later became constant through the nature of the
organism and of the surrounding conditions, as well as through the inter-
crossing of distinct individuals, but not through natural selection; for as
these morphological characters do not affect the welfare of the species, any
slight deviations in them could not have been governed or accumulated
through this latter agency. It is a strange result which we thus arrive at,
namely, that characters of slight vital importance to the species are the
most important to the systematist; but, as we shall hereafter see when we
treat of the genetic principle of classification, this is by no means so para-
doxical as it may at first appear.

Although we have no good evidence of the existence in organic beings
of an innate tendency toward progressive development, yet this necessarily
follows, as I have attempted to show in the fourth chapter, through the
continued action of natural selection. For the best definition which has ever
been given of a high standard of organization, is the degree to which the
parts have been specialized or differentiated; and natural selection tends
toward this end, inasmuch as the parts are thus enabled to perform their
functions more efficiently.


A distinguished zoologist, Mr. St. George Mivart, has recently collected
all the objections which have ever been advanced by myself and others
against the theory of natural selection as propounded by Mr. Wallace and
myself, and has illustrated them with admirable art and force. When thus
marshalled, they make a formidable array; and as it forms no part of Mr.
Mivart's plan to give the various facts and considerations opposed to his
conclusions, no slight effort of reason and memory is left to the reader, who
may wish to weigh the evidence on both sides. When discussing special
cases, Mr. Mivart passes over the effects of the increased use and disuse of
parts, which I have always maintained to be highly important, and have
treated in my "Variation under Domestication" at greater length than, as I
believe, any other writer. He likewise often assumes that I attribute nothing
to variation, independently of natural selection, whereas in the work just
referred to I have collected a greater number of well-established cases than
can be found in any other work known to me. My judgment may not be
trustworthy, but after reading with care Mr. Mivart's book, and comparing
each section with what I have said on the same head, I never before felt
so strongly convinced of the general truth of the conclusions here arrived
at, subject, of course, in so intricate a subject, to much partial error.

All Mr. Mivart's objections will be, or have been, considered in the
present volume. The one new point which appears to have struck many
readers is, "That natural selection is incompetent to account for the in-
cipient stages of useful structures." This subject is intimately connected
with that of the gradation of the characters, often accompanied by a
change of function, for instance, the conversion of a swim-bladder into
lungs, points which were discussed in the last chapter under two headings.
Nevertheless, I will here consider in some detail several of the cases ad-
vanced by Mr. Mivart, selecting those which are the most illustrative, as
want of space prevents me from considering all.

The giraffe, by its lofty stature, much elongated neck, fore legs, head,
and tongue, has its whole frame beautifully adapted for browsing on the
higher branches of trees. It can thus obtain food beyond the reach of the
other Ungulata or hoofed animals inhabiting the same country; and this
must be a great advantage to it during dearths. The Niata cattle in South
America show us how small a difference in structure may make, during
such periods, a great difference in preserving an animal's life. These cattle
can browse as well as others on grass, but from the projection of the lower
jaw they cannot, 3uring the often recurrent droughts, browse on the twigs
of trees, reeds, etc., to which food the common cattle and horses are then
driven; so that at these times the Niatas perish, if not fed by their owners.
Before coming to Mr. Mivart's objections, it may be well to explain once
again how natural selection will act in all ordinary cases. Man has modified
some of his animals, without necessarily having attended to special points of
structure, by simply preserving and breeding from the fleetest individuals,
as with the race-horse and greyhound, or as with the game-cock, by breed-
ing from the victorious birds. So under nature with the nascent giraffe, the


individuals which were the highest browsers and were able during dearth
to reach even an inch or two above the others, will often have been pre
served; for they will have roamed over the whole country in search of fooc
That the individuals of the same species often differ slightly in the relativ
lengths of all their parts may be seen in many works of natural history h
which careful measurements are given. These slight proportional differ
ences, due to the laws of growth and variation, are not of the slightest us(
or importance to most species. But it will have been otherwise with th(
nascent giraffe, considering its probable habits of life; for those individual'
which had some one part or several parts of their bodies rather more
elongated than usual, would generally have survived. These will have inter-
crossed and left offspring, either inheriting the same bodily peculiarities oi
with a tendency to vary again in the same manner; while the individual^
less favored m the same respects will have been the most liable to perish

