very curious physiological facts bearing upon the presence or
absence of white colours in the higher animals have lately been
adduced by Dr. Ogle.[51] It has been found that a coloured or dark
pigment in the olfactory region of the nostrils is essential to
perfect smell, and this pigment is rarely deficient except when
the whole animal is pure white. In these cases the creature is
almost without smell or taste. This, Dr. Ogle believes, explains
the curious case of the pigs in Virginia adduced by Mr. Darwin,
white pigs being killed by a poisonous root which does not affect
black pigs. Mr. Darwin imputed this to a constitutional difference
accompanying the dark colour, which rendered what was poisonous
to the white-coloured animals quite innocuous to the black. Dr.
Ogle, however, observes that there is no proof that the black
pigs eat the root, and he believes the more probable explanation
to be that it is distasteful to them; while the white pigs, being
deficient in smell and taste, eat it and are killed. Analogous
facts occur in several distinct families. White sheep are killed
in the Tarentino by eating _Hypericum crispum_, while black sheep
escape; white rhinoceroses are said to perish from eating _Euphorbia
candelabrum_; and white horses are said to suffer from poisonous
food where coloured ones escape. Now it is very improbable that a
constitutional immunity from poisoning by so many distinct plants
should, in the case of such widely different animals, be always
correlated with the same difference of colour; but the facts are
readily understood if the senses of smell and taste are dependent
on the presence of a pigment which is deficient in wholly white
animals. The explanation has, however, been carried a step further,
by experiments showing that the absorption of odours by dead matter,
such as clothing, is greatly affected by colour; black being the
most powerful absorbent; then blue, red, yellow, and lastly white.
We have here a physical cause for the sense-inferiority of totally
white animals which may account for their rarity in nature; for
few, if any, wild animals are wholly white. The head, the face,
or at least the muzzle or the nose, are generally black; the ears
and eyes are also often black; and there is reason to believe that
dark pigment is essential to good hearing, as it certainly is to
perfect vision. We can therefore understand why white cats with
blue eyes are so often deaf, a peculiarity we notice more readily
than their deficiency of smell or taste.
[51] “Medico-Chirurgical Transactions,” vol. liii. (1870).
If, then, the prevalence of white coloration is generally associated
with some deficiency in the acuteness of the most important senses,
this colour becomes doubly dangerous; for it not only renders its
possessor more conspicuous to its enemies, but at the same time
makes it less ready in detecting the presence of danger. Hence,
perhaps, the reason why white appears more frequently in islands,
where competition is less severe and enemies less numerous and
varied. Hence, also, a reason why _albinoism_, although freely
occurring in captivity, never maintains itself in a wild state,
while _melanism_ does. The peculiarity of some islands in having
all their inhabitants of dusky colours (as the Galapagos) may
also perhaps be explained on the same principles; for poisonous
fruits may there abound which weed out all white- or light-coloured
varieties, owing to their deficiency of smell and taste. We can
hardly believe, however, that this would apply to white-coloured
butterflies; and this may be a reason why the effect of an insular
habitat is more marked in these insects than in birds or mammals.
It is even possible that this relation of sense-acuteness with
colour may have had some influence on the development of the higher
human races. If light tints of the skin were generally accompanied
by some deficiency in the senses of smell, hearing, and vision,
the white could never compete with the darker races so long as
man was in a very low or savage condition, and wholly dependent
for existence on the acuteness of his senses. But as the mental
faculties became more fully developed and more important to his
welfare than mere sense-acuteness, the lighter tints of skin and
hair and eyes would cease to be disadvantageous whenever they were
accompanied by superior brain-power. Such variations would then be
preserved; and thus may have arisen the Xanthochroic race of mankind,
in which we find a high development of intellect accompanied by a
slight deficiency in the acuteness of the senses as compared with
the darker forms.
_Relations of Insular Plants and Insects._--I have now to ask your
attention to a few remarks on the peculiar relations of plants and
insects as exhibited in islands.
