Part 8

The many amateurs of the eighteenth century naturally demanded books written to suit them, and illustrated books with coloured plates, coming out in parts, found a ready sale. Some were devoted to insects, others to microscopic objects. In accordance with prevalent belief, the writers made a point of tracing the hand of Providence in the minutest organisms; many popular treatises were altogether devoted to natural theology. Some few of these natural history miscellanies contained original work, which has not yet lost its interest. The best is Roesel's Insecten-belustigungen (four vols. 4to., 1746-61), memorable among other things for containing the original description of Amœba. For English readers Henry Baker wrote The Microscope Made Easy (1743) and Employment for the Microscope (1753).

Intelligence and Instinct in the Lower Animals.

The period with which we are now concerned (1741-1789) initiated the profitable discussion of the mental powers of animals. We are unable for lack of space to follow the investigation from period to period, and must condense into one short section whatever its history suggests.

In the year 1660 Aristotelians were still discoursing about the vegetative and sensitive souls which bridged the gulf between inanimate matter and the thinking man. Descartes had tried to prove that the bodies of men and animals are machines actuated by springs like watches. Man, however, according to Descartes, possesses a soul wholly different in its properties from his body, and apparently incapable of being acted upon by it. Man only can think; animals are capable only of physical sensations, and have no consciousness. Into speculations like these we shall not venture, being- content, like Locke, "to sit down in quiet ignorance of those things which upon examination are proved to be beyond the reach of our capacities." We shall merely note here and there facts ascertained by observation or experiment, and plain inferences drawn from such facts.

Swammerdam and Réaumur, besides many naturalists of less eminence, recorded a host of observations on the activities of insects. They contributed little to the discussion except new facts, for habit led them to ascribe without reflection every contrivance to the hand of Providence or else to Nature. Some of their facts, however, made a deep impression, none more than the exact agreement of the cells of the honeycomb with the form which calculation showed to be most advantageous. The coincidence has lost some of its interest since the discovery that the theoretically best form of cell is hardly ever realised. Réaumur, in describing the process by which a certain leaf-eating caterpillar makes a case for itself out of the epidermis of an elm-leaf, showed that the caterpillar is not devoid of that kind of intelligence which adapts measures to circumstances. He cut off the margin where the upper epidermis of the leaf passes into the lower one, a margin which the insect had intended to convert into one side of its case; the caterpillar sewed up the gap. He cut off a projection which was meant to form part of the triangular end of the case; the caterpillar altered its plan, and made that the head-end which was originally intended to lodge the tail. This observation anticipates a better-known example taken from the economy of the hive-bee by Pierre Huber, which is mentioned below.

Buffon heard with impatience all expressions of admiration for the works of insects. His poor eyesight and his repugnance to minutiæ disinclined him to pay much attention to creatures so small, and he had set himself up as the rival of Réaumur in physics and natural history. To pour contempt upon insects gratified both feelings at once. Bees, he said, show no intelligence at all; their actions are purely automatic, and their much-vaunted architecture is merely the result of working in a crowd. The cells of the honeycomb are hexagonal, not by reason of forethought or contrivance, but because of mutual pressure; soaked peas in a confined space form hexagonal surfaces wherever they touch.

The elder Huber seems to have denied to bees every trace of intelligence, but his son Pierre found it hard to go so far. He remarked that the storage-cells of a honeycomb are not always exactly alike; they may be lengthened, cut down, or curved, when requisite. Cells which had been rudely trimmed with a knife were repaired with such dexterity and concert as to suggest that even the hive-bee has "le droit de penser." Bees would under compulsion build upwards or sideways, instead of downwards, as they like to do. Finding that they sought to extend their combs in the direction of the nearest support, he covered the support with a sheet of glass, on which they could get no footing. They swerved at once from the straight line, and prolonged their comb towards the nearest uncovered surface, though this obliged them to distort their cells. He was driven to the conclusion that bees possess "a little dose of judgment or reason." In our own time, when all conscious adaptation of means to ends is believed to be worthy of the name of reason, it requires no great courage to ask why we deny such an attribute to all the lower animals.

In spite of examples like this, the favourite expression "blind instinct" helped to strengthen the conviction that the mental processes of animals are unsearchable. It is impossible to deny that the epithet blind is appropriate in many cases. A bird will sit an addled egg all summer, or vainly but repeatedly attempt to make its tunnel in the insufficient breadth of a mud wall (Geositta). Of course such instances do not show that all the acts of the lower animals are devoid of intelligence.

