[BA] _Origin of Species_, p. 577.
[BB] _The Origin of Species_, xiii. 4th ed.
[BC] _The Origin of Species_, p. 1.
[BD] _Malay Archipelago_, 1869.
[BE] The passage is quoted as given by Schmidt in his _Doctrine of
Descent_, p. 132. Italics are inserted to guide the eye of the reader to
the successive stages.
[BF] Referring to organized being.
[BG] _Origin of Species_, p. 577.
[BH] _Doct. of Descent_, p. 161-2.
[BI] _Origin of Species_, 4th ed. p. 143.
[BJ] _Doct. of Descent_, 162.
[BK] _Origin of Species_, 4th ed. p. 141.
[BL] It may be well to mention here that the _third_ American edition is
from the _fifth_ English edition.
[BM] _Origin of Species_, 6th ed., p. 289.
[BN] _Origin of Species_, 6th. ed. chap. iv. p. 100.
[BO] _Contributions to the Natural History of the United States_,
Introduction, Boston, 1857; and _Essay on Classification_, p. 15,
published separately; London, 1859. See Appendix VI.
[BP] _Origin_, 4th ed. p. 143.
[BQ] _Origin of Species_, 6th. ed. chap. iv. p. 100.
LECTURE V.
RELATIONS OF LOWER AND HIGHER ORGANISMS.
From the general aspects of the theory of species, we pass to the
consideration of distinct groups of organism, with the view of
ascertaining their relations to each other. In doing so, it is better to
begin at the lower end of the scale, leaving for a more advanced stage
of inquiry the higher types of organism. In this department of the
subject, special obligations are due to the wide range of investigations
either occasioned or stimulated by the theory of evolution. For,
whatever may be the ultimate award passed on this theory, there will be
a unanimous recognition of the great value to science attending on the
varied forms of inquiry stimulated by the writings of Mr. Charles
Darwin. And one obvious and strong reason for such acknowledgment is
that so many of the results of these researches have an inherent value
quite distinct from their testimony in favor of the theory that the
struggle for existence is the principal factor in the origin of new
species.
One of the most interesting fields of observation thus opened, is that
concerned with the fertilization of plants by the intervention of
insects and birds. A beginning in this department was made by the German
naturalist, Christian Konrad Sprengel, who published in 1793 the report
of his observations. In this he has been followed by Darwin, in 1862; by
Dr. Hooker, Professor Asa Gray whose contributions appeared in the
_American Journal of Science and Art_ in 1862, and 1863, Moggridge,
Fritz Müller, and Sir John Lubbock. The facts now accumulated, rank as
an important contribution to botany and zoölogy, and naturally fall
within the circle of recent advances to which it is desirable that
attention be turned.
The general result is one of great interest, as illustrating a striking
degree of interdependence between lower and higher organisms,--the
vegetable and animal kingdom contributing to each other's subsistence
and propagation. Flowers present special attractions to insects flying
around, alluring them by varied colors, and providing for them by
secreting stores of honey; on the other hand, these insects (flies,
bees, wasps, etc.), seeking the honey which satisfies their wants, at
the same time carry the pollen from one flower to another, thus
providing for the fertilizing of the plants. In some cases,
fertilization is secured by a natural process within the organism
itself; in other cases, the pollen is scattered over a region by the
wind; but the most wonderful, and at the same time efficient mode of
providing for the growth of vigorous plants, is fertilization by the
agency of insect life.
A brief outline of the ordinary structure of the flower will introduce
to a ready appreciation of the scientific interest attaching to this
last mode of fertilization, both as concerning the functions of
different portions of the flowers, and the relation of dependence
established between higher and lower forms of organism, so that each is
dependent on the other.
