and valuable are the results of these researches concerning the
relation of the vegetable kingdom with lower orders of animals. Facts
now recorded in multitudes of scientific journals, and more elaborate
treatises, illustrate wonderful minuteness of contrivance and
completeness of adaptation in the works of nature, giving to the range
of knowledge possessed only a century ago an aspect of insignificance.
What the microscope has done by enlarging the range of human vision,
subdivision of labor among scientific inquirers, and proportionate
concentration, have done, in the way of embracing the vast and
complicated field, of observation lying open to all eyes. The results
exalt to a greatly higher place in our appreciation the evidence of
design in the world. The consequence is that while the line of thought
followed by Paley, in what he designated _Natural Theology_, has become
a thousand-fold more interesting, the familiar and now almost antiquated
illustration of the _watch_, taken as a model of human design, by the
comparative simplicity of its adjustments, seems strangely inadequate to
represent even in the most temporary form, a minuteness of design quite
overwhelming to the human mind in its attempts to bring it within a
uniform scheme. Whether all this was provided for by manifold creative
acts, or by development from a few primordial forms, does not affect the
argument; the latter suggestion only greatly increases its force. To
those who are swayed only by an intellectual interest, the facts of
vegetable and insect life must be full of significance, suggestive of
far-reaching reflection. But to no body of men can these results of
scientific research be so attractive as to those who require for all
nature a supernatural explanation.
FOOTNOTES:
[BR] _Lay Sermons_, chap, vii., p. 134.
[BS] Darwin's _Fertilization of Orchids_, p. 2; Lubbock's _Scientific
Lectures_, p. 8.
[BT] Sir John Lubbock's _Scientific Lectures_, p. 3. Mr. Darwin refers
to Fritz Müller's papers as reported in _Botanische Zeitung_, 1869-70.
Appendix IX.
[BU] _The Various Contrivances by which Orchids are fertilized by
Insects_, 2d ed. p. 293.
[BV] _Fertilization_, p. 5.
[BW] _Fertilization_, p. 12.
[BX] _Fertilization_, p. 102.
[BY] _Ib._ p. 113.
[BZ] _Fertilization_, p. 44.
[CA] _Scientific Lectures_, p. 31.
[CB] _Fertilization_, p. 2.
[CC] _Ib._ p. 284.
[CD] _Fertilization_, p. 285.
[CE] _Fertilization_, p. 246.
[CF] _Botany_, (Science Primers) by Dr. J. D. Hooker, C.B., P.R.S., p.
79.
[CG] _Scientific Lectures_, p. 36.
[CH] See Appendix X.
[CI] See Appendix VIII.
[CJ] _Origin of Species_, 6th ed. p. 230.
[CK] _Origin of Species_, 6th ed. p. 233.
[CL] _Scient. Lects._ p. 73.
[CM] _Ib._ p. 78.
[CN] _Ib._ p. 135.
[CO] _Scientific Lectures_, p. 135.
[CP] _Origin of Species_ 6th ed. p. 207.
[CQ] _Museum of Natural History_ edited by Richardson, Dallas, Cobbold,
Baird, and White, vol. ii. p. 184.
[CR] _Scient. Lects._ p. 68.
LECTURE VI.
HIGHER ORGANISMS.--RESEMBLANCES AND CONTRASTS.--BRAIN STRUCTURE.
The stage of investigation now reached requires us to consider recent
advances in our knowledge of more complicated organisms. This leads into
the line of observation disclosing steadily advancing complexity of
structure, and brings us into contact with the claim that man be
included within the area of scientific inquiry, and regarded as a more
fully organized life to which lower orders are not only pointing, but
actually tending.