Hume in 1739 and again in 1748 appealed to everyday observation of dogs, birds, and other animals of high grade. The facts seemed to him to show that animals as well as men are endowed with reason and able to draw inferences; he did not, however, credit them with the power of framing general statements, holding that experience operates on them, as on children and the generality of mankind, by "custom" alone. It is notorious that the dog and other higher animals learn by experience; Hume tells, for instance, how an old greyhound will leave the more fatiguing part of the chase to younger dogs, and place himself so as to meet the hare in her doubles. On the other hand (though Hume does not say so) man himself possesses non-educable instincts. In short, Hume sees no ground for drawing a line between the mental powers of man and those of the higher animals, though he attributes to man a power of demonstrative reasoning to which animals do not attain. In this he substantially agrees with Aristotle, who maintained that in animals the germs of the psychical qualities of the man are evident, though, as in the child, they are undeveloped. Hume's teaching also accords with modern views; comparative anatomy, for instance, "is easily able to show that, physically, man is but the last term of a long series of forms, which lead by slow gradations from the highest mammal to the almost formless speck of living protoplasm, which lies on the shadowy boundary between animal and vegetable life."

The detailed proofs which Hume was not enough of a naturalist to furnish were at length stated with admirable clearness and force by Leroy, whose Letters on Animals form the most important contribution made to the discussion during our period. Georges Leroy ( 1723-1789) was lieutenant des chasses under the last French kings, and had charge of the parks at Versailles and Marly. He wrote therefore with knowledge about the wolf, fox, deer, rabbit, and dog. His pages are enlivened by many touches of nature, interesting to readers who perhaps care little about psychology. Leroy attributes to the wolf observation, comparison, judgment. The wolf must mark the height of the fold which encloses a flock, and judge whether he can clear it with a sheep in his mouth. Wolf and she-wolf co-operate artfully in the running-down of prey. Sometimes the she-wolf will draw off the sheep-dog in pursuit, thus putting the flock at the mercy of her mate. Or one of the two will chase the quarry till it is out of breath, when the other can take up the running on advantageous terms. An old fox shows knowledge of the properties of traps, and will rather make a new outlet or suffer long famine than encounter them. But when he finds a rabbit already caught, he realises that the trap has lost its power to hurt. Sheep-dogs can be educated to mind things which do not interest wild dogs, or dogs of other breeds; when, for instance, the flock is driven past a patch of wheat, the dog in charge will take care that the sheep do not damage the crop. A trained sporting-dog learns at length to trust his own judgment, even in opposition to that of his master, and sportsmen know that they must direct young dogs, but leave old ones to act for themselves.

From the middle of the eighteenth century to the present day naturalists and psychologists have been labouring to distinguish instinct from intelligence. It is not hard to define well-marked examples of each, and to show that a typical instinct is congenital (not the result of a process of education or self-education), adaptive (conducive to the welfare of the organism), co-ordinated by nerve-centres (thus excluding the superficially similar behaviour of the lowest animals and all plants), actuating the whole organism (thus excluding most, if not all, reflex acts in the higher animals, as well as the wonderful adjustments effected by bone-corpuscles and other parts of organisms), and common to all the members of a species or other group (thus excluding individual aptitudes). In the same way it is easy to point out clear differences between a bird and a tree. But just as a definition which shall separate every animal from every plant has hitherto been sought in vain, so it has hitherto been impossible to frame a definition which while including all instincts shall admit no case of reflex action or intelligence. The most ambiguous cases of all are perhaps to be found in insects, where, as will shortly be explained, our information is ill-fitted to support precise distinctions.

Many naturalists entertain some form of what may be called the use-and-disuse or inherited-memory theory, supposing that the aptitudes of the offspring are influenced by the activities of the parent. Some cling to the belief that habits can be fixed and transmitted, and we must admit that the fixation and transmission of habits might explain a great deal. But all the evidence goes to prove that habits are not inherited at all, and that we must look elsewhere for the origin of instincts. Let naturalists who think differently try to account for the instincts of working bees or ants, which receive their psychical not less than their physical endowment from a long succession of ancestors, none of which worked for their living. Or let them try to explain the instances of spiders, insects, etc., which after egg-laying practise instinctive arts for the defence of their brood, standing over the eggs, carrying them about, blocking the entrance of the burrow, etc. May we not say that it is impossible for the acts of a parent to influence the congenital instincts of offspring which have already lost connection with the mother? But surely a theory of instinct breaks down which fails to account for the expedients by which the worker-bee, the worker-ant, and the spider provide for the safety of the unhatched brood or for the welfare of the community.