Every flower as it unfolds from the bud, consists of a series of whorls,
or layers of substance twined or twirled round in such a manner as to
unfold or coil back, as the flower opens. The _outermost_ of these
whorls (_calyx_) is a mere covering or sheath, usually of a green
color, which protects the bud during the more tender period, curling up
and withering as the flower opens, spreading forth its beauty. The
_second_ whorl (_corolla_) is what we more commonly regard as the flower
proper, the colored leaves, or cup, or bell, according to the specific
shape distinguishing the plant. The _third_ whorl consists of a series
of stalks or filaments (_stamens_) which as the flower matures or ripens
stand up distinct from each other, each one having at its summit a
little tuft or cushion (_anther_) covered with a fine dust or powder
(_pollen_). The _fourth_ or inner-most whorl, the centre piece of the
flower (_pistil_) is that in which the seed is generated and brought to
maturity. We may thus say of the flower, that its outermost whorl is a
temporary covering which withers and shrinks out of view, when the
beauty of the inner structure is laid open; that the second is that
which attracts the eye by the loveliness of its hues; while the two
which belong to the internal structure of the flower are concerned with
the reproduction or propagation of the plant, providing for the healthy
germ from which a fresh plant of the same order may spring up. The
relation of the fine yellow powder produced at the tips of the third
whorl, to the seeds which are gathered together within the fourth whorl,
is the matter to which special attention has been directed by the recent
discoveries which have rewarded patient research. The fine powder or
pollen needs to be carried to the seed, so that its properties may
operate upon that seed, if it is to be fertilized, or so matured, as to
fulfil its function in generating a new plant when it is committed to
the soil. In many cases it is enough that the fine powder should fall
down from its elevation on the seeds below. This is self-fertilization,
and is easily provided for by the mere bending of the head of the flower
as it approaches maturity, or by the swaying of it in the breeze. But a
more difficult, and as we might be inclined to add, more precarious,
because less certain, method for fertilization is required in many
cases. The experiments carried on by all our gardeners, and in a still
more extended scale in all our centres of botanic research, have
established the fact that in many cases, the yellow powder of one plant
must be in some manner carried over to the seed produced within another
flower, if that seed is to yield a satisfactory result to the
horticulturist.
We have thus two prominent facts here. The one is _the essential
importance of the pollen_ for fertilization; and the other, _the need
for the transference of the pollen_ from one plant to another in order
to secure reproduction of vigorous growth by the sowing of the seed. As
to the first, the pollen, which appears a fine powder or flour contains
fluid protoplasm, that which Professor Huxley has described as the
"single physical basis of life under all the diversities of vital
existence."[BR] These pollen grains falling on the seed discharge their
protoplasmic fluid upon it, and by this means contribute to
fertilization. This original or primordial form of vitalising agency is
carried from one part of the flower to another, and this transference is
the law regulating the propagation of flowering plants.
But, just at this point, we come upon the most striking results of
recent research. Though all pollen is of this primary nature, named
protoplasm, it is not found to hold true that pollen is of the same
value for fertilization from whatever quarter it comes. On the
contrary, most important differences result according to the source of
the pollen. There is first the process of self-fertilization. But in
many cases,--Mr. Darwin has shown that this holds of the majority of the
orchids,--transference of the pollen from one plant to another proves to
be a great advantage, if not an actual necessity for propagation of the
plant. This process, known as cross-fertilization, gives a healthy and
vigorous growth; want of it, will lead to degeneration, and ultimate
extinction. This discovery has introduced a whole series of the most
striking observations, throwing a flood of light on the distribution and
interdependence of distinct forms of organism. The necessary relation
between the pollen and the seed having been acknowledged, and next the
value of transference of pollen from one plant to another, the first
step in the line of discovery was made by the observation of a natural
provision to _prevent self-fertilization_ by rendering it impossible
that the pollen of a plant should fall on the seed of that plant. This
entrance on the line of discovery was made by Sprengel so far back as
1790, by whom it was observed that in many plants the pollen and the
seed did not come to ripeness at the same time.[BS] In some cases, the
pollen is ripened before the seeds are ready; in other cases, the order
of events is reversed. This observation naturally suggested transference
of pollen from one plant to another; and this, connected with the
continual coming and going of flies, bees, and butterflies, led to the
further discovery, that _insects_ unwittingly perform a large part of
the work needful in order to _provide for fertilization_. Mr. Darwin has
pointed out that from the paper of Robert Brown in the _Linnean
Transactions_, in 1833; and from that of Dr. Hooker in the _Philosophic
Transactions_ for 1854, the peculiar phenomena had begun to awaken
scientific interest. It was, however, when the researches as to origin
of species had given fresh motive to observation concerning the
relations of different types of organism, that the whole facts were
brought to view, separately recorded, and at length systematized so as
to lead to their full interpretation. Mr. Darwin himself concentrated on
the orchids as peculiarly interesting and suggestive, while a host of
workers all over the world were turning their energies into this new
field of observation which promised ample return for patient research.
