scanty numbers.
But when this cycle of change had taken place, the species would be very
different from the original form. The flower would have been at one time
modified to favour the visits of insects and to secure
cross-fertilisation by their aid, and when the need for this passed
away, some portions of these structures would remain, though in a
reduced or rudimentary condition. But when insect agency became of
importance a second time, the new modifications would start from a
different or more advanced basis, and thus a more complex result might
be produced. Owing to the unequal rates at which the reduction of the
various parts might occur, some amount of irregularity in the flower
might arise, and on a second development towards insect
cross-fertilisation this irregularity, if useful, might be increased by
variation and selection.
The rapidity and comparative certainty with which such changes as are
here supposed do really take place, are well shown by the great
differences in floral structure, as regards the mode of fertilisation,
in allied genera and species, and even in some cases in varieties of the
same species. Thus in the Ranunculaceae we find the conspicuous part of
the flower to be the petals in Ranunculus, the sepals in Helleborus,
Anemone, etc., and the stamens in most species of Thalictrum. In all
these we have a simple regular flower, but in Aquilegia it is made
complex by the spurred petals, and in Delphinium and Aconitum it becomes
quite irregular. In the more simple class self-fertilisation occurs
freely, but it is prevented in the more complex flowers by the stamens
maturing before the pistil. In the Caprifoliaceae we have small and
regular greenish flowers, as in the moschatel (Adoxa); more conspicuous
regular open flowers without honey, as in the elder (Sambucus); and
tubular flowers increasing in length and irregularity, till in some,
like our common honeysuckle, they are adapted for fertilisation by moths
only, with abundant honey and delicious perfume to attract them. In the
Scrophulariaceae we find open, almost regular flowers, as Veronica and
Verbascum, fertilised by flies and bees, but also self-fertilised;
Scrophularia adapted in form and colour to be fertilised by wasps; and
the more complex and irregular flowers of Linaria, Rhinanthus,
Melampyrum, Pedicularis, etc., mostly adapted to be fertilised by bees.
In the genera Geranium, Polygonum, Veronica, and several others there is
a gradation of forms from large and bright to small and obscure coloured
flowers, and in every case the former are adapted for insect
fertilisation, often exclusively, while in the latter self-fertilisation
constantly occurs. In the yellow rattle (Rhinanthus Crista-galli) there
are two forms (which have been named _major_ and _minor_), the larger
and more conspicuous adapted to insect fertilisation only, the smaller
capable of self-fertilisation; and two similar forms exist in the
eyebright (Euphrasia officinalis). In both these cases there are special
modifications in the length and curvature of the style as well as in the
size and shape of the corolla; and the two forms are evidently becoming
each adapted to special conditions, since in some districts the one, in
other districts the other is most abundant.[159]
These examples show us that the kind of change suggested above is
actually going on, and has presumably always been going on in nature
throughout the long geological epochs during which the development of
flowers has been progressing. The two great modes of gaining increased
vigour and fertility - intercrossing and dispersal over wider areas - have
been resorted to again and again, under the pressure of a constant
struggle for existence and the need for adaptation to ever-changing
conditions. During all the modifications that ensued, useless parts were
reduced or suppressed, owing to the absence of selection and the
principle of economy of growth; and thus at each fresh adaptation some
rudiments of old structures were re-developed, but not unfrequently in
a different form and for a distinct purpose.
The chief types of flowering plants have existed during the millions of
ages of the whole tertiary period, and during this enormous lapse of
time many of them may have been modified in the direction of insect
fertilisation, and again into that of self-fertilisation, not once or
twice only, but perhaps scores or even hundreds of times; and at each
such modification a difference in the environment may have led to a
distinct line of development. At one epoch the highest specialisation of
structure in adaptation to a single species or group of insects may have
saved a plant from extinction; while, at other times, the simplest mode
of self-fertilisation, combined with greater powers of dispersal and a
constitution capable of supporting diverse physical conditions, may have
led to a similar result. With some groups the tendency seems to have
been almost continuously to greater and greater specialisation, while
with others a tendency to simplification and degradation has resulted in
such plants as the grasses and sedges.