Darwin's Origin of Species threw a new light upon instinct by showing that natural selection can operate on the subtlest modifications. It can discriminate shades of hardiness to climate, shades of intellectual acuteness, or shades of courage. It can intensify qualities which appear only in adults past bearing or in individuals congenitally incapable of propagation. Human selection, though a blunt tool in comparison with natural selection, can originate a bold and hardy race of dogs, or showy double flowers incapable of producing seed. In the second case fertile single flowers continue the race, as in the garden Stock. Darwin pointed out that the barren double flowers of the Stock answer to the workers of social bees and ants, the fertile single flowers to the functional males and females. Every modification that works to the advantage or disadvantage of the race, whether we classify it as physical, intellectual, or moral, gives scope for the operation of natural selection.

The comparative psychology of small invertebrates, such as insects, is impeded by our imperfect knowledge of their nervous physiology. Introspection is here impossible; experimental physiology and pathology, which have done so much for the psychology of the higher vertebrates, almost impossible; analogy is a treacherous guide where the structures involved differ conspicuously. We have little to guide us in the psychology of insects except their behaviour, and that is often capable of a variety of interpretations. The only course is to adopt Pasteur's watchword, "Travaillons!"—the difficulties will diminish with time and labour.

The Food of Green Plants.

Common observation taught men in very early times that green plants draw nourishment from the soil, and that sunlight is necessary to their health. In the age of Galileo a Belgian physician and chemist, Van Helmont, endeavoured to pursue the subject by experiment. He planted the stem of a live willow in furnace-dried earth, which was enclosed in an earthen vessel. Rain-water or distilled water was supplied when necessary, and dust excluded by a perforated lid. The loss of weight due to the falling-off of leaves was neglected. In the course of five years the tree was found to have increased to more than thirty times its original weight; Van Helmont concluded that this increase was due to water only. Malpighi (1671), being guided mainly by his microscopic studies of the anatomy of the stem and leaf, taught that moisture absorbed by the roots ascends by the wood, becoming (apparently at the same time) aerated by the large, air-conducting vessels; that it enters the leaves, and is there elaborated by evaporation, the action of the sun's rays, and a process of fermentation; lastly, that the elaborated sap passes from the leaves in all directions towards the growing parts. It will be seen that this explanation, though incomplete, makes a fair approximation to the beliefs now held; for more than a hundred years after Malpighi's day less instructed opinions were commonly held. Hales (1727) recognised that green plants are largely nourished at the expense of the atmosphere; he dwelt also on the action of the leaves in drawing water from the soil, and in discharging superfluous moisture by evaporation.

Joseph Priestley, who had been proving that air is necessary both to combustion and respiration, made an experiment in 1771 to discover whether plants affected air in the same way that animals do. He put a sprig of mint into a vessel filled with air in which a candle had burned out, and after ten days found that a candle would now burn perfectly well in the same air. Air kept without a plant, in a glass vessel immersed in water, did not regain its power of supporting combustion. Balm, groundsel, and spinach were found to answer just as well as mint. Air vitiated by the respiration of mice was restored by green plants as readily as air which had been vitiated by combustion.

Priestley did not remark that the glass vessels employed in his experiments had been set in a window, and inattention to this point caused some of his attempts to repeat the experiment to fail. He was further perplexed by using vessels which had become coated with a film of "green matter," probably Euglæna. Such vessels restored vitiated air, though no leaves were present, and when placed in the sun, gave off considerable quantities of a gas, Priestley's "dephlogisticated air" (oxygen). Hardly any oxygen was given off when the green matter was screened by brown paper. Water impregnated with carbonic acid was found to favour the production of the green matter. To us, who have been taught at school something about the properties of green plant-tissues, it seems obvious that Priestley ought to have ascertained by microscopic examination whether his "green matter" was not a living plant. But he had always avoided the use of the microscope, his eyes being weak, and after some imperfect attempts in this way he made up his mind that the green matter was neither animal nor vegetable, but a thing sui generis. Neglecting his most instructive experiments, and not waiting till he could devise new ones, or even disentangle his thoughts, he sent to the press a confused explanation, which seemed to teach that vitiated air may be restored by sunlight alone.

A Dutch physician, named John Ingenhousz, who was then living in England, read Priestley's narrative and began to investigate on his own account. Without detailing his numerous experiments, we may give his own clear summary (condensed). "I observed," Ingenhousz says, "that plants have a faculty to correct bad air in a few hours; that this wonderful operation is due to the light of the sun; that it is more or less brisk according to the brightness of the light; that only the green parts of the plant can effect the change; that leaves pour out the greatest quantity of oxygen from their under surfaces; that the sun by itself has no power to change the composition of air." It will be seen that Priestley started the inquiry, devised and executed the most necessary experiments, and got excellent results. Then he lost his way, and bewildered by conflicting observations, which he was too impatient to reconcile, published a barren and misleading conclusion. Nothing was left for him but to acknowledge that Ingenhousz had cleared up all his perplexities.

