What am I putting between plants and animals? Physical chemistry, I think, because to me it is the filling that holds the sandwich together. I confidently expect that the theories of biological development will eventually embrace all kingdoms, just as, contrary to many expectations, genetic mechanisms proved to be so universal that what the plant-breeder, Gregor Mendel, discovered about peas has become the basis of the understanding of many human diseases, and much else.

Physical chemistry uses a lot of mathematical language, and it is a large part of my evangelistic attitude to suppose that much of developmental biology will some day have to be written in much the same language that physical chemists have been using for decades in their publications. But that is not yet accessible to many biologists. In this part of the book, I have had to struggle to say some things without using mathematical language. I remain unsure that I have done it, or that anybody could do it. In my previous book (Harrison 1993) I gave my reason for what I believe to be the necessary future adoption of much more mathematical language in experimental biology as follows: