8 HISTORICAL INTRODUCTION TO THE 



generation: every individual organism is created by epigenesis 

 during its own life-history. The environment is as important as are 

 the internal and transmitted hereditary factors, and both must be 

 normal for a normal embryo to be developed. If the environment 

 is abnormal, there will either be no development at all, or an ab- 

 normal and abortive development, and the same fate befalls an 

 abnormal hereditary constitution reacting with a normal environ- 

 ment. If both the environmental and hereditary factors are within 

 the bounds of normality, then development will follow the lines 

 which are characteristic for the particular species of organism in 

 question. 



The origin of differentiation and of the epigenetic process are 

 therefore to be found in the processes by which in the first place 

 quantitative differentials are induced in the egg by external factors, 

 and in the second place qualitative structural diversities result from 

 the interaction of the quantitative differentials with the inherited 

 constitution. It is these problems which form the subject-matter 

 of this book. 



§4 



Meanwhile, it is necessary to pause, and to consider for a moment 

 how the causal postulate can be applied to development conceived 

 as an epigenesis. On the preformationist view, the causes of de- 

 velopment present no particular difficulty, for diflferentiation is then 

 supposed to be there all the time and to require nothing but ex- 

 pansion or unrolling (''evolution" in the eighteenth-century sense) 

 in order to become visible. Even after the discomfiture of the pre- 

 formationist view at the hands of Wolff and others, and the 

 acceptance in principle of an epigenetic theory of development, 

 the need for an application of the causal postulate was cloaked by 

 the unfortunate effects of Haeckel's theory of recapitulation. This 

 view, pushed to its ultimate conclusion, maintained that ontogeny 

 or embryonic development was inevitably a recapitulation of 

 phylogeny or racial evolutionary history, and that phylogeny was 

 the mechanical cause of ontogeny, whatever Haeckel may have 

 meant by such a statement. If this was true, then clearly there was no 

 need to look for other causes than the evolutionary history in order 

 to explain development. But, as Wilhelm His saw, it was not true. 



