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as in adapting the general shape of the body of these fishes, and many
other peculiarities, to their habits of life. We should keep in mind, as I
have before insisted, that the inherited effects of the increased use of parts,
and perhaps of their disuse, will be strengthened by natural selection. For
all spontaneous variations in the right direction will thus be preserved;
as will those individuals which inherit in the highest degree the effects
of the increased and beneficial use of any part. How much to attribute
in each particular case to the effects of use, and how much to natural
selection, it seems impossible to decide.

I may give another instance of a structure which apparently owes its
origin exclusively to use or habit. The extremity of the tail in some American
monkeys has been converted into a wonderfully perfect prehensile organ,
and serves as a fifth hand. A reviewer, who agrees with Mr. Mivart in
every detail, remarks on this structure: "It is impossible to believe that
in any number of ages the first slight incipient tendency to grasp could
preserve the lives of the individuals possessing it, or favor their chance
of having and of rearing offspring." But there is no necessity for any such
belief. Habit, and this almost implies that some benefit great or small is
thus derived, would in all probability suffice for the work. Brehm saw the
young of an African monkey (Cercopithecus) clinging to the under surface
of their mother by their hands, and at the same time they hooked their
little tails round that of their mother. Professor Henslow kept in confine-
ment some harvest mice (Mus messorius) which do not possess a structur-
ally prehensile tail; but he frequently observed that they curled their tails
round the branches of a bush placed in the cage, and thus aided them-
selves in climbing. I have received an analogous account from Dr. Giinther,
who has seen a mouse thus suspend itself. If the harvest mouse had been more
strictly arboreal, it would perhaps have had its tail rendered structurally
prehensile, as is the case with some members of the same order. Why
Cercopithecus, considering its habits while young, has not become thus
provided, it would be difficult to say. It is, however, possible that the long
tail of this monkey may be of more service to it as a balancing organ in
making its prodigious leaps, than as a prehensile organ.

The mammary glands are common to the whole class of mammals,
and are indispensable for their existence; they must, therefore, have been
developed at an extremely remote period, and we can know nothing posi-
tively about their manner of development. Mr. Mivart asks: "Is it con-
ceivable that the young of any animal was ever saved from destruction by
accidentally sucking a drop of scarcely nutritious fluid from an accidentally
hypertrophied cutaneous gland of its mother? And even if one was so,
what chance was there of the perpetuation of such a variation?" But the
case is not here put fairly. It is admitted by most evolutionists that mammals
are descended from a marsupial form; and if so, the mammary glands
will have been at first developed within the marsupial sack. In the case
of the fish (Hippocampus) the eggs are hatched, and the young are reared


for a time, within a sack of this nature; and an American naturalist, Mr.
Lockwood, believes from what he has seen of the development of the
young, that they are nourished by a secretion from the cutaneous glands
of the sack. Now, with the early progenitors of mammals, almost before
they deserve to be thus designated, is it not at least possible that the
young might have been similarly nourished? And in this case, the individuals
which secreted a fluid, in some degree or manner the most nutritious, so
as to partake of the nature of milk, would in the long-run have reared
a larger number of well-nourished offspring, than would the individuals
which secreted a poorer fluid; and thus the cutaneous glands, which are
the homologues of the mammary glands, would have been improved or
rendered more effective. It accords with the widely extended principle of
specialization, that the glands over a certain space of the sack should
have become more highly developed than the remainder; and they would
then have formed a breast, but at first without a nipple, as we see in the
Ornithorhynchus, at the base of the mammalian series. Through what
agency the glands over a certain space became more highly specialized
than the others, I will not pretend to decide, whether in part through
compensation of growth, the effects of use, or of natural selection.

