UNIT CHARACTERS IN FOSSILS

Summary. 1. This article is concerned only with visible features, or unit characters, and not with the invisible factors which control their expression. 2. The methods used in establishing a gens are explained, and it is shown that well‐established gentes provide suitable material for the study of unit characters. 3. The work of Ford and Huxley upon Gammarus furnishes an invaluable link between the investigations of geneticists and those of palaeontologists. 4. For the palaeontologist the study of the behaviour of unit characters in development is fundamental. Peculiarities of such behaviour, long known to palaeontologists, are seen to characterise the development of such a typical Mendelian unit as that investigated by Ford and Huxley. It is inferred therefore that the two classes of units are comparable. 5. This inference is greatly strengthened by the fact that conclusions arrived at by Ford and Huxley concerning the relationship of certain Mendelian units to one another in holometabolous insects had already been independently expressed in palaeontological writings upon the same topic. 6. The correlation of the results of palaeontological and experimental methods of enquiry leads to the discovery of evidence that the terms “ mutation” and “transient” merely express different aspects of the behaviour of the same unit. 7. The distinction between a unit character and a combination of unit characters makes it possible to separate the elements of truth and error in the Theory of Recapitulation from one another. 8. It is suggested that parallelism between the behaviour of a character in development and in evolution is a corollary of the factorial hypothesis.