There are hundreds of different kinds of cells in the body, the numbers and sizes of which have to be carefully regulated if the disproportions of cancer and dystrophy are to be avoided. Since there must be as many regulatory mechanisms as thcre are cell types to be controlled, the question may be asked to what extent these mechanisms are alike or different?

One prefers to think, along with William of Occam, that in the economy of nature processes are not multiplied beyond necessity. Indeed, it would be convenient for the student of development if the similarities between the multifarious mechanisms of growth regulation outnumbered their differences. Were this to be the case, it might be possible to identify fundamental principles in those examples which have been more thoroughly explored, and then apply them to other systems the way a secret code can be used to decipher cryptograms. What is the probability that this might be feasible? Natural processes tend to be conservative. So it is that organisms are more alike than different despite the superficial diversity that prevails in the organic world. At the molecular level, for example, they use the same amino acids, possess thc same carbohydrates, fats and proteins, and catalyse their reactions with much the same complement of enzymes. Virtually all cells utilize comparable metabolic processes and are endowed with similar populations of organelles. Mechanisms of inheritance and reproduction are very much alike in all organisms in spite of their intriguing range of variations. Indeed, the very process of evolution itself has remained basically unchanged since life began.