Goodsir developed his theory about the nature and structure of cellular life and organisation. He concluded that all living organisms are formed of microscopic units, cells. Goodsir was not alone in postulating such a concept and the theory that cells form the basic structure of tissues in all plants and animals has been attributed to Matthias Jakob Schleiden and to Theodor Schwann. Goodsir posed and then answered the questions "What is a cell with its walls, contents, nucleus and nucleolus? How is a cell formed? How do cells multiply?" The theory which he developed from these studies was original and won the extravagant praise of Rudolf Virchow (1821–1902), who dedicated his masterpiece Cellular Pathology to Goodsir, describing him as "one of the earliest and most acute observers of cell-life both physiological and pathological." In 1842, Goodsir was elected a Fellow of the Royal Society of Edinburgh his proposer being James Syme. He was elected a Fellow of the Royal Society in 1846. In 1849, he was elected as a member of the American Philosophical Society. Professor of anatomy When Alexander Monro tertius retired in 1846, Goodsir was appointed Professor of Anatomy at the University of Edinburgh. In 1848 he was elected a Fellow of the Royal College of Surgeons of Edinburgh (FRCSEd), applied for the position of Assistant Surgeon to the Royal Infirmary of Edinburgh, but was not appointed. He moved into Edward Forbes's South Cottage at Wardie in north Edinburgh where he spent the last ten years of his life. Such was the improvement that he brought about in anatomy teaching that by 1860–1861 the size of the anatomy class had grown to 354. Later years From 1850, Goodsir became unwell, showing the features of the chronic wasting illness which would eventually prove fatal. It made slow and insidious progress and assumed the characters of locomotor ataxia. Yet despite this, after the death of his friend Edward Forbes in 1854, he took on Forbes's lectures in addition to his own. In 1863, he was invited to assist Sir David Brewster with an article for the North British Review on Faivre's analysis of Goethe's studies. Brewster, a distinguished physicist and mathematician and now Principal of the University of Edinburgh, had known Goodsir from 1839, when both were members of the Literary and Philosophical Society of St Andrews. Goodsir died at South Cottage, Wardie, Edinburgh on 6 March 1867, at the