| Summary: | This thesis presents an examination of the notion of 'normal development' and its role in biological research. It centres on a detailed historical analysis of the experimental embryological work of the American biologist Edmund Beecher Wilson in the early-1890s. Normal development is a fundamental concept in biology, which underpins and facilitates experimental work investigating the processes of organismal development. Concepts of the normal and normality in biology (and medicine) have been fruitfully examined by philosophers. Yet, despite being constantly used and invoked by developmental biologists, the concept of normal development has not been subject to substantial philosophical attention. In this thesis I analyse how the concept of normal development is produced and used in experimental systems, and use this analysis to probe its theoretical and methodological significance. I focus on normal development as a technical condition in experimental practice. In doing so I highlight the work that is required to create and sustain both it and the work that it enables. Variation between embryos can cause problems for scientists trying to produce valid and comparable results. In my study of Wilson's work, I examine how the practices associated with normal development deal with the variation between embryos. In the 1890s, Wilson became increasingly interested in which causes were responsible for the processes of differentiation (the production of different cells and organs) and determination in the process of embryonic development. He performed a series of experiments on the marine invertebrate Amphioxus, which exhibits considerable variability in early development (Wilson, 1893a). Wilson carefully observed his samples and outlined a normal development based on them, which included a considerable range of variation. How Wilson treated variation was reflected in the different way in which he conceived of the process of development compared to other prominent embryologists, such as Hans Driesch and Wilhelm Roux. 4 Having introduced and assessed normal development, I use two analytical approaches to make further sense of it. Furthermore, these approaches identify why appreciating the role of normal development enables us to understand important aspects of scientific practice, such as experimental methodology and making causal attributions based on the results of experimental manipulations. The two main analytical approaches I use are James Woodward's manipulationist theory of causation (Woodward, 2003 and 2010), and Hans- Jörg Rheinberger's experimental systems approach (Rheinberger, 1997). The former assesses the factors involved in assessing proposed causal factors, rather than simply demarcating between causes and non-causes. The latter focuses on the way experimental set-ups are configured by scientists in ongoing series of experiments to frame phenomena of interest: 'epistemic objects'. My experimental systems are produced and reproduced, and to how attributions of causality which arise from experimental work are made.
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