The development of the mammary glands would have been of no service,
and could not have been effected through natural selection, unless the
young at the same time were able to partake of the secretion. There is no
greater difficulty in understanding how young mammals have instinctively
learned to suck the breast, than in understanding how unhatched chickens
have learned to break the egg-shell by tapping against it with their specially
adapted beaks; or how a few hours after leaving the shell they have learned
to pick up grains of food. In such cases the most probable solution seems
to be, that the habit was at first acquired by practice at a more advanced
age, and afterward transmitted to the offspring at an earlier age. But the
young kangaroo is said not to suck, only to cling to the nipple of its mother,
who has the power of injecting milk into the mouth of her helpless, half-
formed offspring. On this head Mr. Mivart remarks: "Did no special pro-
vision exist, the young one must infallibly be choked by the intrusion of
the milk into the windpipe. But there is a special provision. The larynx is
so elongated that it rises up into the posterior end of the nasal passage,
and is thus enabled to give free entrance to the air for the lungs, while
the milk passes harmlessly on each side of this elongated larynx, and so
safely attains the gullet behind it." Mr. Mivart then asks, how did natural
selection removed in the adult kangaroo (and in most other mammals, on
the assumption that they are descended from a marsupial form), "this
at least perfectly innocent and harmless structure?" It may be suggested
in answer, that the voice, which is certainly of high importance to many
animals, could hardly have been used with full force as long as the larynx
entered the nasal passage; and Professor Flower has suggested to me that
this structure would have greatly interfered with an animal swallowing
solid food.


We will now turn for a short space to the lower divisions of the animal
kingdom. The Echinodermata (star-fishes, sea-urchins, etc.) are furnished
with remarkable organs called pedicellarias, which consist, when well de-
veloped, of a tridactyle forceps — that is, of one formed of three serrated arms,
neatly fitting together and placed on the summit of a flexible stem, moved
by muscles. These forceps can seize firm hold of any object; and Alexander
Agassiz has seen an Echinus or sea-urchin rapidly passing particles of
excrement from forceps to forceps down certain lines of its body, in order
that its shell should not be fouled. But there is no doubt that besides re-
moving dirt of all kinds, they subserve other functions; and one of these
apparently is defence.

With respect to these organs, Mr. Mivart, as on so many previous oc-
casions, asks: "What would be the utility of the first rudimentary beginnings
of such structures, and how could such incipient buddings have ever pre-
served the life of a single Echinus?" He adds, "Not even the sudden de-
velopment of the snapping action could have been beneficial without the
freely movable stalk*, nor could the latter have been efficient without the
snapping jaws, yet no minute, merely indefinite variations could simultane-
ously evolve these complex coordinations of structure; to deny this seems
to do no less than to affirm a startling paradox." Paradoxical as this may
appear to Mr. Mivart, tridactyle forcepses, immovably fixed at the base,
but capable of a snapping action, certainly exist on some star-fishes; and
this is intelligible if they serve, at least in part, as a means of defence. Mr.
Agassiz, to whose great kindness I am indebted for much information on
the subject, informs me that there are other star-fishes, in which one of
the three arms of the forceps is reduced to a support for the other two;
and again, other genera in which the third arm is completely lost. In
Echinoneus, the shell is described by M. Perrier as bearing two kinds of
pedicellarias, one resembling those of Echinus, and the other those of
Spatangus; and such cases are always interesting as affording the means
of apparently sudden transitions, through the abortion of one of the two
states of an organ.

With respect to the steps by which these curious organs have been evolved,
Mr. Agassiz infers from his own researches and those of Mr. Miiller, that
both in star-fishes and sea-urchins the pedicellariae must undoubtedly be
looked at as modified spines. This may be inferred from their manner of
development in the individual, as well as from a long and perfect series
of gradations in diflferent species and genera, from simple granules to
ordinary spines, to perfect tridactyle pedicellariae. The gradation extends
even to the manner in which ordinary spines and the pedicellariae,
with their supporting calcareous rods, are articulated to the shell. In certain
genera of star-fishes, "the very combinations needed to show that the pedi-
cellariae are only modified branching spines" may be found. Thus we have
fixed spines, with three equi-distant, serrated, movable branches, articulated
to near their bases; and higher up, on the same spine, three other movable
branches. Now when the latter arise from the summit of a spine they form,


