generally," provides some concluding generalization. In 

 order not to repeat what others had stated previously, Wolff 

 considered it necessary to provide a summarized theory of 

 development, briefly formulated in the form of definite 

 propositions, and to confirm the correctness of the system 

 with his own microscopical investigations. "Only at this 

 time," Wolff wrote, "when all was completed and prepared for 

 publication, did I read books which by chance proved to be 

 identical to the conclusions in my dissertation" (§ 231). Of 

 his predecessors who wrote about epigenesis, Wolff mentioned 

 C. G. Ludwig and (John Turberville) Needham. All the others, 

 according to Wolff, either agreed with Ludwig or defended 

 clear error. Harvey, according to Wolff, adopted his opinion 

 from Ludwig. He confirmed that the male semen affects the 

 uterus in a way analogous to the way impressions arise in the 

 brain of a painter who creates the likeness of the source of 

 his impressions (§ 232) . 



Wolff pointed out that the first principle of development, 

 its moving power which he called the essential power, is 

 a necessary condition of existence of animals and plants. 

 Many authors and investigators had known this power for a 

 long time, although they did not consider it the principle of 

 development. Thus Ludwig, denying the necessity of this power 

 for plants, suggested that the distribution of fluid in the 

 plant organism was due to external causes. Later, Wolff 

 noticed that the expansive and vegetative power, and also the 

 power of resistance, which Needham had mentioned, were not 

 identical with his own principle of development. In his work, 

 "New observations on development, building and destruction 

 of animal and plant substances" Needham called the expansive 

 power of growing matter. Under the influence of this power, 

 each separate point of matter attempts continuously to move 

 away from its neighboring points, and Needham considered growth 

 as extension. However, Wolff designated the essential power to 

 be the ability to distribute and move the fluids somewhere 

 else in the plant, i.e. something more determined than simple 

 extension. Speaking about the extension of plant substance 

 in vesicles and canals, he attributed the extension of 

 substance not having the essential power to a mechanical 

 pressure of penetrating fluid in this substance ($ 233). Wolff 

 later noted that the necessary character of plant and animal 

 substance (here he called this character the decisive cause) 



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