We are now enabled dimly to perceive how the curious anomaly of very
simple and very complex methods of securing cross-fertilisation - both
equally effective - may have been brought about. The simple modes may be
the result of a comparatively direct modification from the more
primitive types of flowers, which were occasionally, and, as it were,
accidentally visited and fertilised by insects; while the more complex
modes, existing for the most part in the highly irregular flowers, may
result from those cases in which adaptation to insect-fertilisation, and
partial or complete degradation to self-fertilisation or to
wind-fertilisation, have again and again recurred, each time producing
some additional complexity, arising from the working up of old rudiments
for new purposes, till there have been reached the marvellous flower
structures of the papilionaceous tribes, of the asclepiads, or of the
orchids.
We thus see that the existing diversity of colour and of structure in
flowers is probably the ultimate result of the ever-recurring struggle
for existence, combined with the ever-changing relations between the
vegetable and animal kingdoms during countless ages. The constant
variability of every part and organ, with the enormous powers of
increase possessed by plants, have enabled them to become again and
again readjusted to each change of condition as it occurred, resulting
in that endless variety, that marvellous complexity, and that exquisite
colouring which excite our admiration in the realm of flowers, and
constitute them the perennial charm and crowning glory of nature.
_Flowers the Product of Insect Agency._
In his _Origin of Species_, Mr. Darwin first stated that flowers had
been rendered conspicuous and beautiful in order to attract insects,
adding: "Hence we may conclude that, if insects had not been developed
on the earth, our plants would not have been decked with beautiful
flowers, but would have produced only such poor flowers as we see on our
fir, oak, nut, and ash trees, on grasses, docks, and nettles, which are
all fertilised through the agency of the wind." The argument in favour
of this view is now much stronger than when he wrote; for not only have
we reason to believe that most of these wind-fertilised flowers are
degraded forms of flowers which have once been insect fertilised, but we
have abundant evidence that whenever insect agency becomes comparatively
ineffective, the colours of the flowers become less bright, their size
and beauty diminish, till they are reduced to such small, greenish,
inconspicuous flowers as those of the rupture-wort (Herniaria glabra),
the knotgrass (Polygonum aviculare), or the cleistogamic flowers of the
violet. There is good reason to believe, therefore, not only that
flowers have been developed in order to attract insects to aid in their
fertilisation, but that, having been once produced, in however great
profusion, if the insect races were all to become extinct, flowers (in
the temperate zones at all events) would soon dwindle away, and that
ultimately all floral beauty would vanish from the earth.
We cannot, therefore, deny the vast change which insects have produced
upon the earth's surface, and which has been thus forcibly and
beautifully delineated by Mr. Grant Allen: "While man has only tilled a
few level plains, a few great river valleys, a few peninsular mountain
slopes, leaving the vast mass of earth untouched by his hand, the insect
has spread himself over every land in a thousand shapes, and has made
the whole flowering creation subservient to his daily wants. His
buttercup, his dandelion, and his meadow-sweet grow thick in every
English field. His thyme clothes the hillside; his heather purples the
bleak gray moorland. High up among the alpine heights his gentian
spreads its lakes of blue; amid the snows of the Himalayas his
rhododendrons gleam with crimson light. Even the wayside pond yields him
the white crowfoot and the arrowhead, while the broad expanses of
Brazilian streams are beautified by his gorgeous water-lilies. The
insect has thus turned the whole surface of the earth into a boundless
flower-garden, which supplies him from year to year with pollen or
honey, and itself in turn gains perpetuation by the baits that it offers
for his allurement."[160]
_Concluding Remarks on Colour in Nature._
In the last four chapters I have endeavoured to give a general and
systematic, though necessarily condensed view of the part which is
played by colour in the organic world. We have seen in what infinitely
varied ways the need of concealment has led to the modification of
animal colours, whether among polar snows or sandy deserts, in tropical
forests or in the abysses of the ocean. We next find these general
adaptations giving way to more specialised types of coloration, by which
each species has become more and more harmonised with its immediate
surroundings, till we reach the most curiously minute resemblances to
natural objects in the leaf and stick insects, and those which are so
like flowers or moss or birds' droppings that they deceive the acutest
eye. We have learnt, further, that these varied forms of protective
colouring are far more numerous than has been usually suspected,
because, what appear to be very conspicuous colours or markings when the
species is observed in a museum or in a menagerie, are often highly
protective when the creature is seen under the natural conditions of its
existence. From these varied classes of facts it seems not improbable
that fully one-half of the species in the animal kingdom possess colours
which have been more or less adapted to secure for them concealment or
protection.
Passing onward we find the explanation of a distinct type of colour or
marking, often superimposed upon protective tints, in the importance of
easy recognition by many animals of their fellows, their parents, or
their mates. By this need we have been able to account for markings that
seem calculated to make the animal conspicuous, when the general tints
and well-known habits of the whole group demonstrate the need of
concealment. Thus also we are able to explain the constant symmetry in
the markings of wild animals, as well as the numerous cases in which the
conspicuous colours are concealed when at rest and only become visible
during rapid motion. In striking contrast to ordinary protective
coloration we have "warning colours," usually very conspicuous and often
brilliant or gaudy, which serve to indicate that their possessors are
either dangerous or uneatable to the usual enemies of their tribe. This
kind of coloration is probably more prevalent than has been hitherto
supposed, because in the case of many tropical animals we are quite
unacquainted with their special and most dangerous enemies, and are also
unable to determine whether they are or are not distasteful to those
enemies. As a kind of corollary to the "warning colours," we find the
extraordinary phenomena of "mimicry," in which defenceless species
obtain protection by being mistaken for those which, from any cause,
possess immunity from attack. Although a large number of instances of
warning colour and of mimicry are now recorded, it is probably still an
almost unworked field of research, more especially in tropical regions
and among the inhabitants of the ocean.
The phenomena of sexual diversities of coloration next engaged our
attention, and the reasons why Mr. Darwin's theory of "sexual
selection," as regards colour and ornament, could not be accepted were
stated at some length, together with the theory of animal coloration and
ornament we propose to substitute for it. This theory is held to be in
harmony with the general facts of animal coloration, while it entirely
dispenses with the very hypothetical and inadequate agency of female
choice in producing the detailed colours, patterns, and ornaments, which
in so many cases distinguish the male sex.
If my arguments on this point are sound, they will dispose also of Mr.
Grant Allen's view of the direct action of the colour sense on the
animal integuments.[161] He argues that the colours of insects and birds
reproduce generally the colours of the flowers they frequent or the
fruits they eat, and he adduces numerous cases in which flower-haunting
insects and fruit-eating birds are gaily coloured. This he supposes to
be due to the colour-taste, developed by the constant presence of bright
flowers and fruits, being applied to the selection of each variation
towards brilliancy in their mates; thus in time producing the gorgeous
and varied hues they now possess. Mr. Allen maintains that "insects are
bright where bright flowers exist in numbers, and dull where flowers are
rare or inconspicuous;" and he urges that "we can hardly explain this
wide coincidence otherwise than by supposing that a taste for colour is
produced through the constant search for food among entomophilous
blossoms, and that this taste has reacted upon its possessors through
the action of unconscious sexual selection."
The examples Mr. Allen quotes of bright insects being associated with
bright flowers seem very forcible, but are really deceptive or
erroneous; and quite as many cases could be quoted which prove the very
opposite. For example, in the dense equatorial forests flowers are
exceedingly scarce, and there is no comparison with the amount of floral
colour to be met with in our temperate meadows, woods, and hillsides.
