98 Pearl and Surface. 



one hand and the adult soma on the other hand, as things given. What 

 comes between the two is neglected. But clearly what goes between 

 is a part of the very essence of heredity itself. 



In our group of adult individuals each one will show some parti- 

 cular variation, in the sense of a deviation from the typical condition 

 of the group. We take it to be one of the final objects of investigations 

 in genetics to find out why (in the sense of locating the essential 

 causal factors involved) a particular individual A exhibits a par- 

 ticular variation a, and not some other variation out of the 

 indefinitely large number of possibilities. Since the discovery 

 of JIendel's laws we have gone a long way towards being able to 

 answer the question for many kinds of qualitative variations. For 

 fluctuating or quantitative variations too, considerable progress has been 

 made through the researches especially of East and his associates, 

 following those of Nilsson-Ehle on colors determined by multiple 

 factors. These studies have made it extremely probable that definite 

 genes determine, for example, how tall a particular Indian corn plant 

 shall be, or how large an ear it shall have. But in these researches 

 only the end elements of the ontogenetic series have received 

 attention. 



What we have tried to do in the present investigation is, by 

 studying the growth of the individual, to analyze the adult variation 

 curve into its component elements. We have attempted, in other words, 

 to make a beginning at an understanding of the developmental physio- 

 logy of the genes concerned in the production of the characters studied. 

 Specifically the angle from which this general problem has been ap- 

 proached in the present case is that of the analysis of the adult 

 variation curve in terms of its component individuals. A given indi- 

 vidual, at an early stage of its growth, may exhibit an exceptional con- 

 dition of a character, as compared with other individuals of the same 

 age or stage of growth. It may be, for example, very short, the 

 shortest plant in the whole population at 3 weeks of age. Will tliis 

 same individual be the shortest plant in the adult population, after all 

 growth is completed? If not, where is it in the adult variation curve, 

 and how did it get there? This example will give a concrete idea of 

 the general mode of approach to the problem of variation followed in 

 this paper. The specific problems will be set forth in detail in the 

 sections where they are discussed 



