Last Words on Evolution: A Popular Retrospect and Summary

by Ernst Haeckel

In the previous chapters I have tried to give a general idea of the present state of the theory of evolution and its victorious struggle with the older legend of creation. We have seen that even the most advanced organism, man, was not brought into being by a creative act, but gradually developed from a long series of mammal ancestors. We also saw that the most man-like mammals, the anthropoid apes, have substantially the same structure as man, and that the evolution of the latter from the former can now be regarded as a fully established hypothesis, or, rather, an historical fact. But in this study we had in view mainly the structure of the body and its various organs. We touched very briefly on the evolution of the human mind, or the immaterial soul that dwells in the body for a time, according to a venerable tradition. To-day we turn chiefly to the development of the soul, and consider whether man's mental development is controlled by the same natural laws as that of his body, and whether it also is inseparably bound up with that of the rest of the mammals.

At the very threshold of this difficult province we encounter the curious fact that there are two radically distinct tendencies in psychology at our universities to-day. On one side we have the metaphysical and professional psychologists. They still cling to the older view that man's soul is a special entity, a unique independent individuality, which dwells for a time only in the mortal frame, leaving it and living on as an immortal spirit after death. This dualistic theory is connected with the doctrine of most religions, and owes its high authority to the fact that it is associated with the most important ethical, social, and practical interests. Plato gave prominence to the idea of the immortality of the soul in philosophy long ago. Descartes at a later date gave emphasis to it by ascribing a true soul to man alone and refusing it to the animals.

This metaphysical psychology, which ruled alone for a considerable period, began to be opposed in the eighteenth, and still more in the nineteenth, century by comparative psychology. An impartial comparison of the psychic processes in the higher and lower animals proved that there were numerous transitions and gradations. A long series of intermediate stages connects the psychic life of the higher animals with that of man on the one side, and that of the lower animals on the other. There was no such thing as a sharp dividing line, as Descartes supposed.

But the greatest blow was dealt at the predominant metaphysical conception of the life of the soul thirty years ago by the new methods of psychophysics. By means of a series of able experiments the physiologists, Theodor Fechner and Ernst Heinrich Weber of Leipsic, showed that an important part of the mental activity can be measured and expressed in mathematical formulæ just as well as other physiological processes, such as muscular contractions. Thus the laws of physics control a part of the life of the soul just as absolutely as they do the phenomena of inorganic nature. It is true that psychophysics has only partially realised the very high expectations that were entertained in regard to its Monistic significance; but the fact remains that a part of the mental life is just as unconditionally ruled by physical laws as any other natural phenomena.

Thus physiological psychology was raised by psychophysics to the rank of a physical and, in principle, exact science. But it had already obtained solid foundations in other provinces of biology. Comparative psychology had traced connectedly the long gradation from man to the higher animals, from these to the lower, and so on down to the very lowest. At the lowest stage it found those remarkable beings, invisible with the naked eye, that were discovered in stagnant water everywhere after the invention of the microscope (in the second half of the seventeenth century) and called "infusoria." They were first accurately described and classified by Gottfried Ehrenberg, the famous Berlin microscopist. In 1838 he published a large and beautiful work, illustrating on 64 folio pages the whole realm of microscopic life; and this is still the base of all studies of the protists. Ehrenberg was a very ardent and imaginative observer, and succeeded in communicating his zeal for the study of microscopic organisms to his pupils. I still recall with pleasure the stimulating excursions that I made fifty years ago (in the summer of 1854) with my teacher, Ehrenberg, and a few other pupils—including my student-friend, Ferdinand von Richthofen, the famous geographer—to the Zoological Gardens at Berlin. Equipped with fine nets and small glasses, we fished in the ponds of the Zoological Gardens and in the Spree, and caught thousands of invisible micro-organisms, which then richly rewarded our curiosity by the beautiful forms and mysterious movements they disclosed under the microscope.

The way in which Ehrenberg explained to us the structure and the vital movements of his infusoria was very curious. Misled by the comparison of the real infusoria with the microscopic but highly organised rotifers, he had formed the idea that all animals are alike advanced in organisation, and had indicated this erroneous theory in the very title of his work: The Infusoria as Perfect Organisms: a Glance at the Deeper Life of Organic Nature. He thought he could detect in the simplest infusoria the same distinct organs as in the higher animals—stomach, heart, ovaries, kidneys, muscles, and nerves—and he interpreted their psychic life on the same peculiar principle of equally advanced organisation.