The forests about Para in the lower Amazon are typical in this respect,
yet they abound with the most gorgeously coloured butterflies, almost
all of which frequent the forest depths, keeping near the ground, where
there is the greatest deficiency of brilliant flowers. In contrast with
this let us take the Cape of Good Hope - the most flowery region probably
that exists upon the globe, - where the country is a complete
flower-garden of heaths, pelargoniums, mesembryanthemus, exquisite
iridaceous and other bulbs, and numerous flowering shrubs and trees; yet
the Cape butterflies are hardly equal, either in number or variety, to
those of any country in South Europe, and are utterly insignificant when
compared with those of the comparatively flowerless forest-depths of the
Amazon or of New Guinea. Neither is there any relation between the
colours of other insects and their haunts. Few are more gorgeous than
some of the tiger-beetles and the carabi, yet these are all carnivorous;
while many of the most brilliant metallic buprestidae and longicorns are
always found on the bark of fallen trees. So with the humming-birds;
their brilliant metallic tints can only be compared with metals or gems,
and are totally unlike the delicate pinks and purples, yellows and reds
of the majority of flowers. Again, the Australian honey-suckers
(Meliphagidae) are genuine flower-haunters, and the Australian flora is
more brilliant in colour display than that of most tropical regions, yet
these birds are, as a rule, of dull colours, not superior on the average
to our grain-eating finches. Then, again, we have the grand pheasant
family, including the gold and the silver pheasants, the gorgeous
fire-backed and ocellated pheasants, and the resplendent peacock, all
feeding on the ground on grain or seeds or insects, yet adorned with the
most gorgeous colours.
There is, therefore, no adequate basis of facts for this theory to rest
upon, even if there were the slightest reason to believe that not only
birds, but butterflies and beetles, take any delight in colour for its
own sake, apart from the food-supply of which it indicates the presence.
All that has been proved or that appears to be probable is, that they
are able to perceive differences of colour, and to associate each colour
with the particular flowers or fruits which best satisfy their wants.
Colour being in its nature diverse, it has been beneficial for them to
be able to distinguish all its chief varieties, as manifested more
particularly in the vegetable kingdom, and among the different species
of their own group; and the fact that certain species of insects show
some preference for a particular colour may be explained by their having
found flowers of that colour to yield them a more abundant supply of
nectar or of pollen. In those cases in which butterflies frequent
flowers of their own colour, the habit may well have been acquired from
the protection it affords them.
It appears to me that, in imputing to insects and birds the same love of
colour for its own sake and the same aesthetic tastes as we ourselves
possess, we may be as far from the truth as were those writers who held
that the bee was a good mathematician, and that the honeycomb was
constructed throughout to satisfy its refined mathematical instincts;
whereas it is now generally admitted to be the result of the simple
principle of economy of material applied to a primitive cylindrical
cell.[162]
In studying the phenomena of colour in the organic world we have been
led to realise the wonderful complexity of the adaptations which bring
each species into harmonious relation with all those which surround it,
and which thus link together the whole of nature in a network of
relations of marvellous intricacy. Yet all this is but, as it were, the
outward show and garment of nature, behind which lies the inner
structure - the framework, the vessels, the cells, the circulating
fluids, and the digestive and reproductive processes, - and behind these
again those mysterious chemical, electrical, and vital forces which
constitute what we term Life. These forces appear to be fundamentally
the same for all organisms, as is the material of which all are
constructed; and we thus find behind the outer diversities an inner
relationship which binds together the myriad forms of life.
Each species of animal or plant thus forms part of one harmonious whole,
carrying in all the details of its complex structure the record of the
long story of organic development; and it was with a truly inspired
insight that our great philosophical poet apostrophised the humble
weed -
Flower in the crannied wall,
I pluck you out of the crannies,
I hold you here, root and all, in my hand,
Little flower - but _if_ I could understand
What you are, root and all, and all in all,
I should know what God and man is.
FOOTNOTES:
[Footnote 136: Burchell's _Travels_, vol. i. p. 10.]
[Footnote 137: _Nature_, vol. iii. p. 507.]
[Footnote 138: _Flowers, Fruits, and Leaves_, p. 128 (Fig. 79).]
[Footnote 139: For a popular sketch of these, see Sir J. Lubbock's
_Flowers, Fruits, and Leaves_, or any general botanical work.]
[Footnote 140: _Nature_, vol. xv. p, 117.]
[Footnote 141: Grant Allen's _Colour Sense_, p. 113.]
[Footnote 142: Canon Tristram's _Natural History of the Bible_, pp. 483,
484.]
[Footnote 143: For a complete historical account of this subject with
full references to all the works upon it, see the Introduction to
Hermann Müller's _Fertilisation of Flowers_, translated by D'Arcy W.
Thompson.]
[Footnote 144: For the full detail of his experiments, see _Cross-and
Self-Fertilisation of Plants_, 1876.]
