The central topic and justification of this book is a conceptual problem: how could orderly or patterned biological form be based on chemical and physical processes. Form is known to be the product of development – but development can lead to tumors rather than organized tissues. Form is also known to depend on the genetic constitution of the organism. Yet this is only a restatement of the problem, since any given gene specifies the structure of a molecule, not the organization of tissues.

The use of new molecular methods is sure to yield a wealth of information. The search for this information requires a conceptual framework of how tissue patterns are formed. Yet many accepted views are implied rather than clearly stated. Furthermore, they consist of unquestioned dogma which is partially contradicted by available facts. An alternative set of concepts is sought here on the basis of a broad, comparative view of available facts, mostly the products of simple techniques. Such a framework should also be important for studies of developmental physiology at non-molecular levels. Finally, though it is the genes that mutate and it is mature structures that are screened by selection, the intervening controls of orderly development must impose constraints on the possibilities exposed to selection – and thus on the course of the evolution of biological form.