As a reward of these investigations important facts have been
established on ample evidence. First, it has been confirmed by varying
lines of evidence that transference of pollen, or cross-fertilization,
is of special value in the development of plant life. Investigation has
strengthened the evidence of disadvantage arising from fertilization by
exclusive dependence on self-produced pollen. Fritz Müller has recorded
a variety of observations that the pollen of some flowers has so little
influence on the seed produced on the same stem that when it falls upon
the seeds no effect is produced; the pollen lies there as if possessed
of no more vitalizing power than grains of dust. And, what is even more
surprising, Müller has found examples in which the pollen does act upon
the seeds of its own flower, but acts _injuriously_, insomuch that the
flower, the pollen, and the seed-producing portion of the plant begin to
decay.[BT] So deep has been the impression made on Mr. Darwin's mind by
the evidence of provision for transference of pollen, that he closes his
valuable and interesting book on the _Fertilization of Orchids_ with the
following statements. Having remarked that "self-fertilization would
have been an incomparably safer and easier process than the transportal
of pollen from flower to flower," he adds these words,--"It is hardly an
exaggeration to say that Nature tells us, in the most emphatic manner,
that she abhors perpetual self-fertilization."[BU]
The next result secured presents an important relation between animal
life and vegetable. These flowers do not depend for their fertilization
upon the action of the wind, which in scattering profusely in all
directions must occasion large waste of pollen. There is found to be
distinct provision for carrying the pollen from one flower to another by
insects, such as the bees, whose industry in gathering honey has been
celebrated from ancient times, specially because of our interest in the
storehouse, but with little suspicion of the double work being done by
the bees, who add to their other industry that of horticulturists. These
bees are the pollen-bearers,--the recognized local carriers, regularly
on the road,--doing the work which the flowers, in lack of locomotive
power, could not do for each other. Or, looking at the relation of
things from another point of view, the bees are at the same time
gathering the honey, and sowing the seed for a future harvest. This
reference to the honey, however, introduces to notice a companion series
of facts, showing the provision in completed form for an interchange of
services. The plants supply an attraction to the animals, while the
animals render a service to the plants. This phase of interdependence is
made more conspicuous by the contrast apparent in the structure and
functions of plants fertilized by the wind, such as the larger shrubs
and trees, which as they present a greater surface to the breeze, do not
call for the same detailed provision for carrying the pollen. In
contrast with these more bulky representatives of the vegetable kingdom,
the more lowly and insignificant in size, as well as more short-lived,
present many attractions in color, scent, and secretion of honey, all
adapted to the nature of insects, suited for the work of pollen
carrying. The attractions of form, color, and scent in the flowers are
well known to us; but they are also appreciated by the insects,--a fact
which may possibly suggest that a high degree of intellectual power is
not required for appreciation of these qualities, as no one professes
that bees rank high intellectually. At the same time, if comparisons are
to be made at this point, the farther suggestion may also be introduced,
that there is little testimony to intelligence where search for food is
concerned, and while the human race do not feed on flowers, insects are
constantly feeding from them. The attractions in the two cases therefore
vary considerably in their significance. Restricting attention, however,
to the special field of observation now before us, with the view simply
of ascertaining the relations of plants and insects, color and honey
present the two most prominent attractions accounting for the perpetual
hum of life heard amongst the flowering plants. Different parts of the
flower provide for variegated coloring, and stores of honey; these
present attractions to the insects; and the structure of the flowers as
they provide for the landing of the insects, and require that they
penetrate to their centre for the secreted honey, secures that the work
needful for fertilization be effectually done. This last feature of
adaptation is that on which attention may be specially concentrated
here. Mr. Darwin in treating of orchids has described this part of their
structure in these words;--"In almost all the species, one of the petals
(or leaves of the flower) which is properly the upper one, is larger
than the others and stands on the lower side of the flower, where it
offers a landing-place for insects."[BV] Towards the inner or root end
of this leaf (_labellum_) is the gland, in some flowers appearing only
as a slit, in others forming like a tube, (_nectary_) which secrets the
honey. Just over the entrance to the part where the honey is to be found
stands that which secrets the pollen prepared for fertilizing some other
flower. So soon as the bee or other insect presses its head well into
the centre of the flower, some of the pollen adheres to it; when the
head is withdrawn, this pollen is borne off to the flower which the bee
next visits; and as the head is pressed into the core of this flower
the pollen is deposited, and provision for fertilization is complete.
