The Riddle of the Universe at the close of the nineteenth century

by Ernst Haeckel

Every effort of genuine science makes for a knowledge of the truth. Our only real and valuable knowledge is a knowledge of nature itself, and consists of presentations which correspond to external things. We are incompetent, it is true, to penetrate into the innermost nature of this real world—the “thing in itself”—but impartial critical observation and comparison inform us that, in the normal action of the brain and the organs of sense, the impressions received by them from the outer world are the same in all rational men, and that in the normal function of the organs of thought certain presentations are formed which are everywhere the same. These presentations we call true, and we are convinced that their content corresponds to the knowable aspect of things. We know that these facts are not imaginary, but real.

All knowledge of the truth depends on two different, but intimately connected, groups of human physiological functions: firstly, on the sense-impressions of the object by means of sense-action, and, secondly, on the combination of these impressions by an association into presentations in the subject. The instruments of sensation are the sense-organs (sensilla or aestheta); the instruments which form and link together the presentations are the organs of thought (phroneta). The latter are part of the central, and the former part of the peripheral, nervous system—that important and elaborate system of organs in the higher animals which alone effects their entire psychic activity.

Man’s sense-activity, which is the starting-point of all knowledge, has been slowly and gradually developed from that of his nearest mammal relatives, the primates. The sense-organs are of substantially the same construction throughout this highest animal group, and their function takes place always according to the same physical and chemical laws. They have had the same historical development in all cases. In the mammals, as in the case of all other animals, the sensilla were originally parts of the skin; the sensitive cells of the epidermis are the sources of all the different sense-organs, which have acquired their specific energy by adaptation to different stimuli (light, heat, sound, chemical action, etc.). The rod-cells in the retina of the eye, the auditory cells in the cochlea of the ear, the olfactory cells in the nose, and the taste-cells on the tongue, are all originally derived from the simple, indifferent cells of the epidermis, which cover the entire surface of the body. This significant fact can be directly proved by observation of the embryonic development of man or any of the higher animals. And from this ontogenetic fact we confidently infer, in virtue of the great biogenetic law, the important phylogenetic proposition, that in the long historical evolution of our ancestors, likewise, the higher sense-organs with their specific energies were originally derived from the epidermis of lower animals, from a simple layer of cells which had no trace of such differentiated sensilla.

A particular importance attaches to the circumstance that different nerves are qualified to perceive different properties of the environment, and these only. The optic nerve accomplishes only the perception of light, the auditory nerve the perception of sound, the olfactory nerve the perception of smell, and so on. No matter what stimuli impinge on and irritate a given sense-organ, its reaction is always of the same character. From this specific energy of the sense-nerves, which was first fully appreciated by Johannes Müller, very erroneous inferences have been drawn, especially in favor of a dualistic and à priori theory of knowledge. It has been affirmed that the brain, or the soul, only perceives a certain condition of the stimulated nerve, and that, consequently, no conclusion can be drawn from the process as to the existence and nature of the stimulating environment. Sceptical philosophy concluded that the very existence of an outer world is doubtful, and extreme idealism went on positively to deny it, contending that things only exist in our impressions of them.

In opposition to these erroneous views, we must recall the fact that the “specific energy” was not originally an innate, special quality of the various nerves, but it has arisen by adaptation to the particular activity of the epidermic cells in which they terminate. In harmony with the great law of “division of labor” the originally indifferent “sense-cells of the skin” undertook different tasks, one group of them taking over the stimulus of the light rays, another the impress of the sound waves, a third the chemical impulse of odorous substances, and so on. In the course of a very long period these external stimuli effected a gradual change in the physiological, and later in the morphological, properties of these parts of the epidermis, and there was a correlative modification of the sensitive nerves which conduct the impressions they receive to the brain. Selection improved, step by step, such particular modifications as proved to be useful, and thus eventually, in the course of many million years, created those wonderful instruments, the eye and the ear, which we prize so highly; their structure is so remarkably purposive that they might well lead to the erroneous assumption of a “creation on a preconceived design.” The peculiar character of each sense-organ and its specific nerve has thus been gradually evolved by use and exercise—that is, by adaptation—and has then been transmitted by heredity from generation to generation. Albrecht Rau has thoroughly established this view in his excellent work on Sensation and Thought, a physiological inquiry into the nature of the human understanding (1896). It points out the correct significance of Müller’s law of specific sense-energies, adding searching investigations into their relation to the brain, and in the last chapter there is an able “philosophy of sensitivity” based on the ideas of Ludwig Feuerbach. I thoroughly agree with his convincing work.

Critical comparison of sense-action in man and the other vertebrates has brought to light a number of extremely important facts, the knowledge of which we owe to the penetrating research of the nineteenth century, especially of the second half of the century. This is particularly true of the two most elaborate “æsthetic” organs, the eye and the ear. They present a different and more complicated structure in the vertebrates than in the other animals, and have also a characteristic development in the embryo. This typical ontogenesis and structure of the sensilla of all the vertebrates is only explained by heredity from a common ancestor. Within the vertebrate group, however, we find a great variety of structure in points of detail, and this is due to adaptation to their manner of life on the part of the various species, to the increasing or diminishing use of various parts.

In respect of the structure of his sense-organs man is by no means the most perfect and most highly-developed vertebrate. The eye of the eagle is much keener, and can distinguish small objects at a distance much more clearly than the human eye. The hearing of many mammals, especially of the carnivora, ungulata, and rodentia of the desert, is much more sensitive than that of man, and perceives slight noises at a much greater distance; that may be seen at a glance by their large and very sensitive cochlea. Singing birds have attained a higher grade of development, even in respect of musical endowment, than the majority of men. The sense of smell is much more developed in most of the mammals, especially in the carnivora and the ungulata, than in man; if the dog could compare his own fine scent with that of man, he would look down on us with compassion. Even with regard to the lower senses—taste, sex-sense, touch, and temperature—man has by no means reached the highest stage in every respect.

We can naturally only pass judgment on the sensations which we ourselves experience. However, anatomy informs us of the presence in the bodies of many animals of other senses than those we are familiar with. Thus fishes and other lower aquatic vertebrates have peculiar sensilla in the skin which are in connection with special sense-nerves. On the right and left sides of the fish’s body there is a long canal, branching into a number of smaller canals at the head. In this “mucous canal” there are nerves with numerous branches, the terminations of which are connected with peculiar nerve-aggregates. This extensive epidermic sense-organ probably serves for the perception of changes in the pressure, or in other properties, of the water. Some groups are distinguished by the possession of other peculiar sensilla, the meaning of which is still unknown to us.