We here see that there is no need to separate single pairs, as man does
when he methodically improves a breed: natural selection will preserve and
thus separate all the superior individuals, allowing them freely to intercross
and will destroy all the inferior individuals. By this process long continued'
which exactly corresponds with what I have called unconscious selection by
man, combined, no doubt, in a most important manner with the inherited
effects of the increased use of parts, it seems to me almost certain that an
ordinary hoofed quadruped might be converted into a giraffe.

To this conclusion Mr. Mivart brings forward two objections. One is that
the increased size of the body would obviously require an increased supply
of food, and he considers it as "very problematical whether the disad^
vantages thence arising would not, in times of scarcity, more than counter-
balance the advantages." But as the giraffe does actually exist in large num-
bers m Africa, and as some of the largest antelopes in the world taller
than an ox, abound there, why should we doubt that, as far as size 'is con-
cerned, intermediate gradations could formerly have existed there sub-
jected as now to severe dearths? Assuredly the being able to reach at each
stage of increased size, to a supply of food left untouched by the other
hoofed quadrupeds of the country, would have been of some advantage to
the nascent giraffe. Nor must we overlook the fact, that increased bulk
would act as a protection against almost all beasts of prey excepting the
hon; and against this animal, its tall neck— and the taller the better-
would, as Mr. Chauncey Wright has remarked, serve as a watch-tower It
is from this cause, as Sir S. Baker remarks, that no animal is more difficult
to stalk than the giraffe. This animal also uses its long neck as a means of
offence or defence, by violently swinging its head armed with stump-like
horns. The preservation of each species can rarely be determined by any one
advantage, but by the union of all, great and small.

^ Mr. Mivart then asks (and this is his second objection), if natural selec-
tion be so potent, and if high browsing be so great an advantage, why has
not any other hoofed quadruped acquired a long neck and lofty stature,
besides the giraffe, and, in a lesser degree, the camel, guanaco, and


jnacrauchenia? Or, again, why has not any member of the group acquired
a long proboscis? With respect to South Africa, which was formerly in-
habited by numerous herds of the giraffe, the answer is not difficult, and
can best be given by an illustration. In every meadow in England, in which
trees grow, we see the lower branches trimmed or planed to an exact level
by the browsing of the horses or cattle; and what advantage would it be,
for instance, to sheep, if kept there, to acquire slightly longer necks? In
every district some one kind of animal will almost certainly be able to
browse higher than the others; and it is almost equally certain that this
one kind alone could have its neck elongated for this purpose, through
natural selection and the effects of increased use. In South Africa the com-
petition for browsing on the higher branches of the acacias and other trees
must be between giraffe and giraffe, and not with the other ungulate

Why, in other quarters of the world, various animals belonging to this
same order have not acquired either an elongated neck or a proboscis, can-
not be distinctly answered; but it is as unreasonable to expect a distinct
answer to such a question as why some event in the history of mankind did
not occur in one country while it did in another. We are ignorant with
respect to the conditions which determine the numbers and range of each
species, and we cannot even conjecture what changes of structure would
be favorable to its increase in some new country. We can, however, see in
a general manner that various causes might have interfered with the de-
velopment of a long neck or proboscis. To reach the foliage at a considerable
height (without climbing, for which hoofed animals are singularly ill-
constructed) implies greatly increased bulk of body; and we know that
some areas support singularly few large quadrupeds, for instance South
America, though it is so luxuriant, while South Africa abounds with them
to an unparalleled degree. Why this should be so, we do not know; nor why
the later tertiary periods should have been much more favorable for their

Online LibraryCharles DarwinThe origin of species → online text (page 20 of 50)