Ever since Mr. Darwin showed the immense importance of insects in
the fertilization of flowers, great attention has been paid to
the subject, and the relation of these two very different classes
of natural objects has been found to be more universal and more
complex than could have been anticipated. Whole genera and families
of plants have been so modified as, first to attract and then to be
fertilized by, certain groups of insects; and this special adaptation
seems in many cases to have determined the more or less wide range
of the plants in question. It is also known that some species of
plants can be fertilized only by particular species of insects; and
the absence of these from any locality would necessarily prevent
the continued existence of the plant in that area.
In this direction, I believe, will be found the clue to much of the
peculiarity of the floras of oceanic islands; since the methods by
which these have been stocked with plants and with insects will be
often quite different. Many seeds are, no doubt, carried by oceanic
currents, others probably by aquatic birds. Mr. H. N. Moseley informs
me that the albatrosses, gulls, puffins, tropic birds and many
others, nest inland, often amidst dense vegetation; and he believes
they often carry seeds, attached to their feathers, from island to
island for great distances. In the tropics they often nest on the
mountains far inland, and may thus aid in the distribution even of
mountain-plants. Insects, on the other hand, are mostly conveyed by
aerial currents, especially by violent gales; and it may thus often
happen that totally unrelated plants and insects may be brought
together, in which case the former must often perish for want of
suitable insects to fertilize them. This will, I think, account for
the strangely fragmentary nature of these insular floras, and the
great differences that often exist between those which are situated
in the same ocean; as well as for the preponderance of certain
orders and genera.
In Mr. Pickering’s valuable work on the “Geographical Distribution
of Animals and Plants” (founded on his researches during the
United States exploring expedition), he gives a list of no less
than sixty-six natural orders of plants _unexpectedly_ absent from
Tahiti, or which occur in many of the surrounding lands; some being
abundant in other islands--as the Labiatæ at the Sandwich Islands.
In these latter islands the flora is much richer, yet a large number
of families which abound in other parts of Polynesia are totally
wanting. Now much of the poverty and exceptional distribution of
the plants of these islands is probably due to the great scarcity
of flower-frequenting insects. Lepidoptera and Hymenoptera are
exceedingly scarce in the eastern islands of the Pacific, and it
is almost certain that many plants which require these insects for
their fertilization have been thereby prevented from establishing
themselves. In the western islands, such as the Fijis, several
species of butterflies occur in tolerable abundance, and no doubt
some flower-haunting Hymenoptera accompany them; and in these
islands the flora appears to be much more varied, and especially to
be characterized by a much greater variety of showy flowers, as may
be seen by examining the plates of Dr. Seeman’s “Flora Vitiensis.”
Darwin and Pickering both speak of the great preponderance of ferns
at Tahiti; and Mr. Moseley, who spent several days in the interior
of the island, informs me that “at an elevation of from 2,000 to
3,000 feet the dense vegetation is composed almost entirely of
ferns. A tree fern (_Alsophila tahitensis_) forms a sort of forest
to the exclusion of almost every other tree, and, with huge plants
of two other ferns (_Angiopteris evecta_ and _Aspelenium nidus_),
forms the main mass of the vegetation.” And he adds, “I have nowhere
seen ferns in so great proportionate abundance.” This unusual
proportion of ferns is a general feature of insular as compared
with continental floras; but it has, I believe, been generally
attributed to favourable conditions, especially to equable climate
and perennial moisture. In this respect, however, Tahiti can hardly
differ greatly from many other islands, which yet have no such vast
preponderance of ferns. This is a question that cannot be decided by
mere lists of species, since it is probable that in Tahiti they are
less numerous than in some other islands where they form a far less
conspicuous feature in the vegetation. The island most comparable
with Tahiti in this respect is Juan Fernandez. Mr. Moseley writes
to me:--“In a general view of any wide stretch of the densely
clothed mountainous surface of the island, the ferns, both tree
ferns and the unstemmed forms, are seen at once to compose a very
large proportion of the mass of foliage.” As to the insects of Juan
Fernandez, Mr. Edwyn C. Reed, who made two visits and spent several
weeks there, has kindly furnished me with some exact information. Of
butterflies there is only one (_Pyrameis carie_), and that rare--a
Chilian species and probably an accidental straggler. Four species
of moths of moderate size were observed (all Chilian), and a few
larvæ and pupæ. Of bees there were none, except one very minute
species (allied to Chilicola), and of other Hymenoptera a single
specimen of _Ophion luteus_ a cosmopolitan ichneumon. About twenty
species of flies were observed, and these formed the most prominent
feature of the entomology of the island.
