THE HEREDITARY FACTORS AND DIFFERENTIATION 413 



§8 



The last example shows how a detailed analysis is often required to 

 discover the mechanism by which genes exert their effect. Indeed, 

 it is necessary to think in terms of development before it is possible 

 to discover what is the fundamental process with which a given gene 

 is concerned. In such an analysis, the old concept of Mendelian 

 characters will disappear. The visible character is not Mendelian 

 in any real sense : it is the resultant of the interaction of a particular 

 gene-complex with a particular set of environmental conditions. In 

 investigating the effect of a given gene, it is usual to study the 

 difference in development and end-result obtained by substituting 

 one allelomorph of the given gene for another in the gene-complex. 

 By doing so in different environmental conditions, it is possible to 

 obtain an idea of the fundamental process influenced by the gene 

 in question. By paying proper attention to the development in this 

 analysis, this fundamental process is seen to be something very 

 different from what would have been expected if only the end- 

 results in the adult had been studied. The resolution of the red- 

 black series of adult eye-colours in Gammarus into the effects of 

 genes controlling relative rates of melanin-deposition is a case in 

 point; and this in all probability has a bearing upon other eye- 

 colour series, as in Drosophila and in man. 



Again, the fundamental process resulting in white ("albino") 

 eyes in Gafmnarus concerns the failure of the embryonic eye to 

 differentiate any rudiment of the retinula region : only a close study 

 of the developmental physiology of the eye-region will be able to 

 shed further light on the processes involved. 



This is, in a certain sense, obvious. What has not been ade- 

 quately recognised, however, is that the converse holds true, and 

 that the study of developmental processes will of itself shed Hght 

 upon genetics. To illustrate this point, an example may be taken 

 from among growth-processes. The empirical study of relative 

 growth has shown that a change in relative growth in an organ or 

 region appears always to be brought about by a change in a growth- 

 gradient affecting that region. For instance, in Crustacea, the 

 differences between a small purely female type and a large male 

 type of chela, and between the small male abdomen and the large 



