Without attempting to enter upon the argument yet to be conducted through the wider relations concerned, it must be obvious that the facts bearing on insect life must erelong have a larger share than they have yet had in influencing our generalizations. By reference to these, it becomes apparent, that anatomical structure is not in itself an adequate guide in determining comparative importance on the scale of organic existence; and, what is still more startling, that even comparative brain structure can not be taken as the sole test of the measure of intelligence belonging to animals. The whole orders of ants, taken collectively, must be regarded as presenting quite exceptional difficulties, not only for a theory of evolution regarded as an all-embracing science of life; but also for that theory of intelligence which seeks to account for diversities of power by the comparative complexity of brain structure.
Pa.s.sing from more detailed discussion, it is needful to observe how wide and valuable are the results of these researches concerning the relation of the vegetable kingdom with lower orders of animals. Facts now recorded in mult.i.tudes of scientific journals, and more elaborate treatises, ill.u.s.trate 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 ill.u.s.tration 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 Muller"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 a.s.sociated. As, therefore, we rise in the scale, pa.s.sing 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 pa.s.s 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 cooperative, unitedly contributing to the management of the living organism. This appears even if we take for ill.u.s.tration 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 pa.s.sing 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 ma.s.s 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, antennae 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 ill.u.s.tration 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 pa.s.ses 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 const.i.tutes 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 cla.s.ses of nerves. The nerves of sensibility have a peculiarly sensitive arrangement spread under the skin, const.i.tuting 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 cl.u.s.ter. The terminal arrangements of the motor nerves are quite different. The nerve fibres pa.s.s 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.
[Ill.u.s.tration: DIAGRAM OF CEREBRO-SPINAL NERVE CENTRES. DARK REPRESENTING SENSORY; THE LIGHT, MOTOR CENTRES. THE ARROWS INDICATE THE DIRECTION OF THE CURRENT OF INFLUENCE.]
[Ill.u.s.tration: 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 pa.s.s over to the motor system and terminate in muscular activity. This is most simply ill.u.s.trated by the circ.u.mstance 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 ill.u.s.trating 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 ident.i.ty 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 ill.u.s.tration, we may find in the most complex organism known to us ill.u.s.tration 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 ma.s.s_ consisting of a mult.i.tude 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 const.i.tuting the external ma.s.s, 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 a.s.signed 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 pa.s.s 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 ma.s.s 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.
[Ill.u.s.tration: HUMAN BRAIN, WITH CENTRES OF ELECTRIC EXCITATION.]