In the early twentieth century, Paul Kammerer conducted a series of experiments to demonstrate that organisms could transmit characteristics acquired in their lifetimes to their offspring. In his 1809 publication, zoologist Jean-Baptiste Lamarck had hypothesized that living beings can inherit features their parents or ancestors acquired throughout life. By breeding salamanders, as well as frogs and other organisms, Kammerer tested Lamarck's hypothesis in an attempt to provide evidence for Lamarck's theory of the inheritance of acquired characteristics. In particular, Kammerer argued that the inheritance of acquired characteristics caused species to evolve, and he claimed that his results provided an explanation for evolutionary processes through developmental phenomena.

In the first decade of the twentieth century, Paul Kammerer, a zoologist working at the Vivarium in Vienna, Austria, conducted research on developmental mechanisms, including a series of breeding experiments on toads (Alytes obstetricans). Kammerer claimed that his results demonstrated that organisms could transmit acquired characteristics to their offspring. To explain how evolution occurred, biologist Jean-Baptiste Lamarck in France suggested in his 1809 book that offspring inherited the features their ancestors acquired throughout the lives of those ancestors, a process termed the inheritance of acquired characteristics. Kammerer conducted breeding experiments to test the theory of inheritance of acquired characteristics, which he said described the mechanics of evolution. Additionally, Kammerer's experiments aimed at explaining how development shaped evolutionary processes.

Friedrich Leopold August Weismann published Das
Keimplasma: eine Theorie der Vererbung (The Germ-Plasm: a
Theory of Heredity, hereafter The Germ-Plasm) while
working at the University of Freiburg in Freiburg, Germany in 1892.
William N. Parker, a professor in the University College of South
Wales and Monmouthshire in Cardiff, UK, translated The
Germ-Plasm into English in 1893. In The Germ-Plasm,
Weismann proposed a theory of heredity based on the concept of the
germ plasm, a substance in the germ cell that carries hereditary information. The
Germ-Plasm compiled Weismann's theoretical work and analyses of
other biologists' experimental work in the 1880s, and it provided a
framework to study development, evolution and heredity. Weismann
anticipated that the germ-plasm theory would enable researchers to
investigate the functions and material of hereditary substances.

In the early twentieth century, Paul Kammerer, a zoologist working at the Vivarium in Vienna, Austria, experimented on sea-squirts (Ciona intestinalis). Kammerer claimed that results from his experiments demonstrated that organisms could transmit characteristics that they had acquired in their lifetimes to their offspring. Kammerer conducted breeding experiments on sea-squirts and other organisms at a time when Charles Darwin's 1859 theory of evolution lacked evidence to explain how offspring inherited traits from their parents. In 1809, zoologist Jean-Baptiste Lamarck in France theorized that living beings can inherit the features their parents or ancestors acquired during those ancestor's lifetime, a theory called the inheritance of acquired characteristics. Kammerer attempted to provide evidence for the theory of inheritance of acquired characteristics, which constituted, he argued, the mechanics of evolution. Kammerer claimed that his results could explain evolutionary processes through developmental phenomena.

Ontogeny and Phylogeny is a book published in 1977, in which the author Stephen J. Gould, who worked in the US, tells a history of the theory of recapitulation. A theory of recapitulation aims to explain the relationship between the embryonic development of an organism (ontogeny) and the evolution of that organism's species (phylogeny). Although there are several variations of recapitulationist theories, most claim that during embryonic development an organism repeats the adult stages of organisms from those species in it's evolutionary history. Gould suggests that, although fewer biologists invoked recapitulation theories in the twentieth century compared to those in the nineteenth and eighteenth centuries, some aspects of the theory of recapitulation remained important for understanding evolution. Gould notes that the concepts of acceleration and retardation during development entail that changes in developmental timing (heterochrony) can result in a trait appearing either earlier or later than normal in developmental processes. Gould argues that these changes in the timing of embryonic development provide the raw materials or novelties upon which natural selection acts.

