affords us an almost infallible guide in doubtful cases; because it
appears to change its form with extreme slowness, and thus indicates
deeper-seated affinities than those shown by organs which are in
direct connection with the outside world, and are readily modified
in accordance with varying conditions of existence. Another, though
less valuable guide is afforded, in the case of birds, by the eggs.
These often have a characteristic form and colour, and a peculiar
texture of surface, running unchanged through whole genera and
families which are nearly related to each other, however much they
may differ in outward form and habits. Another detail of structure
which has no direct connection with habits and economy, is the
manner in which the plumage is arranged on the body. The feathers
of birds are by no means set uniformly over their skin, but grow
in certain definite lines and patches, which vary considerably in
shape and size in the more important orders and tribes, while the
mode of arrangement agrees in all which are known to be closely
related to each other; and thus the form of the feather-tracts or
the “pterylography” as it is termed, of a bird, is a valuable aid
in doubtful cases of affinity.
Now, if we apply these three tests to the humming-birds, we find
them all pointing in the same direction. The sternum or breast-bone
is not notched behind; and this agrees with the swifts, and not
with the sun-birds, whose sternum has two deep notches behind, as
in all the families of the vast order of Passeres to which the
latter belong. The eggs of both swifts and humming-birds are white,
only two in number, and resembling each other in texture. And in
the arrangement of the feather-tracts the humming-birds approach
more nearly to the swifts than they do to any other birds; and
altogether differ from the sun-birds, which, in this respect as in
so many others, resemble the honey-suckers of Australia and other
true passerine birds.
_Resemblances of Swifts and Humming-birds._--Having this clue to
their affinities, we shall find other peculiarities common to these
two groups, the swifts and the humming-birds. They have both ten
tail-feathers, while the sun-birds have twelve. They have both only
sixteen true quill-feathers, and they are the only birds which have
so small a number. The humming-birds are remarkable for having, in
almost all the species, the first quill the longest of all, the only
other birds resembling them in this respect being a few species
of swifts; and, lastly, in both groups the plumage is remarkably
compact and closely pressed to the body. Yet, with all these points
of agreement, we find an extreme diversity in the bills and tongues
of the two groups. The swifts have a short, broad, flat bill,
with a flat horny-tipped tongue of the usual character; while the
humming-birds have a very long, narrow, almost cylindrical bill,
containing a tubular and highly extensible tongue. The essential
point however is, that whereas hardly any of the other characters
we have adduced are adaptive, or strictly correlated with habits
and economy, this character is pre-eminently so; for the swifts are
pure aërial insect-hunters, and their short, broad bills, and wide
gape, are essential to their mode of life. The humming-birds, on the
other hand, are floral insect-hunters, and for this purpose their
peculiarly long bills and extensile tongues are especially adapted;
while they are at the same time honey-suckers, and for this purpose
have acquired the tubular tongue. The formation of such a tubular
tongue out of one of the ordinary kind is easily conceivable, as
it only requires to be lengthened, and the two laminæ of which it
is composed curled in at the sides; and these changes it probably
goes through in the young birds.
When on the Amazon I once had a nest brought me containing two little
unfledged humming-birds, apparently not long hatched. Their beaks
were not at all like those of their parents, but short, triangular,
and broad at the base; just the form of the beak of a swallow or
swift slightly lengthened. Thinking (erroneously) that the young
birds were fed by their parents on honey, I tried to feed them with
a syrup made of honey and water, but though they kept their mouths
constantly open as if ravenously hungry, they would not swallow
the liquid, but threw it out again and sometimes nearly choked
themselves in the effort. At length I caught some minute flies, and
on dropping one of these into the open mouth it instantly closed,
the fly was gulped down and the mouth opened again for more; and
each took in this way fifteen or twenty little flies in succession
before it was satisfied. They lived thus three or four days, but
required more constant care than I could give them. These little
birds were in the “swift” stage; they were pure insect-eaters, with
a bill and mouth adapted for insect-eating only. At that time I
was not aware of the importance of the observation of the tongue;
but as the bill was so short and the tubular tongue not required,
there can be little doubt that the organ was, at that early stage
of growth, short and flat, as it is in the birds most nearly allied
to them.
_Differences between Sun-birds and Humming-birds._--In respect of all
the essential and deep-seated points of structure, which have been
shown to offer such remarkable similarities between the swifts and
the humming-birds, the sun-birds of the Eastern hemisphere differ
totally from the latter, while they agree with the passerine birds
generally, or more particularly with the creepers and honey-suckers.