in fact, a rude tridactyle pedicellaria, and such may be seen on the same
spine together with the three lower branches. In this case the identity in
nature between the arms of the pedicellariae and the movable branches
of a spine, is unmistakable. It is generally admitted that the ordinary spines
serve as a protection; and if so, there can be no reason to doubt that
those furnished with serrated and movable branches likewise serve for the
same purpose; and they would thus serve still more effectively as soon as
by meeting together they acted as a prehensible or snapping apparatus.
Thus every gradation, from an ordinary fixed spine to a fixed pedicellaria,
would be of service.

In certain genera of star-fishes, these organs, instead of being fixed or
borne on an immovable support, are placed on the summit of a flexible and
muscular, though short, stem; and in this case they probably subserve some
additional function besides defence. In the sea-urchins the steps can be
followed by which a fixed spine becomes articulated to the shell, and is
thus rendered movable. I wish I had space here to give a fuller abstract
of Mr. Agassiz's interesting observations on the development of the pedi-
cellariae. All possible gradations, as he adds, may likewise be found be-
tween the pedicellariae of the star-fishes and the hooks of the Ophiurians,
another group of the Echinodermata ; and again between the pedicellariae
of sea-urchins and the anchors of the Holothuriae, also belonging to the
same great class.

Certain compound animals, or zoophytes, as they have been termed,
namely the Polyzoa, are provided with curious organs called avicularia.
These differ much in structure in the different species. In their most perfect
condition they curiously resemble the head and beak of a vulture in minia-
ture, seated on a neck and capable of movement, as is likewise the lower
jaw or mandible. In one species observed by me, all the avicularia on the
same branch often moved simultaneously backward and forward, with the
lower jaw widely open, through an angle of about go degrees, in the course
of five seconds ; and their movement caused the whole polyzoary to tremble.
When the jaws are touched with a needle they seize it so firmly that the
branch can thus be shaken.

Mr. Mivart adduces this case, chiefly on account of the supposed difficulty
of organs, namely the avicularia of the Polyzoa and the pedicellariae of
the Echinodermata, which he considers as "essentially similar," having been
developed through natural selection in widely distinct divisions of the
animal kingdom. But, as far as structure is concerned, I can see no similarity
between tridactyle pedicellariae and avicularia. The latter resembles some-
what more closely the chelae or pincers of Crustaceans; and Mr. Mivart
might have adduced, with equal appropriateness, this resemblance as a
special difficulty, or even their resemblance to the head and beak of a bird.
The avicularia are believed by Mr. Busk, Dr. Smitt, and Dr. Nitsche —
naturalists who have carefully studied this group — to be hornologous with
the zooids and their cells which compose the zoophyte, the movable lip or
lid of the cell corresponding with the lower and movable mandible of the


avicularium. Mr. Busk, however, does not know of any gradations now
existing between a zooid and an avicularium. It is therefore impossible to
conjecture by what serviceable gradations the one could have been con-
verted into the other, but it by no means follows from this that such grada-
tions have not existed.

As the chelas of Crustaceans resemble in some degree the avicularia of
Polyzoa, both serving as pincers, it may be worth while to show that with
the former a long series of serviceable gradations still exists. In the first and
simplest stage, the terminal segment of a limb shuts down either on the
square summit of the broad penultimate segment, or against one whole side,
and is thus enabled to catch hold of an object, but the limb still serves as
an organ of locomotion. We next find one corner of the broad penultimate
segment slightly prominent, sometimes furnished with irregular teeth, and
against these the terminal segment shuts down. By an increase in the size
of this projection, with its shape, as well as that of the terminal segment,
slightly modified and improved, the pincers are rendered more and more
perfect, until we have at last an instrument as efficient as the chelae of a
lobster. And all these gradations can be actually traced.