Special features appearing in certain classes of the orchids illustrate
how it is possible for the bee so laden to visit many flowers without
depositing the pollen, yet a little later accomplish the object quite
simply. The following illustration from Mr. Darwin's account of the
first orchid selected, will suffice. Just _above_ the entrance to the
honey store, lies a pouch connected with the pollen store. As the head
of the bee is pressed down towards the honey, this pouch is burst open,
and from it issues a little sticky gland or disc, or it may be two of
these discs. These adhere at once to the head of the bee, and being
connected by a slight band with packets of pollen grains so soon as the
animal retires the pollen is drawn with it, standing out like a seed
vessel on the head. The strangest part of the contrivance appears in
what thereafter follows. "The viscid matter has the peculiar chemical
quality, of setting like cement, hard and dry in a few minutes." Suppose
both the little viscid balls have been withdrawn, the bands bearing the
pollen will appear "projecting up like horns." "How then can the flower
be fertilized? This is effected by a beautiful contrivance; though the
viscid surface remains immovably affixed, the apparently insignificant
and minute disc of membrane to which the caudicle adheres is endowed
with a remarkable power of contraction, which causes the pollinium to
sweep through an angle of about ninety degrees, always in one direction
towards the apex of the proboscis, in the course of thirty seconds on an
average."[BW] That is, the two erections bearing packets of pollen which
formerly stood up almost perpendicular, like horns, begin to lower until
they reach the horizontal; in this way when the bee enters a flower the
packets of pollen inevitably touch the seed stores, communicating what
is required for their fertilization. Nor have we even yet the whole of
the contrivances adapted for this end. "Here comes into play another
pretty adaptation." The seed vessel to be fertilized is very sticky,
"but not so viscid as when touched by a pollinium to pull the whole off
an insect's head." But it is sufficiently adhesive "to break the elastic
threads by which the packets of pollen grains are tied together." In
this way, it tears off so much from the store adhering to the head of
the bee, and still leaves there what may supply the requirements of many
flowers besides. The description thus given will suffice to indicate how
close is the relation of the lower orders of animal life with vegetable
life, and will illustrate how the lower organism may be dependent for
existence on the higher, an illustration in some respect the converse of
the facts illustrating origin of species by development.
There remains in this department of inquiry only one additional set of
facts, to which reference may be made, as illustrating _distribution of
work among insects and birds_, assigning them to different orders of
plants. This will illustrate contrivance on a still wider scale,
discovering distinct sets of affinity, which imply common localization
for given plants and animals. In this it appears that flies, humble
bees, and birds with long slender bills, such as the humming birds, all
have a share in the work required for fertilizing plants.
There is one example, _epipactis latifolia_, with a cup-shaped labellum,
in which honey is secreted, and which bees are never seen to frequent.
What, however, the bees pass, the wasps suck eagerly, and by them it is
fertilized. Of this flower, Mr. Darwin says,--"It is very remarkable
that the sweet nectar of this _epipactis_ should not be attractive to
any kind of bee. If wasps were to become extinct in any district, so
probably would the _Epipactis Latifolia_."[BX]
Another example there is of an orchid _(Spiranthes Autumnalis_),
commonly known as _Ladies' Tresses_, having a series of spikes, of which
the lowest flowers are first matured, the others following in order as
they rise towards the summit. This plant is frequented by bees, whose
practice it is to begin with the lowest flowers and ascend gradually to
the top. This order in seeking to extract the honey, proves to be the
proper one for fertilizing of the plant, because the pollen which the
bee brings will be received by the riper flower on which it lands, and
when that has been deposited, fresh pollen will adhere to the bee as it
rises to the less matured flowers, and thus it departs laden with pollen
destined for the lowest flowers of the next plant it visits.[BY]
[Illustration: 1 SECTION OF ORCHID FLOWER SHOWING POLLEN VESSEL, SEED
STORE AND HONEY STORE 2 BEE ENTERING 3 POLLEN VESSEL 4 BEE RETREATING 5
BEE PREPARED FOR ENTERING ANOTHER FLOWER]
These examples introduce us to a general plan for fertilization of
plants by the intervention of insects, so complete in the order of
distribution that we may classify the plants according to the insect by
which they are fertilized, making it natural to speak of fly orchids,
spider orchids, wasp orchids, and bee orchids. In all cases the search
for honey determines the visits made, leaving still unexplained,
however, the fact that the nectar of some plants is shunned by certain
insects, and eagerly absorbed by others. With the general source of
attraction in the flowering plants, there are diversities of arrangement
among the insects, according to the comparative size of the flower, and
strength required in order to penetrate to the inner chamber where the
honey is stored. Because of the minuteness of the aperture, there are
flowers from which the bee can not draw supplies; on the other hand,
because of the size and strength of the flower, there are cases in which
the ordinary bee is incapable of reaching the store, and the stronger
humble bee alone succeeds in effecting an entrance.