As to this last claim, about which more must be said as we approach the
close of these investigations, it may be remarked by way of preliminary,
that as man belongs to nature, all the characteristics of his life must
come within the area of scientific inquiry, and indeed the test of any
theory of existence which may be offered, will be found in the measure
of success with which it explains our own nature. That man stands
highest in the scale of organism belonging to this world admits of no
doubt, therefore the explanation of human nature may be regarded as the
supreme effort of science. Around this subject, however, serious
differences have arisen among scientific men, but these differences do
not concern the very simple question whether all that belongs to nature
comes within the range of the science of nature. This is granted by all,
whether there be a preference for including all such inquiry under the
single name of science, or for distinguishing between physical science
and mental philosophy. This is simply a matter of defining terms, and
tracing the boundaries of recognized departments of inquiry. But whether
a continued study of organism will conduct us to an adequate
understanding of human nature, must be a matter of observation and
inference. If it do, science has completed its work. If it do not, there
remains a still higher question, how shall we account for features of
life for which organism affords no scientific explanation? The whole
field is certainly free to science, and the whole task which this
immense field of research imposes must be undertaken, and persistently
prosecuted to a rational issue.
Entering now, therefore, on the contemplation of animal life, regarded
as a higher order, distinguishable from vegetable life, we have the
outstanding characteristics of sensibility and locomotion. Whether there
is a distinct line of demarcation between vegetable and animal does not
require special attention, for no matter of controversy on this point
can delay procedure. There is, as already remarked, in the vegetable
kingdom a singular approximation towards animal life, in so far as we
have evidence of sensibility to touch among the plants, to a degree
which appears wonderful chiefly by contrast with the common
characteristics of the vegetable kingdom.
On the other hand, sensibility to influences operating from without is a
common feature of animal life. Even the very lowest orders of animals
are sensitive to touch, and as this form of experience is closely
connected with power of locomotion, all animals have the conditions of
their life largely affected by interference with their own movements, or
resistance offered, whether by objects lying in their way, or by some
force restraining their progress, or causing movement in an opposite
direction. Now these two characteristics--sensibility to impression from
without, and movement caused by an exercise of energy from within the
organism itself--are both provided for by means of the nerve system
belonging to the animal. This nerve system varies in the number and
complexity of its arrangements, according to the complexity of the
organism with which it is associated. As, therefore, we rise in the
scale, passing from the soft pulpy form of the lowest orders, to those
formed in segments or rings, next to those with distinct portions of
organism fulfilling separate functions, as in insect life, with head,
body, and legs; and next pass up to the vertebrates, with back-bone and
skeleton, on which is built up a more or less complicated muscular
system, we find a nerve system, growing in complexity along with the
appearance of different organs of the body. And in all cases, this
system fulfils these two functions--sensibility to touch, and movement
of the body. These two are provided for by distinct lines or nerve
fibres; and in all cases, these two sets are combined in a centre,
thereby securing that the two sets be coöperative, unitedly
contributing to the management of the living organism. This appears even
if we take for illustration an organism so low as the _ascidian
mollusk_, which floats in the water as if it were a sack drawn together
towards the top, bulging out below; and which is nourished simply by the
passing of a current of water in at the mouth, and out at a vent towards
the lower end of the sac. A series of nerve lines comes from the mouth;
a distinct ramification spreads over the lower portion of the sac; and
these two are united in a single knot or ganglion, a little above the
vent. By these contrivances, this little body, though for the most part
stationary, is sensitive to the approach of any thing injurious, and by
contraction of its mass expels the water with considerable force,
driving the injurious matter to a distance. This combination of the two
sets of nerves appears more strikingly in such an animal as the
_centipede_, along whose body are successive groups of nerves, combined
in regular order in a series of knots, and united longitudinally by
connecting threads, attaching the successive knots. The same plan is
carried up into a more articulated form in the case of the _winged
insect_, with head, antennæ or feelers projecting from the head, wings,
and legs, leading to a more marked appearance of separate combinations,
giving greater prominence to the head. When from this we rise to the
_fish_, thence to the _bird,_ thence to the _quadruped_, we find the
head made conspicuously the central organ of the entire nerve system of
the animal, while it occupies the front position in the body. It is no
longer one of a set or series of knots; nor even the largest or more
conspicuous in a graduated order of centres; but in the head of the
animal is found that which is the true nerve centre for the whole nerve
system, designated the brain. In the case of the vertebrates, not only
does the skeleton afford the solid frame-work on which the muscular
system is built, but the back-bone contains within it the main column of
nerve fibres, which are given out at the several joints according to the
requirements of the body.