Besides the avicularia, the polyzoa possesses curious organs called vibrac-
ula. These generally consist of long bristles, capable of movement and
easily excited. In one species examined by me the vibracula were slightly
curved and serrated along the outer margin, and all of them on the same
polyzoary often moved simultaneously; so that, acting like long oars, they
swept a branch rapidly across the object-glass of my microscope. When a
branch was placed on its face, the vibracula became entangled, and they
made violent efforts to free themselves. They are supposed to serve as a
defence, and may be seen, as Mr. Busk remarks, "to sweep slowly and
carefully over the surface of the polyzoary, removing what might be noxious
to the delicate inhabitants of the cells when their tentacula are protruded.'*
The avicularia, like the vibracula, probably serve for defence, but they ^Iso
catch and kill small living animals, which, it is believed, are afterward swept
by the currents within reach of the tentacula of the zooids. Some species
are provided with avicularia and vibracula, some with avicularia alone,
and a few with vibracula alone.

It is not easy to imagine two objects more widely different in appearance
than a bristle or vibraculum, and an avicularium like the head of a bird;
yet they are almost certainly homologous and have been developed from
the same common source, namely a zooid with its cell. Hence, we can under-
stand how it is that these organs graduate in some cases, as I am informed
by Mr. Busk, into each other. Thus, with the avicularia of several species of
Lepralia, the movable mandible is so much produced and is so like a bristle
that the presence of the upper or fixed beak alone serves to determine its
avicularian nature. The vibracula may have been directly developed from
the lips of the cells, without having passed through the avicularian stage;
but it seems more probable that they have passed through this stage, as
during the early stages of the transformation, the other parts of the cell,


with the included zooid, could hardly have disappeared at once. In many
cases the vibracula have a grooved support at the base, which seems to
represent the fixed beak; though this support in some species is quite absent.
This view of the development of the vibracula, if trustworthy, is interesting;
for supposing that all the species provided with avicularia had become
extinct, no one with the most vivid imagination would ever have thought
that the vibracula had originally existed as part of an organ, resembling a
bird's head, or an irregular box or hood. It is interesting to see two such
widely different organs developed from a common origin; and as the mov-
able lip of the cell serves as a protection to the zooid, there is no difficulty
in believing that all the gradations, by which the lip became converted
first into the lower mandible of an avicularium, and then into an elongated
bristle, likewise served as a protection in different ways and under different

In the vegetable kingdom Mr. Mivart only alludes to two cases, namely
the structure of the flowers of orchids, and the movements of climbing
plants. With respect to the former, he says: "The explanation of their
origin is deemed thoroughly unsatisfactory, — utterly insufficient to explain
the incipient, infinitesimal beginnings of structures which are of utility
only when they are considerably developed." As I have fully treated this
subject in another work, I will here give only a few details on one alone of
the most striking peculiarities of the flowers of orchids, namely, their
pollinia. A pollinium, when highly developed,, consists of a mass of pollen-
grains, affixed to an elastic foot-stalk or caudicle, and this to a little mass
of extremely viscid matter. The pollinia are by this means transported by
insects from one flower to the stigma of another. In some orchids there is
no caudicle to the pollen-masses, and the grains are merely tied together
by fine threads; but as these are not confined to orchids, they need not here
be considered ; yet I may mention that at the base of the orchidaceous series,
in Cypripedium, we can see how the threads were probably first developed.
In other orchids the threads cohere at one end of the pollen-masses; and
this forms the first or nascent trace of a caudicle. That this is the origin of
the caudicle, even when of considerable length and highly developed, we
have good evidence in the aborted pollen-grains which can sometimes be
detected embedded within the central and solid parts.

