August Weismann (1834–1914), one of the greatest evolutionists of the nineteenth century, is considered second only to Charles Darwin. He was born in Frankfurt am Main in the Germanic state of Hessen, on 17 January 1834 and died on 5 November 1914. Weismann studied medicine at Goettingen between 1852 and1856 and graduated with a highly praised dissertation: De acidi hippurici in corpora humano generatione. Dissertatio inauguralis Goettingen (The origin of hipuric acid in the human body) (Weismann Reference Weismann1857).Footnote ¹ After several years of medical practice, his dislike of clinical work led him to turn to biological research, which enabled him to investigate questions pertaining to embryological development. His first studies were embryological works of the dipterans. During these investigations, in the 1860s, he made important discoveries, discovering the pole cells and the imaginal discs.Footnote ² His empirical discoveries and the conclusions he reached on the basis of his studies had an impact on his future experimental work which eventually led to the consolidation of his theories on heredity and evolution. He gained his habilitation in 1863 at the University of Freiburg i. Br and stayed there until his death.

Weismann became a convinced Darwinist in 1861 after he read Darwin's Origin of Species, and most of his subsequent experimental and theoretical studies were inspired by Darwin's evolutionary theory. Evolutionary theory and in particular Darwin's theory of natural selection, was the subject of the public lecture “Über die Berechtigung der Darwin'schen Theory” which he delivered at the inauguration of the Zoological Institute in the auditorium of the Freiburg i. Br. University on the 7^th of July 1868. He was the first occupant of the newly created chair of zoology at the Institute, and he also became a regular Extraordinarius in the philosophical faculty of Freiburg i. Br. This lecture was his first opportunity to publicly discuss some of the questions posed by Darwin on the role of natural selection in evolution. He sent his lecture (in German) to Charles Darwin in 1868, but it was lost, and Darwin asked for another copy. The first translation of this lecture into English is presented in this issue (pages 181–201).

The lecture was prepared and delivered during a period when Weismann had to give up microscopy because of his eye problem. In his autobiography he described the years between 1864 and 1874, when he could not make microscopic examinations:

Weismann had met Ernst Haeckel (1834–1919) around 1864. During the exchange of letters between the two men, Haeckel, who was aware of Weismann's poor eye condition, encouraged him to consider the theoretical analysis of natural phenomena using the criteria set by Darwin's theory of descent. He wrote to Weismann in a letter from 29 October 1865:

In a letter sent to Haeckel on 23 July 1868, Weismann told him about his lecture:

Weismann's inaugural lecture is of historical interest because it contains the seeds of his ideas and sheds light on the development of his later theories. At the age of 34, at the very beginning of his academic career, Weismann engaged in a debate about evolution and argued for the centrality of natural selection in evolution. He defended these ideas in the decade when Darwin's theory of evolution through natural selection was met with resistance in some circles of the scientific community in Germany. The opponents of Darwin's theory belonged to the older generation. Most were in their 60s, held well-established positions, and were orthodox in their outlook. Their beliefs oscillated between indifference towards religion, as with Albert Kölliker or Rudolph Virchow (who was a non-believer but objected to debating religion in public), and the views of those who thought that species transmutation required the direct intervention of the Creator. In general, however, Darwin's central idea of descent with modification was widely accepted when Weismann delivered his inaugural lecture in 1868. It was the mechanism of evolution, natural selection, that still raised many objections. The influence of typology and idealism, together with a distaste for non-purposive and random mechanisms, made natural selection unacceptable. However, Weismann was committed not only to transformism, but also to Darwin's theory of natural selection, which he expressed strongly in the inaugural lecture. He argued against any teleological process, opposed any view of a gradual ascent of life toward progressively complex forms, and argued against any vitalistic view of variation caused by inner forces.

In his public lecture, Weismann started by arguing against the holders of belief in creation by a supernatural power. He compared the scientific characteristics of Darwin's natural selection theory to the Copernican revolution, which made Darwin's theory the only scientific foundation on which biological phenomena could be understood. Moreover, he outlined the main elements of his own thoughts about evolution, which later developed into his theory of heredity-development-evolution. For example, the lecture contains the first versions of his ideas about the causes of variation, the importance of developmental constraints and the nature of the organism in evolution, and the nature of heredity.

When Weismann delivered his lecture the interest in Darwinism was already ignited in Germany. Heinrich Georg Bronn (1800–1862) translated Darwin's Origin, and Ernst Haeckel enthusiastically promoted Darwinism (Haeckel Reference Haeckel1866).Footnote ³ Weismann did not concern himself with the issue of the origin of life, which bothered Bronn, and started from Haeckel's standpoint, which did not require an assumption about an initial creative force. However, he did try to deal with some of Bronn's concerns, especially those related to the issue of intermediate links between different groups and the limits of variation. He realized that Darwin's theory could not be vindicated until the causes of variation had been clarified and the right relation between chance and necessity had been established. The resolution of these questions was crucial for the acceptance by naturalists of evolution driven by natural selection.