Now, as far as we know, this extreme entomological poverty agrees
closely with that of Tahiti; and there are probably no other
portions of the globe equally favoured in soil and climate, and
with an equally luxuriant vegetation, where insect-life is so
scantily developed. It is curious, therefore, to find that these
two islands also agree in the wonderful predominance of ferns over
the flowering plants--in individuals even more than in species; and
there is no difficulty in connecting the two facts. The excessive
minuteness and great abundance of fern-spores causes them to be
far more easily distributed by winds than the seeds of flowering
plants; and they are thus always ready to occupy any vacant places in
suitable localities, and to compete with the less vigorous flowering
plants. But where insects are so scarce, all plants which require
insect-fertilization, whether constantly to enable them to produce
seed at all or occasionally to keep up their constitutional vigour
by crossing, must be at a great disadvantage; and thus the scanty
flora which oceanic islands must always possess, peopled as they
usually are by waifs and strays from other lands, is rendered still
more scanty by the weeding out of all such as depend largely on
insect-fertilization for their full development. It seems probable,
therefore, that the preponderance of ferns in islands (considered
in mass of individuals rather than in number of species) is largely
due to the absence of competing phænogamous plants, and that this
is in great part due to the scarcity of insects. In other oceanic
islands, such as New Zealand and the Galapagos, where ferns,
although tolerably abundant, form no such predominant feature in
the vegetation, but where the scarcity of flower-haunting insects
is almost equally marked, we find a great preponderance of small,
green, or otherwise inconspicuous flowers, indicating that only
such plants have been enabled to flourish there as are independent
of insect-fertilization. In the Galapagos (which are perhaps even
more deficient in flying insects than Juan Fernandez) this is so
striking a feature that Mr. Darwin speaks of the vegetation as
consisting in great part of “wretched-looking weeds,” and states
that “it was some time before he discovered that almost every plant
was in flower at the time of his visit.” He also says that he “did
not see one beautiful flower” in the islands. It appears, however,
that Compositæ, Leguminosæ, Rubiaceæ, and Solanaceæ form a large
proportion of the flowering plants; and as these are orders which
usually require insect-fertilization, we must suppose, either that
they have become modified so as to be self-fertilized, or that they
are fertilized by the visits of the minute Diptera and Hymenoptera
which are the only insects recorded from these islands.
In Juan Fernandez, on the other hand, there is no such total
deficiency of showy flowers. I am informed by Mr. Moseley that a
variety of the Magnoliaceous winter-bark abounds and has showy white
flowers, and that a Bignoniaceous shrub with abundance of dark blue
flowers was also plentiful; while a white-flowered Liliaceous plant
formed large patches on the hill-sides. Besides these, there were
two species of woody Compositæ with conspicuous heads of yellow
blossoms, and a species of white-flowered myrtle also abundant;
so that, on the whole, flowers formed a rather conspicuous feature
in the aspect of the vegetation of Juan Fernandez.
But this fact--which at first sight seems entirely at variance
with the view we are upholding of the important relation between
the distribution of insects and plants--is well explained by the
existence of two species of humming-birds in Juan Fernandez, which,
in their visits to these large and showy flowers, fertilize them
as effectually as bees, moths, or butterflies. Mr. Moseley informs
me that “these humming-birds are _extraordinarily abundant_, every
tree or bush having one or two darting about it.” He also observed
that “nearly all the specimens killed had the feathers round the
base of the bill and front of the head clogged and coloured yellow
with pollen.” Here, then, we have the clue to the perpetuation
of large and showy flowers in Juan Fernandez; while the total
absence of humming-birds in the Galapagos may explain why no such
large-flowered plants have been able to establish themselves in
those equatorial islands.