With respect to the second chief peculiarity, namely, the little mass of
viscid matter attached to the end of the caudicle, a long series of gradations
can be specified, each of plain service to the plant. In most flowers belong-
ing to other orders the stigma secretes a little viscid matter. Now, in certain
orchids similar viscid matter is secreted, but in much larger quantities, by
one alone of the three stigmas; and this stigma, perhaps in consequence of
the copious secretion, is rendered sterile. When an insect visits a flower
of this kind, it rubs off some of the viscid matter, and thus at the same
time drags away some of the pollen-grains. From this simple condition,
which differs but little from that of a multitude of common flowers, there


are endless gradations — to species in which the pollen-mass terminates in a
very short, free caudicle — to others in which the caudicle becomes firmly
attached to the viscid matter, with the sterile stigma itself much modified.
In this latter case we have a pollinium in its most highly developed and
perfect condition. He who will carefully examine the flowers of orchids for
himself will not deny the existence of the above series of gradations —
from a mass of pollen-grains merely tied together by threads, with the
stigma differing but little from that of an ordinary flower, to a highly
complex pollinium, admirably adapted for transportal by insects; nor will
he deny that all the gradations in the several species are admirably adapted
in relation to the general structure of each flower for its fertilization by
different insects. In this, and in almost every other case, the inquiry may
be pushed further backward; and it may be asked how did the stigma of
an ordinary flower become viscid; but as we do not know the full history of
any one group of beings, it is as useless to ask, as it is hopeless to attempt
answering, such questions.

We will now turn to climbing plants. These can be arranged in a long
series, from those which simply twine round a support, to those which I
have called leaf-climbers, and to those provided with tendrils. In these two
latter classes the stems have generally, but not always, lost the power of
twining, though they retain the power of revolving, which the tendrils like-
wise possess. The gradations from leaf -climbers to tendril bearers are won-
derfully close, and certain plants may be indifferently placed in either
class. But in ascending the series from simple twiners to leaf-climbers, an
important quality is added, namely sensitiveness to a touch, by which means
the foot-stalks of the leaves or flowers, or these modified and converted into
tendrils, are excited to bend round and clasp the touching object. He who
will read my memoir on these plants will, I think, admit that all the many
gradations in function and structure between simple twiners and tendril-
bearers are in each case beneficial in a high degree to the species. For
instance, it is clearly a great advantage to a twining plant to become a
leaf -climber; and it is probable that every twiner which possessed leaves
with long foot-stalks would have been developed into a leaf-climber, if the
foot-stalks had possessed in any slight degree the requisite sensitiveness to
a touch.

As twining is the simplest means of ascending a support, and forms the
basis of our series, it may naturally be asked how did plants acquire this
power in an incipient degree, afterward to be improved and increased
through natural selection. The power of twining depends, firstly, on the
stems while young being extremely flexible (but this is a character common
to many plants which are not climbers) ; and, secondly, on their continually
bending to all points of the compass, one after the other in succession, in
the same order. By this movement the stems are inclined to all sides, and
are made to move round and round. As soon as the lower part of a stem
strikes against any object and is stopped, the upper part still goes on bend-
ing and revolving, and thus necessarily twines round and up the support.


The revolving movement ceases after the early growth of each shoot. As
in many widely separated families of plants, single species and single genera
possess the power of revolving, and have thus become twiners, they must
have independently acquired it, and cannot have inherited it from a com-
mon progenitor. Hence, I was led to predict that some slight tendency to a
movement of this kind would be found to be far from uncommon with
plants which did not climb; and that this had afforded the basis for natural
selection to work on and improve. When I made this prediction, I knew of
only one imperfect case, namely, of the young flower-peduncles of a
Maurandia which revolved slightly and irregularly, like the stems of twin-
ing plants, but without making any use of this habit. Soon afterward Fritz
Miiller discovered that the young stems of an Alisma and of a Linum —
plants which do not climb and are widely separated in the natural system —
revolved plainly, though irregularly: and he states that he has reason to
suspect that this occurs with some other plants. These slight movements
appear to be of no service to the plants in question; anyhow, they are not
of the least use in the way of climbing, which is the point that concerns us.
Nevertheless we can see that if the stems of these plants had been flexible,

Online LibraryCharles DarwinThe origin of species → online text (page 22 of 50)