If meanwhile we concentrate attention on our own bodies, we may by the
aid of personal experience find easy illustration of the prominent
features of the nerve system. We shall take first the _two distinct
lines_ of nerves already mentioned, the one set concerned with
sensibility, the other with movement of the muscles. From the tips of
the fingers there run lines of nerve fibre, which are brought into
combination at the wrist, and are carried up the arm, and onward by the
shoulder and upper portion of the back-bone to the head. These are the
nerves of _sensibility_, by means of which, as by telegraph wires, the
slightest impression made on the tips of the fingers is instantly
conveyed to the great nerve centre in the brain. Distinct from these is
another set of nerves issuing from the brain, and descending the arm,
giving off its fibres as it passes to the several muscles above the
elbow, next to those above the wrist, and next to the muscles of the
hand and fingers. These are the nerves of _movement_, by means of which
the whole arm may be brought into action at pleasure, or the hand may be
set to work, while the arm is at rest.
These two sets of nerves--the sensory and motor--are exactly _similar in
structure_, consisting of an outer covering, within which floating in a
white fluid is a thread which constitutes the nerve proper. The outer
covering provides for _isolation_ of the fibre, from other fibres laid
alongside of it, just as copper wire is isolated by a gutta-percha
covering when the two connecting lines from an electric battery are laid
down in close proximity as in the arrangement for electric bells. By
this provision the nerve fibres are completely isolated making it
possible to distinguish sensory impressions so as to tell which finger
has been touched. The similarity of structure in the two lines of nerves
is a striking fact in view of the completely distinct functions
fulfilled. This leads to a special explanation of the provision for
different modes of action. This is secured by _diversity in the terminal
arrangements_ for the two classes of nerves. The nerves of sensibility
have a peculiarly sensitive arrangement spread under the skin,
constituting an end-bulb or touch organ. In certain parts of the body
more sensitive than others, such as the tips of the fingers, there are
additional minute corpuscles, grouped alongside of the nerve, liable to
contract under the slightest pressure, and which add greatly to the
sensitiveness of the particular parts about which they cluster. The
terminal arrangements of the motor nerves are quite different. The nerve
fibres pass into the substance of the muscle to be moved by them, and
the nerve fibre is subdivided and distributed, so as to bring the
several parts of the muscle under control. These fibres are so laid and
connected, that a whole set of muscles can be moved simultaneously,
being made to work in perfect harmony.
_The vital activity_ of this whole arrangement of nerve fibres,
including sensory and motor in one system, depends upon living
connection of all with the great nerve centre in the brain, where the
nerve energy is provided which keeps all in functional activity. Only,
there is this striking difference with the two sets of fibres, that in
the case of the sensory nerve the pulsation of energy is upwards to the
brain, in the case of the motor nerve it is downwards towards the
muscle. There is no scientific explanation yet reached of this contrast
of molecular action. But by means of it the one order of nerves plays
the part of a vehicle of impression providing for knowledge of what is
without, the other order fulfils the part of an instrument for moving
the muscular system which is part of the organism itself.
[Illustration: DIAGRAM OF CEREBRO-SPINAL NERVE CENTRES. DARK
REPRESENTING SENSORY; THE LIGHT, MOTOR CENTRES. THE ARROWS INDICATE THE
DIRECTION OF THE CURRENT OF INFLUENCE.]
[Illustration: NERVE SYSTEM OF THE INSECT, SHOWING DISTINCT CENTRES.]
These two orders are not, however, to be regarded as separate systems
quite apart from each other, but as two sides of one system, which are
essentially and closely related to each other. There is a provision for
_combined action_ of the two sets, so that an impulse communicated along
a sensory nerve or set of nerves, may pass over to the motor system and
terminate in muscular activity. This is most simply illustrated by the
circumstance that the nerves of sensibility become instruments of
_pain_, when a severe shock or blow is given, or some injury is
inflicted. Suffering becomes a signal of risk and instantly the injured
part shrinks or starts away from the source of suffering. This is a
phase of sensori-motor activity illustrating a law which has a wide
range of application in animal life. This sketch of the arrangements and
functions of the two sides of the nerve system though traced in view of
its application to human nature, will suffice to indicate the general
plan in accordance with which sensibility and muscular activity are
provided for in the animal kingdom generally. The ramification of the
nerve lines will in each case be according to the simplicity or
complexity of structure belonging to the animal; but the provisions for
sensitiveness to touch, and power of movement are in all cases the same.