[Footnote 145: See Darwin's _Fertilisation of Orchids_ for the many
extraordinary and complex arrangements in these plants.]
[Footnote 146: The English reader may consult Sir John Lubbock's
_British Wild Flowers in Relation to Insects_, and H. Müller's great and
original work, _The Fertilisation of Flowers_.]
[Footnote 147: Müller's _Fertilisation of Flowers_, p. 248.]
[Footnote 148: "Alpenblumen," by D.H. Müller. See _Nature_, vol. xxiii.
p. 333.]
[Footnote 149: This peculiarity of local distribution of colour in
flowers may be compared, as regards its purpose, with the recognition
colours of animals. Just as these latter colours enable the sexes to
recognise each other, and thus avoid sterile unions of distinct species,
so the distinctive form and colour of each species of flower, as
compared with those that usually grow around it, enables the fertilising
insects to avoid carrying the pollen of one flower to the stigma of a
distinct species.]
[Footnote 150: See H. Müller's _Fertilisation of Flowers_, p. 18.]
[Footnote 151: The above examples are taken from Rev. G. Henslow's paper
on "Self-Fertilisation of Plants," in _Trans. Linn. Soc._ Second series,
_Botany_, vol. i. pp. 317-398, with plate. Mr. H.O. Forbes has shown
that the same thing occurs among tropical orchids, in his paper "On the
Contrivances for insuring Self-Fertilisation in some Tropical Orchids,"
_Journ. Linn. Soc._, xxi. p. 538.]
[Footnote 152: These are the numbers given by Darwin, but I am informed
by Mr. Hemsley that many additions have been since made to the list, and
that cleistogamic flowers probably occur in nearly all the natural
orders.]
[Footnote 153: For a full account of cleistogamic flowers, see Darwin's
_Forms of Flowers_, chap. viii.]
[Footnote 154: Henslow's "Self-Fertilisation," _Trans. Linn. Soc._
Second series, _Botany_, vol. i. p. 391.]
[Footnote 155: The Rev. George Henslow, in his _Origin of Floral
Structures_, says: "There is little doubt but that all wind-fertilised
angiosperms are degradations from insect-fertilised flowers....
_Poterium sanguisorba_ is anemophilous; and _Sanguisorba officinalis_
presumably was so formerly, but has reacquired an entomophilous habit;
the whole tribe Poterieae being, in fact, a degraded group which has
descended from Potentilleae. Plantains retain their corolla but in a
degraded form. Junceae are degraded Lilies; while Cyperaceae and
Gramineae among monocotyledons may be ranked with Amentiferae among
dicotyledons, as representing orders which have retrograded very far
from the entomophilous forms from which they were possibly and probably
descended" (p. 266).
"The genus Plantago, like _Thalictrum minus_, Poterium, and others, well
illustrate the change from an entomophilous to the anemophilous state.
_P. lanceolata_ has polymorphic flowers, and is visited by
pollen-seeking insects, so that it can be fertilised either by insects
or the wind. _P. media_ illustrates transitions in point of structure,
as the filaments are pink, the anthers motionless, and the pollen grains
aggregated, and it is regularly visited by _Bombus terrestris_. On the
other hand, the slender filaments, versatile anthers, powdery pollen,
and elongated protogynous style are features of other species indicating
anemophily; while the presence of a degraded corolla shows its ancestors
to have been entomophilous. _P. media_, therefore, illustrates, not a
primitive entomophilous condition, but a return to it; just as is the
case with _Sanguisorba officinalis_ and _Salix Caprea_; but these show
no capacity of restoring the corolla, the attractive features having to
be borne by the calyx, which is purplish in Sanguisorba, by the pink
filaments of Plantago, and by the yellow anthers in the Sallow willow"
(p. 271).
"The interpretation, then, I would offer of inconspicuousness and all
kinds of degradations is the exact opposite to that of conspicuousness
and great differentiations; namely, that species with minute flowers,
rarely or never visited by insects, and habitually self-fertilised, have
primarily arisen through the neglect of insects, and have in consequence
assumed their present floral structures" (p. 282).
In a letter just received from Mr. Henslow, he gives a few additional
illustrations of his views, of which the following are the most