Nicholas Theodore de Saussure, son of the Alpine explorer, showed in 1804 that when carbon is separated from the carbonic acid of the air by green plants, the elements of water are also assimilated, a result which owes its importance to the fact that starch is a combination of carbon with the elements of water. Saussure also proved that salts derived from the soil are essential ingredients of plant-food, and that green plants are unable to fix the free nitrogen of the air; all the nitrogen which they require is obtained from the ground.

We are unable to follow the history further. Though the main facts were established as early as the beginning of the nineteenth century, experimental results of scientific and practical interest have never ceased to accumulate down to the present time.

The Metamorphoses of Plants.

Speculations concerning the nature of the flower roused at one time an interest far beyond that felt in most botanical questions. The literary eminence of Goethe, who took a leading part in the discussion, heightened the excitement, and to this day often prompts the inquiry: What does modern science think of the Metamorphoses of Plants?

Let us first briefly notice some anticipations of Goethe's famous essay. In the last years of the sixteenth century Cesalpini, taking a hint from Aristotle, tried to establish a relation between certain parts of the flower and the component layers of the stem. Linnæus worked out the same notion more elaborately, and with a confidence which sought little aid from evidence. His wonderful theory of Prolepsis (Anticipation) need not be described, far less discussed, here. He also borrowed and adapted an analogy which had been thrown out by Swammerdam. The bark of a tree, which according to the theory of Prolepsis gives rise to the calyx of the flower, he compared to the skin of a caterpillar, the expansion of the calyx to the casting of the skin, and the act of flowering to the metamorphosis by which the caterpillar is converted into a moth or butterfly. More rational than the speculations just cited, and more suggestive to the morphologists of the future, are his words: "Principium florum et foliorum idem est" (Flower and leaf have a common origin)—which was not, however, a very novel remark in the eighteenth century. Long before Linnæus early botanists had remarked the resemblance of sepals, petals, and seed-leaves to foliage-leaves; Cesalpini has a common name for all (folium).

At the very time when Linnæus was occupied with his fanciful analogies, a young student of medicine named Caspar Friedrich Wolff, who was destined to become a biologist of great note, published a thesis which he called Theoria Generationis (Halle, 1759). This thesis marks an epoch in the history of animal embryology, but what concerns us here is that Wolff examined the growing shoot, and there studied the development of leaf and flower. He found that in early stages foliage-leaves and floral-leaves may be much alike, and thought that he could trace both to a soft or even fluid substance, which is afterwards converted into a mass of cells. It seemed to him possible to resolve the flowering shoot into stem and leaves only. Wolff's thesis, or at least that part of it which dealt with the plant, was little read and soon forgotten; his studies of the development of animals were carried further and became famous.

Goethe in 1790 revived Wolff's theory of the flower, without suspicion that he had been anticipated. It is only our ignorance, he said, when the fact came to his knowledge, that ever deludes us into believing that we have put forth an original view. As soon as he realised the true state of the case, he spared no pains to do Wolff full justice.

The aim of Goethe's Metamorphoses of Plants was to determine the Idea or theoretical conception of the plant, and also to trace the modifications which the Idea undergoes in nature. These two inquiries constituted what he called the Morphology of the plant, a useful, nay, indispensable term, which is still in daily use. He thought that he could discover in the endless variety of the organs of the flowering plant one structure repeated again and again, which gradually attained, as by the steps of a ladder, what he called the crowning purpose of nature—viz., the sexual propagation of the race. This fundamental structure was the leaf. The proposition that all the parts of the flower are modifications of the leaf he defended by three main arguments—viz., (1) the structural similarity of seed-leaves, foliage-leaves, bracts, and floral organs; (2) the existence of transitions between leaves of different kinds; and (3) the occasional retrogression, as he called it, of specially modified parts to a more primitive condition. These lines of argument were illustrated by many well-chosen examples, the result of long and patient observation. Goethe did not, however, fortify his position by the likeness of developing floral organs to developing foliage-leaves, which had been Wolff's starting-point. He arrived independently at Wolff's opinion that the conversion of foliage-leaves into floral organs is due to diminished nutrition.

Linnæus's exposition of the nature of the flower had been read attentively by Goethe, who must have remarked that the conversion of organs to new uses was there described as a metamorphosis. That word had been, long before the time of Linnæus, appropriated to a particular kind of change—viz., an apparently sudden change occurring in the life-history of one and the same animal. It was therefore unlucky that Goethe should have been led by the example of Linnæus to employ the word in the general sense of adaptation to new purposes. He did not, however, expressly compare flower-production with the transformation of an insect, as Linnæus had done.