Fish, bird, and quadruped are alike sensitive to touch, and they are
alike capable of movement, though the mechanical contrivances by which
locomotion is secured vary greatly; but a double distribution of nerve
fibres in all cases provides for these two characteristics of animal
life.
From this, we advance to the nerve centre,--the brain,--to which the
nerves of sensibility run up, and from which the nerves of motion come
forth. Here also there is identity in the nature of the organ, while
there is variety in its size, with more or less complicated plans of
arrangement, according to the extent of the nerve system of which it is
the central organ. Still keeping to the human body for illustration, we
may find in the most complex organism known to us illustration of what
holds good in the main so far as essential structure is concerned.
The brain is made up of two entirely distinct substances. In the
interior of the organ, and altogether concealed from view when a drawing
of it is made, or the organ itself is exposed to observation, is _a
white mass_ consisting of a multitude of fibres. These are simply
crowds of nerve lines gathered together, led up from the extremities and
trunk, or provided for intercommunication with the several parts of this
central organ. Gathered all round about this, and constituting the
external mass, on the summit, sides, and base of the brain, is a
completely distinct substance known as _the grey matter_, folded up in
wavings, twistings, or convolutions, enclosing myriads of cells from
which nerve energy is discharged. These cells differ considerably in
form and size, suggesting the possibility of distinct functions being
assigned to cells of different structure, some being smaller and less
intimately connected with those around, others so much larger and more
important as to have suggested the name of pyramidal cells, and also
having lines of connection between themselves and other parts much more
numerous than in the case of the smaller cells. Every cell has a nucleus
or central point, which is the centre of vitality, while the fibres
which they send out, varying in number from one to four or five,
establish connection between cells, or pass into the nerves proper.
These cells are packed together in a soft glutinous substance, in the
outer layer of which they are fewer in number; approaching the interior,
they become more numerous; and they are both more abundant, larger in
size, and more distinguished by the number of their protoplasmic[CS]
fibres as they lie nearer to the mass of nerve fibres. In this crowd of
nerve cells are the stores of nerve energy supplied to the nerve system,
with every exercise of which molecular changes in the brain are believed
to take place. On this account there must be regular and ample supply of
nourishment for the brain, for which such provision has been made that,
according to Haller's computation, one fifth part of the whole blood
supply goes to the brain.
[Illustration: HUMAN BRAIN, WITH CENTRES OF ELECTRIC EXCITATION.]
Regarded as the great central organ, the brain is divided into two
halves or hemispheres, from each one of which goes forth supply of nerve
fibres and nerve energy for the opposite side of the body. Its greatest
depth is in the central part, the front and back being rounded down, the
frontal region being, however, considerably more massive than the rear.
Besides this great central body, there are several dependent subordinate
bodies, placed underneath, and directly above the upper part of the
spine. Most important of these is the _cerebellum_, or little brain,
whose functions are now generally believed to be closely connected with
the equilibrium of the body when moving. Somewhat nearer the centre, and
quite under the brain proper is the _pons_ or bridge, providing for the
interlacing of the fibres on their way out from the the central organ,
and just below that are certain elongated bodies (_medulla oblongata_),
consisting of masses of fibre just above the spinal cord.
Before closing this very brief and hasty description of the nerve
system, there is one peculiarly striking arrangement to which special
reference may be made. The mass of nerve fibre which passes down within
the back-bone constituting the spinal column, which is formed in two
divisions equivalent to the hemispheres of the brain, gives out at each
of the vertebræ or spinal joints a supply of nerve for the portion of
the body contiguous. This supply is sent out from each side of the
column, and issues in two roots, a posterior and anterior; the posterior
root being a body of sensory nerves, the anterior root of motor nerves.