Weismann considered variation to be the result of a combination of an inherited developmental direction (Entwicklungsrichtung) and the effects of external influences. Thus, the direction of variation was itself variable, but was limited by the specific nature of the organism. Weismann argued that species are the result of both natural selection and the nature of variation of their ancestors, since natural selection chooses only among pre-existing variations. In this context the role of external influences was two-fold: first as a direct modifier of heredity, and second as a process regulating variation. The latter, he claimed, was natural selection. Thus, the struggle for existence was the driving force in evolution. Later on Weismann carried out empirical research on the effect of external conditions on the marking and coloration in seasonally dimorphic butterflies trying to rule out an inner developmental vitalistic force. He returned to this kind of experiment along most of his career in his endeavor to foster his belief that evolutionary change was gradual and was dependent on variation which occurred at the level of nuclear components. Moreover, with the consolidation of his theory of heredity, Weismann formulated the concept of reduction division during which molecular complexes separate, resulting in the creation of non-identical gametes. He regarded this type of division to be a source of hereditary germ line variation on which natural selection operates. Recombination of different genotypes during sexual reproduction increased the range of variation.

Weismann showed an early understanding of the developmental constraints on evolution. In his lecture in 1868 he suggested that the physical and physiological constitution of the organism was fixed in its developmental direction and, though it had a wide margin of possible variations in all directions, it was nonetheless constrained. These limits were very difficult to transgress. This idea is reminiscent of the traditional study of “varieties,” based on the work of Linnaeus, according to which species were not considered in terms of absolutely rigid characters, but as spheres of variation with limits which could not be transcended (Gayon Reference Gayon1998, 34). However, unlike Linnaeus, Weismann believed that over very long periods of time, very gradually, extensive evolutionary transformations can occur because developmental constraints themselves evolve. The nature of the organism and the constraints on its development can therefore change during evolution. Weismann adhered to his views and in his essay on “The Significance of Sexual Reproduction in the Theory of Natural Selection,” in 1885, he argued:

Only in the late 1970s with the re-emergence of developmental perspective on evolution (Evo-Devo) were these insights recognized and extended.

Weismann understood from the beginning that heredity was central for a theory of evolution. He thought that what was missing in Darwin's theory was a good explanation of heredity. In the inaugural lecture he defined heredity as the transmission of parental developmental tendencies to the offspring, and he believed that its extent and range were determined by the developmental direction inherent in the organism. At this time Weismann accepted epigenesis, the idea that a formative matter was passed from the parent to the offspring. This idea was first expressed by Kaspar Wolff, and Haeckel promoted it in his Generelle Morphologie. Weismann's theory of the continuity of the germ plasm has its roots in this conceptualization, since it accounted not only for heredity but also for the development of the individual.

Similarities in the adult forms of parent and offspring are caused by similarities in their patterns of differentiation, and heredity was considered to be an aspect of development. Such hereditary transmission of developmental tendencies was possible because of the material continuity between generations, which was discussed by Haeckel (Weismann Reference Weismann1868, 26, n. 23).Footnote ⁴ Weismann elaborated on Haeckel's idea: “I believe, however, that heredity is the communication of a very specific developmental direction to the germ of the organism through the mixture of its components; the same developmental direction is possessed by the parental organism at the beginning” (Weismann 1865, 193). Weismann developed his views which originally implied a continuity of germ cells into a continuity of germ substance, germ plasm, and arrived finally at the idea that it was the germ plasm that was the bearer of heredity. He assumed that part of the germ plasm was in charge of maintaining species’ characteristics and passed unchanged to the offspring through the germ cells. Another part played a regulative role as it participated in development and was used up. Weismann's first comprehensive synthesis The germ plasm. A theory of heredity, was published in 1893. It was a coherent and complex theoretical model presenting Weismann's views of the structure of the hereditary apparatus, the mechanism of hereditary transmission and nucleus-directed development. Weismann synthesized his and others’ views of vital hereditary particles: he suggested a definite and complex architecture for the germ plasm, the components of which participated in all nuclear processes. Only such a definite structure could in his view, account for the phenomenon of development, differentiation, and evolution.

Weismann added an appendix to the printed version of the inaugural lecture. In it he reacted to Moritz Wagner's essay, The Darwinian Theory and the Migration Law of the Organisms, published in 1868 (Wagner Reference Wagner and Laird1868). This appendix provided Weismann with the opportunity to focus on the factors leading to speciation, which he recognized as crucial for legitimizing Darwin's theory, and on the role of isolation in this process. Wagner suggested that the isolation of a colony from its original source is a preliminary and necessary condition for speciation. Weismann accepted a secondary role for isolation, but argued that natural selection is the primary driving force of speciation, and is always a necessary and often a sufficient reason for its occurrence. This challenge to Wagner's ideas started a debate between the two naturalists over the mechanism of evolution, and led to the crystallization of Weismann's ideas on the central and essential role of selection, both natural and sexual, in all processes of evolution.Footnote ⁵ The debate between Wagner and Weismann is an introduction to Weismann's contribution to evolutionary thinking because it reveals his selectionistic agenda and his complex mode of theorizing. Throughout his life, Weismann retained his conviction that natural selection for survival, for functional traits, is necessary for the attainment of a distinct status which taxonomists will accept as a species. The utilitarian principle of natural selection, which stressed the usefulness of each character as it contributed to the adaptation of the species to its environment, was for him a central explanatory principle for evolutionary change, and the most important element driving his thought. Weismann defended selectionism all his life, subordinated all his evolutionary considerations to selection, leaving no aspect of the history of life that would not be explained by selectionistic processes.

At a later stage in his theorizing he also added germinal selection (a selection at the molecular level of the components of the hereditary substance, the determinants) as an additional selection process that can initiate or accelerate the fixation of characters in the process of speciation (Weismann Reference Weismann and McCormack1896).