They have a deeply-notched sternum; they have twelve tail-feathers
in place of ten; they have nineteen quills in place of sixteen; and
the first quill instead of being the longest is the very shortest
of all, while the wings are short and round, instead of being
excessively long and pointed; their plumage is arranged differently;
and their feet are long and strong, instead of being excessively
short and weak. There remain only the superficial characters of small
size and brilliant metallic colours to assimilate them with the
humming-birds, and one structural feature--a tubular and somewhat
extensile tongue. This, however, is a strictly adaptive character,
the sun-birds feeding on small insects and the nectar of flowers,
just as do the humming-birds; and it is a remarkable instance of a
highly peculiar modification of an organ occurring independently
in two widely-separate groups. In the sun-birds the hyoid or
tongue-muscles do not extend so completely over the head as they
do in the humming-birds, so that the tongue is less extensible;
but it is constructed in exactly the same way by the inrolling of
the two laminæ of which it is composed.
The tubular tongue of the sun-birds is a special adaptive
modification acquired within the family itself, and not inherited
from a remote ancestral form. This is shown by the amount of
variation this organ exhibits in different members of the family.
It is most highly developed in the Arachnotheræ, or spider-hunters,
of Asia, which are sun-birds without any metallic or other
brilliant colouring. These have the longest bills and tongues,
and the most developed hyoid muscles; they hunt much about the
blossoms of palm-trees, and may frequently be seen probing the
flowers while fluttering clumsily in the air, just as if they had
seen and attempted to imitate the aërial gambols of the American
humming-birds. The true metallic sun-birds generally cling about
the flowers with their strong feet; and they feed chiefly on minute
hard insects, as do many humming-birds. There is, however, one
species (Chalcoparia phœnicotis) always classed as a sun-bird, which
differs entirely from the rest of the species in having the tongue
flat, horny, and forked at the tip; and its food seems to differ
correspondingly, for small caterpillars were found in its stomach.
More remotely allied, but yet belonging to the same family, are
the little flower-peckers of the genus Diceum, which have a short
bill and a tongue twice split at the end; and these feed on small
fruits, and perhaps on buds and on the pollen of flowers. The little
white-eyes (Zosterops), which are probably allied to the last, eat
soft fruits and minute insects.
Here then we have an extensive group of birds, considerably varied
in external form, yet undoubtedly closely allied to each other,
one division of which is specially adapted to feed on the juices
secreted by flowers and the minute insects that harbour in them; and
these alone have a lengthened bill and double tubular tongue, just
as in the humming-birds. We can hardly have a more striking example
of the necessity of discriminating between adaptive and purely
structural characters. The same adaptive character may coexist in
two groups which have a similar mode of life, without indicating
any affinity between them, because it may have been acquired by
each independently, to enable it to fill a similar place in nature.
In such cases it is found to be an almost isolated character,
apparently connecting two groups which otherwise differ radically.
Non-adaptive, or purely structural characters, on the other hand, are
such as have probably been transmitted from a remote ancestor; and
thus indicate fundamental peculiarities of growth and development.
The changes of structure rendered necessary by modifications of the
habits or instincts of the different species, have been made, to a
great extent, independently of such characters; and as several of
these may always be found in the same animal their value becomes
cumulative. We thus arrive at the seeming paradox, that the _less_
of direct use is apparent in any peculiarity of structure, the
_greater_ is its value in indicating true, though perhaps remote,
affinities; while any peculiarity of an organ which seems essential
to its possessor’s well-being is often of very little value in
indicating its affinity for other creatures.
This somewhat technical discussion will, it is hoped, enable the
general reader to understand some of the more important principles
of the modern or natural classification of animals, as distinguished
from the artificial system which long prevailed. It will also
afford him an easily remembered example of those principles, in
the radical distinctness of two families of birds often confounded
together,--the sun-birds of the Eastern Hemisphere, and the
humming-birds of America; and in the interesting fact that the
latter are essentially swifts--profoundly modified, it is true, for
an aërial and flower-haunting existence, but still bearing in many
important peculiarities of structure the unmistakable evidences of
a common origin.
V.
THE COLOURS OF ANIMALS AND SEXUAL SELECTION.
General Phenomena of Colour in the Organic World--Theory of Heat and
Light as producing Colour--Changes of Colour in Animals produced
by Coloured Light--Classification of Organic Colours--Protective
Colour--Warning Colours--Sexual Colours--Typical Colours--The
Nature of Colour--Colour a normal product of Organization--Theory
of Protective Colours--Theory of Warning Colours--Theory of
Sexual Colours--Colour as a means of Recognition--Colour
proportionate to Integumentary Development--Selection by
Females not a cause of Colour--Probable use of the Horns
of Beetles--Cause of the greater brilliancy of some Female
Insects--Theory of display of Ornaments by Males--Natural
Selection as neutralizing Sexual Selection--Theory of Typical
Colours--Colour-development as illustrated by Humming-birds--Local
causes of Colour-development--Summary on Colour-development in
Animals.