In 2004, a team of researchers at Tufts-New England
Medical Center in Boston, Massachusetts, investigated the fetal
cells that remained in the maternal blood stream after pregnancy.
The results were published in Transfer of Fetal Cells with
Multilineage Potential to Maternal Tissue. The team working on that
research included Kiarash Khosrotehrani, Kirby L. Johnson, Dong
Hyun Cha, Robert N. Salomon, and Diana W. Bianchi. The researchers
reported that the fetal cells passed to a pregnant woman during
pregnancy could develop into multiple cell types in her organs. They
studied these differentiated fetal cells in a cohort of women
fighting different diseases. The researchers found that the fetal
cells in the women differentiated into different cell types under
the influence of maternal tissues, and that those differentiated
cells concentrated in the tissue surrounding diseased tissues.
According to the team, this response could be a therapeutic response
to the disease in the once pregnant woman. The research indicated the long
lasting effects of pregnancy in a woman's body.

The Spandrels of San Marco and the Panglossian Paradigm:
A Critique of the Adaptationist Programme, hereafter called
The Spandrels, is an article written by Stephen J. Gould and
Richard C. Lewontin published in the Proceedings of the Royal
Society of London in 1979. The paper emphasizes issues with
what the two authors call adaptationism or the adaptationist
programme as a framework to explain how species and traits evolved. The paper
is one in a series of works in which Gould emphasized the
role of development in evolutionary theories. The article suggests
that constraints on how organisms can develop and constraints on how species can evolve from others play a
central role in explaining the how species and traits evolve. The
authors note that organisms from different species develop as
embryos through stages similar across species, genera, and higher
classes. Gould and Lewontin hypothesize that those stages
constrained the possible pathways of evolution and has therefore
guided the history of life. Throughout the paper, the authors rely on analogy of some parts of organisms to architectural structures called spandrels, marked in this image as 'a'."

Paul Kammerer conducted experiments on amphibians and marine animals at the Vivarium, a research institute in Vienna, Austria, in the early twentieth century. Kammerer bred organisms in captivity, and he induced them to develop particular adaptations, which Kammerer claimed the organismss offspring would inherit. Kammerer argued that his results demonstrated the inheritance of acquired characteristics, or Lamarckian inheritance. The Lamarckian theory of inheritance posits that individuals transmit acquired traits to their offspring. Kammerer worked during a period in which scientists debated how variation between organisms and within species was caused, and how organisms could inherit that variation from their parents. Kammerer contended that the inheritance of acquired characteristics occurs during embryological development, but several scientists argued that he provided poor evidence for his claims.

In 1868 in England, Charles Darwin proposed his pangenesis theory to describe the units of inheritance between parents and offspring and the processes by which those units control development in offspring. Darwin coined the concept of gemmules, which he said referred to hypothesized minute particles of inheritance thrown off by all cells of the body. The theory suggested that an organism's environment could modify the gemmules in any parts of the body, and that these modified gemmules would congregate in the reproductive organs of parents to be passed on to their offspring. Darwin's theory of pangenesis gradually lost popularity in the 1890s when biologists increasingly abandoned the theory of inheritance of acquired characteristics (IAC), on which the pangenesis theory partially relied. Around the turn of the twentieth century, biologists replaced the theory of pangenesis with germ plasm theory and then with chromosomal theories of inheritance, and they replaced the concept of gemmules with that of genes.

Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs is the thirteenth chapter of Charles Darwin's book The Origin of Species, first published in England in 1859. The book details part of Darwin's argument for the common ancestry of life and natural selection as the cause of speciation. In this chapter, Darwin summarizes the evidence for evolution by connecting observations of development in organisms to the processes of natural selection. Darwin shows how the theory of special creation, which claims that God directly created all organisms in their current form, is inferior to the theory of natural selection for its ability to explain the diversity of life. In this chapter, Darwin also discusses classification and homology as they relate to natural selection.