Among the many pupils of Johannes Müller who, either during his lifetime or after his death, labored hard for the advancement of the various branches of biology, one of the most fortunate—if not the most important—was Theodor Schwann. When the able botanist Schleiden, in 1838, indicated the cell as the common elementary organ of all plants, and proved that all the different tissues of the plant are merely combinations of cells, Johannes Müller recognized at once the extraordinary possibilities of this important discovery. He himself sought to point out the same composition in various tissues of the animal body—for instance, in the spinal cord of vertebrates—and thus led his pupil, Schwann, to extend the discovery to all the animal tissues. This difficult task was accomplished by Schwann in his Microscopic Researches into the Accordance in the Structure and Growth of Plants and Animals (1839). Thus was the foundation laid of the “cellular theory,” the profound importance of which, both in physiology and anatomy, has become clearer and more widely recognized in each subsequent year. Moreover, it was shown by two other pupils of Johannes Müller that the activity of all organisms is, in the ultimate analysis, the activity of the components of their tissues, the microscopic cells—these were the able physiologist Ernst Brücke, of Vienna, and the distinguished histologist Albert Kölliker, of Würzburg. Brücke correctly denominated the cells the “elementary organisms,” and showed that, in the body of man and of all other animals, they are the only actual, independent factors of the life process. Kölliker earned special distinction, not only in the construction of the whole science of histology, but particularly by showing that the animal ovum and its products are simple cells.
Still, however widely the immense importance of the cellular theory for all biological research was acknowledged, the “cellular physiology” which is based on it only began an independent development very recently. In this Max Verworn (of Jena) earned a twofold distinction. In his Psycho-physiological Studies of the Protistae (1889) he showed, as a result of an ingenious series of experimental researches, that the “theory of a cell-soul” which I put forward in 1866[11] is completely established by an accurate study of the unicellular protozoa, and that “the psychic phenomena of the protistæ form the bridge which unites the chemical processes of inorganic nature with the mental life of the highest animals.” Verworn has further developed these views, and based them on the modern theory of evolution, in his General Physiology. This distinguished work returns to the comprehensive point of view of Johannes Müller, in opposition to the one-sided and narrow methods of those modern physiologists who think to discover the nature of the vital phenomena by the exclusive aid of chemical and physical experiments. Verworn showed that it is only by Müller’s comparative method and by a profound study of the physiology of the cell that we can reach the higher stand-point which will give us a comprehensive survey of the wonderful realm of the phenomena of life. Only thus do we become convinced that the vital processes in man are subject to the same physical and chemical laws as those of all other animals.
The fundamental importance of the cellular theory for all branches of biology was made clear in the second half of the nineteenth century, not only by the rapid progress of morphology and physiology, but also by the entire reform of that biological science which has always been deemed most important on account of its relation to practical medicine—pathology, or the science of disease. Many even of the older physicians were convinced that human diseases were natural phenomena, like all other manifestations of life, and should be studied scientifically, like other vital functions. Particular schools of medicine—the Iatrophysical and the Iatrochemical—had already, in the seventeenth century, attempted to trace the sources of disease to certain physical and chemical changes. However, the imperfect condition of science at that period precluded any lasting results of these efforts. Many of the older theories, which sought the nature of disease in supernatural and mystical causes, were almost universally accepted down to the middle of the nineteenth century.
It was then that Rudolf Virchow, another pupil of Müller, conceived the happy idea of transferring the cellular theory from the healthy to the diseased organism; he sought in the more minute metamorphoses of the diseased cells and the tissues they composed the true source of those larger changes which, in the form of disease, threaten the living organism with peril and death. Especially during the seven years of his professorship at Würzburg (1849-56) Virchow pursued his great task with such brilliant results that his Cellular Pathology (published in 1858) turned, at one stroke, the whole of pathology and the dependent science of practical medicine into new and eminently fruitful paths. This reform of medicine is significant for our present purpose in that it led us to a monistic and purely scientific conception of disease. In sickness, no less than in health, man is subject to the same eternal “iron laws” of physics and chemistry as all the rest of the organic world.
Among the numerous classes of animals which modern zoology distinguishes the mammals occupy a pre-eminent position, not only on morphological grounds, but also for physiological reasons. As man belongs to the class of mammals (see [p. 27]) by every portion of his frame, we must expect him to share his characteristic functions with the rest of the mammals. Such we find to be the case. The circulation of the blood and respiration are accomplished in man under precisely the same laws and in the same manner as in all the other mammals—and in these alone; they are determined by the peculiar structure of their heart and lungs. In mammals only is all the arterial blood conducted from the left ventricle of the heart to the body by one, the left, branch of the aorta, while in birds it passes along the right branch, and in reptiles along both branches. The blood of mammals is distinguished from that of any other vertebrate by the circumstance that its red cells have lost their nucleus (by reversion). The respiratory movements are effected largely by the diaphragm in this class of animals alone, because only in them does it form a complete partition between the pectoral and abdominal cavities. Special importance, however, in this highest class of animals, attaches to the production of milk in the breasts (mammae), and to the peculiar method of the rearing of the young, which entails the supplying of the offspring with the mother’s milk. As this nutritive process reacts most powerfully on the other vital functions, and the maternal affection of mammals must have arisen from this intimate form of rearing, the name of the class justly reminds us of its great importance. In millions of pictures, most of them produced by painters of the highest rank, the “madonna with the child” is revered as the purest and noblest type of maternal love—the instinct which is found in its extreme form in the exaggerated tenderness of the mother-ape.
As the apes approach nearest to man of all the mammals in point of structure, we shall expect to hear the same of their vital functions; and that we find to be the case. Everybody knows how closely the habits, the movements, the sense activity, the mental life, and the parental customs of apes resemble those of man. Scientific physiology proves the same significant resemblance in other less familiar processes, particularly in the working of the heart, the division of the breasts, and the sexual life. In the latter connection it is especially noteworthy that the mature females of many kinds of apes suffer a periodical discharge of blood from the womb, which corresponds to the menstruation of the human female. The secretion of the milk in the glands and the suctorial process also take place in the female ape in precisely the same fashion as in women.
Finally, it is of especial interest that the speech of apes seems on physiological comparison to be a stage in the formation of articulate human speech. Among living apes there is an Indian species which is musical; the hylobates syndactylus sings a full octave in perfectly pure, harmonious half-tones. No impartial philologist can hesitate any longer to admit that our elaborate rational language has been slowly and gradually developed out of the imperfect speech of our Pliocene simian ancestors.