There is probably no one quality of natural objects from which we
derive so much pure and intellectual enjoyment as from their colours.
The heavenly blue of the firmament, the glowing tints of sunset,
the exquisite purity of the snowy mountains, and the endless shades
of green presented by the verdure-clad surface of the earth, are a
never-failing source of pleasure to all who enjoy the inestimable
gift of sight. Yet these constitute, as it were, but the frame and
background of a marvellous and ever-changing picture. In contrast
with these broad and soothing tints, we have presented to us in the
vegetable and animal worlds, an infinite variety of objects adorned
with the most beautiful and most varied hues. Flowers, insects and
birds, are the organisms most generally ornamented in this way; and
their symmetry of form, their variety of structure, and the lavish
abundance with which they clothe and enliven the earth, cause them
to be objects of universal admiration. The relation of this wealth
of colour to our mental and moral nature is indisputable. The child
and the savage alike admire the gay tints of flower, bird, and
insect; while to many of us their contemplation brings a solace and
enjoyment which is both intellectually, and morally beneficial. It
can then hardly excite surprise that this relation was long thought
to afford a sufficient explanation of the phenomena of colour in
nature; and although the fact that--
“Full many a flower is born to blush unseen,
And waste its sweetness on the desert air--”
might seem to throw some doubt on the sufficiency of the explanation,
the answer was easy,--that in the progress of discovery, man would,
sooner or later, find out and enjoy every beauty that the hidden
recesses of the earth have in store for him. This theory received
great support, from the difficulty of conceiving any other use
or meaning in the colours with which so many natural objects are
adorned. Why should the homely gorse be clothed in golden raiment,
and the prickly cactus be adorned with crimson bells? Why should
our fields be gay with buttercups, and the heather-clad mountains
be clad in purple robes? Why should every land produce its own
peculiar floral gems, and the alpine rocks glow with beauty, if
not for the contemplation and enjoyment of man? What could be
the use to the butterfly of its gaily-painted wings, or to the
humming-bird of its jewelled breast, except to add the final touches
to a world-picture, calculated at once to please and to refine
mankind? And even now, with all our recently-acquired knowledge of
this subject, who shall say that these old-world views were not
intrinsically and fundamentally sound; and that, although we now
know that colour has “uses” in nature that we little dreamt of, yet
the relation of those colours--or rather of the various rays of
light--to our senses and emotions, may not be another, and perhaps
more important use which they subserve in the great system of the
universe?
We now propose to lay before our readers a general account of the
more recent discoveries on this interesting subject; and in doing so,
it will be necessary first to give an outline of the more important
facts as to the colours of organised beings; then to point out the
cases in which it has been shown that colour is of use; and lastly,
to endeavour to throw some light on its nature, and on the general
laws of its development.
Among naturalists, colour was long thought to be of little import,
and to be quite untrustworthy as a specific character. The numerous
cases of variability of colour led to this view. The occurrence
of white blackbirds, white peacocks, and black leopards; of white
blue-bells, and of white, blue, or pink milkworts, led to the belief
that colour was essentially unstable, that it could therefore be of
little or no importance, and belonged to quite a different class of
characters from form or structure. But it now begins to be perceived
that these cases, though tolerably numerous, are, after all,
exceptional; and that colour, as a rule, is a constant character.
The great majority of species, both of animals and plants, are
each distinguished by peculiar tints which vary very little, while
the minutest markings are often constant in thousands or millions
of individuals. All our field buttercups are invariably yellow, and
our poppies red; while many of our butterflies and birds resemble
each other in every spot and streak of colour through thousands of
individuals. We also find that colour is constant in whole genera and
other groups of species. The Genistas are all yellow, the Erythrinas
all red; many genera of Carabidæ are entirely black; whole families
of birds--as the Dendrocolaptidæ--are brown; while among butterflies
the numerous species of Lycæna are all more or less blue, those of
Pontia white, and those of Callidryas yellow. An extensive survey of
the organic world thus leads us to the conclusion that colour is by
no means so unimportant or inconstant a character as at first sight
it appears to be; and the more we examine it the more convinced we
shall become that it must serve some purpose in nature, and that,
besides charming us by its diversity and beauty, it must be well
worthy of our attentive study, and have many secrets to unfold to
us.
_Theory of Heat and Light as producing Colour._--In commencing our
study of the great mass of facts relating to the colours of the
organic world, it will be necessary to consider first, how far the
chief theories already proposed will account for them. One of the
most obvious and most popular of these theories, and one which is
still held, in part at least, by many eminent naturalists, is--that
colour is due to some direct action of the heat and light of the
sun--thus at once accounting for the great number of brilliant
birds, insects, and flowers, which are found between the tropics.