Shortly after passing out, these two form into one, uniting to
constitute a nerve trunk. Just after they have thus united, the trunk
again opens up into two, and in each one of these two a share of the
sensory and motor roots finds a place, and thus preparation is made for
sending out towards both the front and back of the body suitable
proportion of both sets of nerves. The two roots drawn together as if to
bind them into one, are by some inexplicable process subdivided, and the
two bands issuing from the united band are found to have each a share of
the contents of each root. Of all the singular occurrences coming under
scientific observation there is nothing more surprising. The fact is
certain, but there is no scientific explanation of the contrivance by
which such a singular result is secured.
[Illustration: BRAIN OF THE CRAB.]
[Illustration: BRAIN OF THE COD, THE TWO LARGER LOBES BEING THOSE OF
VISION, THE BRAIN BEING IN FRONT OF THESE.]
Having now before us in outline a representation of the nervous system
of man, and having in this a guide to the understanding of the prominent
features involved in the distribution of two orders of nerves over the
body, and their concentration in a central organ, we are prepared for
considering the comparative brain development presented to view as we
ascend the scale of animal life. The main features of gradation may be
shortly stated. In all cases, the brain is a soft pulpy body, composed
as described, the exterior portion being cellular tissue, the interior
fibrous, from the gathering of nerve lines. In the lowest orders of
animals, the brain is of very small size. In the _insects_, such as the
ant, bee, and wasp, it is only a slight band stretching from eye to eye.
In the whole order of _fishes_ an advance in organization appears,
though the brain is small relatively to the size of the body, a fact
which seems readily explained by the fact that there is little
articulation in the structure of the fish, the whole body moving in one
mass, by simple management of the fins and tail. The brain as a rule is
simply two small round lobes of smooth surface laid together; and what
is most to be remarked is that the brain proper is quite inferior in
size to lobes of vision.[CT] In front of the brain are slight strands
connected with the organ of smell; and behind it are the two large lobes
known as optic lobes, before which the brain appears comparatively
insignificant. This is the ordinary arrangement, but in the case of the
shark the brain extends to much larger proportions, greatly surpassing
the optic lobes, and having in front of it unusually ample provision for
the organ of smell.[CU]
When we reach the _reptiles_ the normal order appears which continues
thereafter up the whole range of animal life. The brain takes precedence
of the lobes of special sense, and is the most important organ. This
appears quite decisively in the brain of the frog. On account of the
possession of four limbs, and its power of locomotion by forward leaps,
provided for by the superior size and strength of the hind legs, there
is much greater need for distribution of nerve lines, to place distinct
muscles under control, and as a consequence the brain or central organ
assumes a position of greater importance.
[Illustration: BRAIN OF THE BIRD.]
[Illustration: BRAIN OF THE CAT, WITH BULB OF SMELL IN FRONT, AND LITTLE
BRAIN BEHIND.]
Passing next to _birds_, we find a marked advance in the structure of
the brain. The two hemispheres are considerably extended towards the
rear, and the two optic lobes underneath the back part of the brain are
separated from each other, being placed somewhat to the side. The
cerebellum, or little brain, regulating equilibrium becomes more
important in size and form, being laid up in transverse furrows. These
important advances indicate a life of much more varied activity than in
the lower orders. This animal walks, hops, perches on branches by the
clutching of its claws, and flies from place to place. To provide for
these varied forms of activity, there must be a more detailed
arrangement of nerve system, which is clearly indicated in the
complexity of the central organ.
The next advance introduces to notice the _smaller quadrupeds_, known as
the rodents, of which the rat, rabbit, and hare may be taken as the most
familiar examples. Here we still have the smooth surface of the brain,
without any subdivision and twining into folds such as afterwards
appears, but it is somewhat elongated in shape. An additional element
here comes into view, that is, extra provision for acuteness of smell,
in accordance with the well-known characteristics of the class of
animals. Set out in front of the brain are two distinct lobes, which are
the olfactory lobes. Wherever these are so placed in front of the
brain, it is a clear proof that the life of the animal is largely
directed by smell, that is, in a relatively greater degree than by
sight, though constantly using the organs of vision with rapidity and
acuteness. The cerebellum is in all cases prominent to the rear,
presenting the laminated appearance always distinctive of the organ.