This leads to the observation that many other groups of birds
also, no doubt, aid in the fertilization of flowers. I have often
observed the beaks and faces of the brush-tongued lories of the
Moluccas covered with pollen; and Mr. Moseley noted the same fact
in a species of Artamus, or swallow-shrike, shot at Cape York,
showing that this genus also frequents flowers and aids in their
fertilization. In the Australian region we have the immense group
of the Meliphagidæ, which all frequent flowers; and as these range
over all the islands of the Pacific, their presence will account
for a certain proportion of showy flowers being found there, such
as the scarlet Metrosideros, one of the few conspicuous flowers
in Tahiti. In the Sandwich Islands, too, there are forests of
Metrosideros; and Mr. Charles Pickering writes me, that they are
visited by honey-sucking birds, one of which is captured by sweetened
bird-lime, against which it thrusts its extensile tongue. I am also
informed that a considerable number of flowers are occasionally
fertilized by humming-birds in North America; so that there can, I
think, be little doubt that birds play a much more important part in
this respect than has hitherto been imagined. It is not improbable
that in Tropical America, where the humming-bird family is so
enormously developed, many flowers will be found to be expressly
adapted to fertilization by them, just as so many in our own country
are specially adapted to the visits of certain families or genera
of insects.[52]
[52] The probable influence of fertilization by birds on the
flowers of the Auckland Isles has been referred to at p. 238. Mr.
Darwin, in his book on _Cross and Self-Fertilisation of Plants_ (p.
371), gives in a note numerous cases in which birds are known to
fertilise flowers, the most important being that of several species
of _Abutilon_ in South Brazil, which, according to Fritz Müller,
are sterile unless fertilised by humming-birds. This proves, not
only that birds fertilise flowers in the same manner as insects,
but that the two classes of organisms have become so correlated as
to be mutually necessary to each other; and it completely justifies
us in imputing the fertilization of flowers to flower-frequenting
birds wherever these are present and suitable insects are notoriously
scarce, as is the case in so many of the islands here referred to.
It must also be remembered, as Mr. Moseley has suggested to me, that
a flower which has acquired a brilliant colour to attract insects
might, on transference to another country and becoming so modified
as to be capable of self-fertilization, retain the coloured petals
for an indefinite period. Such is probably the explanation of the
Pelargonium of Tristan d’Acunha, which forms masses of bright colour
near the shore during the flowering season; while most of the other
plants of the island have colourless flowers in accordance with the
almost total absence of winged insects. The presence of many large
and showy flowers among the indigenous flora of St. Helena must be
an example of a similar persistence. Mr. Melliss indeed states it
to be “a remarkable peculiarity that the indigenous flowers are,
with very slight exceptions, all perfectly colourless;”[53] but
although this may apply to the general aspect of the remains of
the indigenous flora, it is evidently not the case as regards the
_species_, since the interesting plates of Mr. Melliss’s volume
show that about one third of the indigenous flowering plants have
more or less coloured or conspicuous flowers, while several of them
are exceedingly showy and beautiful. Among these are a Lobelia,
three Wahlenbergias, several Compositæ, and especially the handsome
red flowers of the now almost extinct forest-trees, the ebony and
redwood (species of Melhania, Byttneriaceæ). We have every reason to
believe, however, that when St. Helena was covered with luxuriant
forests, and especially at that remote period when it was much
more extensive than it is now, it must have supported a certain
number of indigenous birds and insects, which would have aided in
the fertilization of these gaily-coloured flowers. The researches
of Dr. Hermann Müller have shown us by what minute modifications
of structure or of function, many flowers are adapted for partial
insect and self-fertilization in various degrees; so that we have
no difficulty in understanding how, as the insects diminished and
finally disappeared, self-fertilization may have become the rule,
while the large and showy corollas remain to tell us plainly of a
once different state of things.
[53] Melliss’s _St. Helena_, p. 226, note.
