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This is the third time thai I have presented the sub j eel matter 

 of this book. Each time it has been in differenl form. The first 

 little book, "Die quantitative Grundlage von Vererbung und 

 Artbildung," written in L917 and published in 1920, consisted of 

 a number of essays in which I developed my views on the prob- 

 lems of gene and development as derived from my own work on 

 Lymantria dispar. There was not much material from other 

 sources to be quoted. Ten years later I returned to the same 

 subject. Meanwhile my own work had amplified, experimental 

 embryology had entered a new phase, and a number of other 

 geneticists had become interested in the problems of genie action. 

 Notable among these were S. Wright, whose interest in the 

 subject dates back as far as 1916; Zeleny and his school, who 

 furnished most exact quantitative data; J. S. Huxley, who had 

 contributed to the theoretical analysis and had started experi- 

 mental work; Sturtevant, who had just cleared up the case of 

 Bar eye in Drosophila; and Plunkett, who had applied his 

 physicochemical knowledge to certain problems. This second 

 book, "Physiologische Theorie der Vererbung," was meant to 

 elaborate in all details my own views regarding the action of the 

 gene in development , but it also reported whatever relevant work 

 of others was available. Again ten years have passed, and these 

 have witnessed an ever-growing interest in the field and a corre- 

 spondingly increasing amount of work, which, with the introduc- 

 tion of new methods, is expanding more and more. As it is 

 emphasized over and over again by writers of texts and by general 

 speakers that we know next to nothing of the action of the 

 hereditary material in controlling development, it seems advis- 

 able to present the entire material available. Not only has this 

 material been assembled and reviewed, but an attempt has been 

 made to organize it into the skeleton of a future science of 



