In these pages, the reader will find neither a treatise nor a textbook on biochemistry, but a number of essays grouped around ideas of the unity and diversity of organisms in the biochemical sphere. "The manifold and the one" are eternal preoccupations of the human intellect, and we must not be surprised that, from the time biochemistry has been able to gather together a sufficient number of facts, the search for the lowest common denominator of all organisms or a "unity of biochemical plan" has been confused in many minds with the idea of a comparative biochemistry. The latter is a problem which is perhaps more relevant to natural philosphy than to scientific investigation, for we are becoming more and more aware of the extreme diversity of biochemical function arising during cellular differentiation in a single organism, as well as in the multiplicity of species and even of individuals. The biosphere, by which we understand the total amount of living matter, behaves like a chemist of a very special type. All the organic compounds present in the many regions of the biosphere and resulting from its biosynthetic activities have structures lying within certain definite limits. The first part of this book provides a concise catalogue of these structures but is not coincident with the contents of a textbook of organic chemistry provided that the latter is not defined as it was by Berzelius at the beginning of the 19th century, when he wrote that organic chemistry is that section of physiology describing the composition of living things and the chemical reactions going on therein. This definition of organic chemistry is no longer valid today; beginning with the synthesis of a naturally occurring substance, urea, organic chemistry has extended its domain to the synthesis of a tremendous number of non- natural substances. One of the objectives of biochemistry is to define and understand the nature of the collection of compounds composing living matter and to distinguish them from those originating from non- living sources and human inventiveness, all of which are described by the broad generalizations of chemistry.
The biosphere is not only a chemist of a special type, but also one of great antiquity whose methods have been developed over a long period of time since long before there were laboratories of organic chemistry, and are of an efficiency far from being paralleled in these laboratories. This point is developed further in the two essays which make up Parts 2 and 3 of this book with the intention of demonstrating the originality of this organic chemist who has laboured since the dawn of time and comparing his methods with those of the laboratory chemist. The essays making up . . .