By H. KACSER Department of Animal Genetics, Edinburgh
There are many ways of discussing the phenomena which are usually described under the heading of differentiation, development, and growth. Such discussions usually involve the description of classical embryological material which, owing to its suitability as experimental material, has largely dominated work in this field as well as the thoughts and concepts which we have. For the purposes of this symposium, however, I shall be concerned with much more general questions. These may be described very briefly, and therefore very inaccurately, as 'the interpretation of biology in terms of chemistry'.
In order that it may be clearly understood what I am attempting, I must begin by discussing the very nature of my approach. This may lead into the fields of semantics and epistemology, but the nature of the symposium is such that this can hardly be avoided. There are a large number of words currently in use which are relevant to discussion of this kind. Some of these are, 'hypothesis', 'theory', 'law', 'explanation', 'computer', 'experimental animal', 'analogue' and 'model'. It is my view that all these terms are synonyms of only two of these. The most appropriate are 'analogue' and 'model'. All other terms are synonyms of either of these two. Analogue and model differ fundamentally from each other. They describe two entirely different phenomena and I shall show a sensible and consistent way in which these two terms should be used. It is certainly the way I shall use these words here.
'An analogue is any device or objects in which entities are related to one another'. This definition includes, of course, all perceptible objects which can be apprehended by man. While it is perfectly true that all such objects can be regarded as analogues, I shall add a qualification which will make the definition more useful. An analogue is clearly an analogue of something. Billiard balls on a table may be an analogue of gas molecules in a vessel, or they may be a source of relaxation for tired University Professors, or they may be the beginning of the ruin of a promising student. It therefore appears that it is the use to which objects are put that marks them out as analogues. I shall therefore qualify the definition of the word analogue by saying that 'an analogue is a device (in which entities are related to one another) which can be used for the purposes of making a model'. We therefore must know what