Ehrenberg's theory of life was entirely wrong, and was radically destroyed in the hour of its birth (1838) by the cell-theory which was then formulated, and to which he never became reconciled. Once Matthias Schleiden had shown the composition of all the plants, tissues, and organs from microscopic cells, the last structural elements of the living organism, and Theodor Schwann had done the same for the animal body, the theory attained such an importance that Kölliker and Leydig based on it the modern science of tissues, or histology, and Virchow constructed his cellular pathology by applying it to diseased human beings. These are the most important advances of theoretical medicine. But it was still a long time before the difficult question of the relation of these microscopic beings to the cell was answered. Carl Theodor von Siebold had already maintained (in 1845) that the real infusoria and the closely related rhizopods were unicellular organisms, and had distinguished these protozoa from the rest of the animals. At the same time, Carl Naegeli had described the lowest algæ as "unicellular plants." But this important conception was not generally admitted until some time afterwards, especially after I brought all the unicellular organisms under the head of "protists" (1872), and defined their psychic functions as the "cell-soul."

I was led to make a very close study of these unicellular protists and their primitive cell-soul through my research on the radiolaria, a very remarkable class of microscopic organisms that float in the sea. I was engaged most of my time for more than thirty of the best years of my life (1856-87) in studying them in every aspect, and if I came eventually to adopt a strictly Monistic attitude on all the great questions of biology, I owe it for the most part to my innumerable observations and uninterrupted reflections on the wonderful vital movements that are disclosed by these smallest and frailest, and at the same time most beautiful and varied, of living things.

I had undertaken the study of the radiolaria as a kind of souvenir of my great master, Johannes Müller. He had loved to study these animals (of which only a few species were discovered for the first time in the year of my birth, 1834) in the last years of his life, and had in 1855 set up the special group of the rhizopods (protozoa). His last work, which appeared shortly after his death (1858), and contained a description of 50 species of radiolaria, went with me to the Mediterranean when I made my first long voyage in the summer of 1859. I was so fortunate as to discover about 150 new species of radiolaria at Messina, and based on these my first monograph of this very instructive class of protists (1862). I had no suspicion at that time that fifteen years afterwards the deep-sea finds of the famous Challenger expedition would bring to light an incalculable wealth of these remarkable animals. In my second monograph on them (1887), I was able to describe more than 4,000 different species of radiolaria, and illustrate most of them on 140 plates. I have given a selection of the prettiest forms on ten plates of my Art-forms in Nature.

I have not space here to go into the forms and vital movements of the radiolaria, of the general import of which my friend, Wilhelm Bölsche, has given a very attractive account in his various popular works. I must restrict myself to pointing out the general phenomena that bear upon our particular subject, the question of the mind. The pretty flinty skeletons of the radiolaria, which enclose and protect the soft unicellular body, are remarkable, not only for their extraordinary gracefulness and beauty, but also for the geometrical regularity and relative constancy of their forms. The 4,000 species of radiolaria are just as constant as the 4,000 known species of ants; and, as the Darwinian Jesuit, Father Wasmann, has convinced himself that the latter have all descended by transformation from a common stem-form, I have concluded on the same principles that the 4,000 species of radiolaria have developed from a primitive form in virtue of adaptation and heredity. This primitive form, the stem-radiolarian (Actissa) is a simple round cell, the soft living protoplasmic body of which is divided into two different parts, an inner central capsule (in the middle of which is the solid round nucleus) and an outer gelatinous envelope (calymma). From the outer surface of the latter, hundreds and thousands of fine plasmic threads radiate; these are mobile and sensitive processes of the living internal substance, the plasm (or protoplasm). These delicate microscopic threads, or pseudopodia, are the curious organs that effect the sensations (of touch), the locomotion (by pushing), and the orderly construction of the flinty house; at the same time, they maintain the nourishment of the unicellular body, by seizing infusoria, diatoms, and other protists, and drawing them within the plasmic body, where they are digested and assimilated. The radiolaria generally reproduce by the formation of spores. The nucleus within the protoplasmic globule divides into two small nuclei, each of which surrounds itself with a quantity of plasm, and forms a new cell.