He then moved back to Anstruther to work in his father's medical practice, which allowed him to resume his boyhood hobby of searching the local coastline along the Firth of Forth for all forms of wildlife.

Together with Goodsir's brother Joseph they rented a flat at 21 Lothian Street close to the university, which became a meeting place for scientists, writers and artists, who together called themselves the Brotherhood of the Friends of Truth.

[10] The following year he gave a paper to the British Association for the Advancement of Science entitled "Dentition in the ruminants",[11] with some assistance from the University of Edinburgh Professor of Natural History, Robert Jameson.

His lectures on pathology in 1841 and 1842 presented his innovative ideas on cell theory which were read and later developed by the German pathologist Rudolf Virchow.

[12] He gave the first description of the stomach bacterium Sarcina ventriculi which demonstrated his status as a shrewd observer and innovative thinker.

Two years later, he was appointed curator of the University of Edinburgh natural history collection and he was succeeded as RCSEd Museum Conservator by his brother, Harry Goodsir, who continued in this post until 1845.

His lectures attracted large numbers of students and did much to restore the University of Edinburgh's reputation for anatomical teaching which had suffered under Monro.

[13] The following year he published, jointly with his brother Harry, Anatomical and Pathological Observations[14] based on his earlier lectures at the RCSEd.

On the basis of his studies using the compound microscope Goodsir developed his theory about the nature and structure of cellular life and organisation.

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[16] and to Theodor Schwann.

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.

[21] 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.