Another interesting fact in connexion with this subject is the
presence of arborescent forms of Compositæ in so many of the remotest
oceanic islands. They occur in the Galapagos, in Juan Fernandez,
in St. Helena, in the Sandwich Islands, and in New Zealand; but
they are not directly related to each other; representatives
of totally different tribes of this extensive order becoming
arborescent in each group of islands. The immense range and almost
universal distribution of the Compositæ is due to the combination
of a great facility of distribution (by their seeds) with a great
attractiveness to insects; and to the capacity of being fertilized
by a variety of species of all orders, and especially by flies and
small beetles. Thus they would be among the earliest of flowering
plants to establish themselves on oceanic islands; but where insects
of all kinds were very scarce, it would be an advantage to gain
increased size and longevity, so that fertilization at an interval
of several years might suffice for the continuance of the species.
The arborescent form would combine with increased longevity the
advantage of increased size in the struggle for existence with
ferns and other early colonists; and these advantages have led to
its being independently produced in so many distant localities,
whose chief feature in common is their remoteness from continents
and the extreme poverty of their insect life.
As the sweet odours of flowers are known to act in combination
with their colours, as an attraction to insects, it might be
anticipated that where colour was deficient scent would be so also.
On applying to my friend Sir Joseph Hooker for information as to
the odoriferous qualities of New-Zealand plants, he informed me,
that the New-Zealand flora is, speaking generally, as strikingly
deficient in sweet odours as it is in conspicuous colours. Whether
this peculiarity occurs in other islands I have not been able to
obtain information; but we may certainly expect to find it where
colour is so strikingly deficient as in the flora of the Galapagos
Islands.
Another question which here comes before us, is the origin and
meaning of the odoriferous glands of leaves. Sir Joseph Hooker
informed me that not only are New-Zealand plants deficient in
bright-coloured and sweet-smelling flowers, but equally so in scented
leaves. This led me to think that perhaps such leaves were in some
way an additional attraction to insects--though it is not easy to
understand how this could be, except by adding a general attraction
to the special attraction of the flowers, or by supporting the
larvæ which, as perfect insects, aid in fertilization. Mr. Darwin,
however, informs me that he considers that leaf-glands bearing
essential oils are a protection against the attacks of insects
where these abound, and would thus not be required in countries
where insects were very scarce. But it seems opposed to this view
that highly aromatic plants are characteristic of deserts all
over the world, and in such places insects are not abundant. Mr.
Stainton informs me that the aromatic Labiatæ enjoy no immunity
from insect attacks. The bitter leaves of the cherry-laurel are
often eaten by the larvæ of moths that abound on our fruit-trees;
while in the Tropics the leaves of the orange tribe are favourites
with a large number of lepidopterous larvæ; and our northern firs
and pines, although abounding in a highly aromatic resin, are very
subject to the attacks of beetles. My friend Dr. Richard Spruce--who
while travelling in South America allowed nothing connected with
plant-life to escape his observation--informs me that trees whose
leaves have aromatic and often resinous secretions in immersed
glands abound in the plains of tropical America, and that such are
in great part, if not wholly, free from the attacks of leaf-eating
ants, except where the secretion is only slightly bitter, as in
the orange tribe, orange-trees being sometimes entirely denuded
of their leaves in a single night. Aromatic plants abound in the
Andes up to about 13,000 feet, as well as in the plains, but hardly
more so than in Central and Southern Europe. They are perhaps more
plentiful in the dry mountainous parts of Southern Europe; and
as neither here nor in the Andes do leaf-eating ants exist, Dr.
Spruce infers that, although in the hot American forests where such
ants swarm the oil-bearing glands serve as a protection, yet they
were not originally acquired for that purpose. Near the limits of
perpetual snow on the Andes such plants as occur are not, so far as
Dr. Spruce has observed, aromatic; and as plants in such situations
can hardly depend on insect visits for their fertilization, the
fact is comparable with that of the flora of New Zealand, and would
seem to imply some relation between the two phenomena, though what
it exactly is cannot yet be determined.