The monumental greatness of Charles Darwin, who surpasses every other student of science in the nineteenth century by the loftiness of his monistic conception of nature and the progressive influence of his ideas, is perhaps best seen in the fact that not one of his many successors has succeeded in modifying his theory of descent in any essential point or in discovering an entirely new standpoint in the interpretation of the organic world. Neither Nageli nor Weismann, neither De Vries nor Roux, has done this. Nageli, in his "Mechanisch-Physiologische Theorie der Abstammungslehre" (Munich, 1884.), which is to a great extent in agreement with Weismann, constructed a theory of the idioplasm, that represents it (like the germ-plasm) as developing continuously in a definite direction from internal causes. But his internal "principle of progress" is at the bottom just as teleological as the vital force of the Vitalists, and the micellar structure of the idioplasm is just as hypothetical as the "dominant" structure of the germ-plasm. In 1889 Moritz Wagner sought to explain the origin of species by migration and isolation, and on that basis constructed a special "migration-theory." This, however, is not out of harmony with the theory of selection. It merely elevates one single factor in the theory to a predominant position. Isolation is only a special case of selection, as I had pointed out in the fifteenth chapter of my "Natural history of creation". The "mutation-theory" of De Vries ("Die Mutationstheorie", Leipzig, 1903.), that would explain the origin of species by sudden and saltatory variations rather than by gradual modification, is regarded by many botanists as a great step in advance, but it is generally rejected by zoologists. It affords no explanation of the facts of adaptation, and has no causal value.
Much more important than these theories is that of Wilhelm Roux ("Der Kampf der Theile im Organismus", Leipzig, 1881.) of "the struggle of parts within the organism, a supplementation of the theory of mechanical adaptation."He explains the functional autoformation of the purposive structure by a combination of Darwin's principle of selection with Lamarck's idea of transformative heredity, and applies the two in conjunction to the facts of histology. He lays stress on the significance of functional adaptation, which I had described in 1866, under the head of cumulative adaptation, as the most important factor in evolution. Pointing out its influence in the cell-life of the tissues, he puts "cellular selection" above "personal selection," and shows how the finest conceivable adaptations in the structure of the tissue may be brought about quite mechanically, without preconceived plan. This "mechanical teleology" is a valuable extension of Darwin's monistic principle of selection to the whole field of cellular physiology and histology, and is wholly destructive of dualistic vitalism.
The most important advance that evolution has made since Darwin and the most valuable amplification of his theory of selection is, in my opinion, the work of Richard Semon: "Die Mneme als erhaltendes Prinzip im Wechsel des organischen Geschehens" (Leipzig, 1904.). He offers a psychological explanation of the facts of heredity by reducing them to a process of (unconscious) memory. The physiologist Ewald Hering had shown in 1870 that memory must be regarded as a general function of organic matter, and that we are quite unable to explain the chief vital phenomena, especially those of reproduction and inheritance, unless we admit this unconscious memory.
In my essay "Die Perigenesis der Plastidule" (Berlin, 1876.) I elaborated this far-reaching idea, and applied the physical principle of transmitted motion to the plastidules, or active molecules of plasm. I concluded that "heredity is the memory of the plastidules, and variability their power of comprehension." This "provisional attempt to give a mechanical explanation of the elementary processes of evolution" I afterwards extended by showing that sensitiveness is (as Carl Nageli, Ernst Mach, and Albrecht Rau express it) a general quality of matter. This form of panpsychism finds its simplest expression in the "trinity of substance."To the two fundamental attributes that Spinoza ascribed to substance--Extension (matter as occupying space) and Cogitation (energy, force)--we now add the third fundamental quality of Psychoma (sensitiveness, soul). Ifurther elaborated this trinitarian conception of substance in the nineteenth chapter of my "Die Lebenswunder" (1904) ("Wonders of Life", London, 1904.), and it seems to me well calculated to afford a monistic solution of many of the antitheses of philosophy.
This important Mneme-theory of Semon and the luminous physiological experiments and observations associated with it not only throw considerable light on transformative inheritance, but provide a sound physiological foundation for the biogenetic law. I had endeavoured to show in 1874, in the first chapter of my "Anthropogenie" (English translation; "The Evolution of Man", 2 volumes, London, 1879 and 1905.), that this fundamental law of organic evolution holds good generally, and that there is everywhere a direct causal connection between ontogeny and phylogeny.
"Phylogenesis is the mechanical cause of ontogenesis"; in other words, "The evolution of the stem or race is--in accordance with the laws of heredity and adaptation--the real cause of all the changes that appear, in a condensed form, in the development of the individual organism from the ovum, in either the embryo or the larva."It is now fifty years since Charles Darwin pointed out, in the thirteenth chapter of his epoch-making "Origin of Species", the fundamental importance of embryology in connection with his theory of descent: