In his 1939 preface to the work, the biologist J. B. S. Haldane states "most of the manuscript seems to have been written between 1872 and 1882, that is to say it refers to the science" of that era. "Hence it is often hard to follow if one does not know the history of the scientific practice of that time. The idea of what is now called the conservation of energy was beginning to permeate physics, chemistry and biology, but it was still very incompletely realised, and still more incompletely applied.