But before proceeding to discuss this supposed explanation
of the colours of living things we must ask the preliminary
question,--whether it is really the fact that colour is more
developed in tropical than in temperate climates, in proportion to
the whole number of species; and even if we find this to be so, we
have to inquire whether there are not so many and such striking
exceptions to the rule, as to indicate some other causes at work
than the direct influence of solar light and heat. As this is a
most important branch of the inquiry, we must go into it somewhat
fully.
It is undoubtedly the case that there are an immensely greater
number of richly-coloured birds and insects in tropical than in
temperate and cold countries, but it is by no means so certain that
the _proportion_ of coloured to obscure species is much or any
greater. Naturalists and collectors well know that the majority of
tropical birds are dull-coloured; and there are whole families,
comprising hundreds of species, not one of which exhibits a particle
of bright colour. Such are, for example, the Timaliidæ, or babbling
thrushes of the Eastern, and the Dendrocolaptidæ, or tree-creepers
of the Western hemispheres. Again, many groups of birds, which are
universally distributed, are no more adorned with colour in the
tropical than in the temperate zones; such are the thrushes, wrens,
goatsuckers, hawks, grouse, plovers, and snipe; and if tropical
light and heat have any direct colouring effect, it is certainly
most extraordinary that in groups so varied in form, structure,
and habits as those just mentioned, the tropical should be in no
wise distinguished in this respect, from the temperate species.
It is true that brilliant tropical birds mostly belong to groups
which are wholly tropical--as the chatterers, toucans, trogons,
and pittas; but as there are perhaps an equal number of groups
which are wholly dull-coloured, while others contain dull and
bright-coloured species in nearly equal proportions, the evidence
is by no means strong that tropical light and heat have anything
to do with the matter. But there are other groups in which the
cold and temperate zones produce finer-coloured species than the
tropics. Thus the arctic ducks and divers are handsomer than those
of the tropical zone; while the king-duck of temperate America and
the mandarin-duck of North China are the most beautifully coloured
of the whole family. In the pheasant family we have the gorgeous
gold and silver pheasants in North China and Mongolia; and the
superb Impeyan pheasant in the temperate North-Western Himalayas, as
against the peacock and fire-backed pheasants of tropical Asia. Then
we have the curious fact that most of the bright-coloured birds of
the tropics are denizens of the forests, where they are shaded from
the direct light of the sun, and that they abound near the equator
where cloudy skies are very prevalent; while, on the other hand,
places where light and heat are at a maximum have often dull-coloured
birds. Such are the Sahara and other deserts, where almost all the
living things are sand-coloured; but the most curious case is that
of the Galapagos islands, situated under the equator, and not far
from South America where the most gorgeous colours abound, but
which are yet characterized by prevailing dull and sombre tints in
birds, insects, and flowers, so that they reminded Mr. Darwin of
the cold and barren plains of Patagonia rather than of any tropical
country. Insects are wonderfully brilliant in tropical countries
generally; and any one looking over a collection of South American
or Malayan butterflies would scout the idea of their being no more
gaily-coloured than the average of European species, and in this he
would be undoubtedly right. But on examination we should find that
all the more brilliantly-coloured groups were exclusively tropical,
and that, where a genus has a wide range, there is little difference
in coloration between the species of cold and warm countries.
Thus the European Vanessides, including the beautiful “peacock,”
“Camberwell beauty,” and “red admiral” butterflies, are quite up to
the average of tropical colour in the same group; and the remark
will equally apply to the little “blues” and “coppers;” while the
alpine “apollo” butterflies have a delicate beauty that can hardly
be surpassed. In other insects, which are less directly dependent
on climate and vegetation, we find even greater anomalies. In the
immense family of the Carabidæ or predaceous ground-beetles, the
northern forms fully equal, if they do not surpass, all that the
tropics can produce. Everywhere, too, in hot countries, there are
thousands of obscure species of insects which, if they were all
collected, would not improbably bring down the average of colour
to much about the same level as that of temperate zones.
But it is when we come to the vegetable world that the greatest
misconception on this subject prevails. In abundance and variety
of floral colour the tropics are almost universally believed to
be pre-eminent, not only absolutely, but relatively to the whole
mass of vegetation and the total number of species. Twelve years of
observation among the vegetation of the eastern and western tropics
has, however, convinced me that this notion is entirely erroneous,
and that, in proportion to the whole number of species of plants,
those having gaily-coloured flowers are actually more abundant in
the temperate zones than between the tropics. This will be found to
be not so extravagant an assertion as it may at first appear, if we
consider how many of the choicest adornments of our greenhouses and
flower-shows are really temperate as opposed to tropical plants.
The masses of colour produced by our Rhododendrons, Azaleas, and
Camellias, our Pelargoniums, Calceolarias, and Cinerarias,--all
strictly temperate plants--can certainly not be surpassed, if they
can be equalled, by any productions of the tropics.