SECT. 3] AND EIGHTEENTH CENTURIES 217 



cessors until the eighteenth century a.d. treated it as a psychological 

 rather than a physiological or morphological theory, and lost them- 

 selves in speculations about the vegetative, sensitive, and rational 

 souls. Yet the other aspect of the theory was only asleep, and was 

 destined to be of the greatest value as soon as investigators began to 

 direct their attention more to the material than to the spiritual 

 aspect of the developing being. 



Hunter did not absolutely reject preformationism, but regarded it 

 as holding good for some species in the animal kingdom ; he therefore 

 attached no philosophical importance to it. 



Although Wolff's work did not lead to the immediate morphological 

 advances which might have been expected, it was in many ways 

 fruitful. It produced J. F. Blumenbach's Uber den Bildungstrieb of 

 1789, a work which elaborated the Wolffian vis essentialis into 

 the nisus formativus, a directing morphogenetic force peculiar to 

 living bodies. It is interesting to note that Blumenbach passed through 

 an exactly opposite succession of opinions to that of Haller, i.e. he 

 was first attracted by preformationism, but, being convinced by 

 Wolff's work, abandoned it in favour of epigenesis. Blumenbach 

 compares his nisus formativus with the force of gravity, regarding 

 them as exactly similar conceptions and using them simply as 

 definitions of a force whose constant effects are recognised in 

 everyday experience. Blumenbach says that his nisus formativus 

 differs from Wolff's vis essentialis because it actively does the shaping 

 and does not merely add suitable material from time to time to a 

 heap of material which is already engaged in shaping itself. Wolff 

 was still alive at this time, but he did not make any comment on 

 Blumenbach, though he might very well have said that Blumenbach 

 had misunderstood him, and that their forces were really alike in 

 every particular. Both Blumenbach and Wolff were mentioned by 

 Kant in the Critique of Judgement where he adopted the epigenetic 

 theory in his discussion of embryogeny. 



A word must be said at this point about the opinions of the 

 eighteenth century on foetal nutrition. At the beginning of it, there 

 was, as has been shown, a welter of conflicting theories; and though, 

 later on, writers on this subject were fewer, the progress made was 

 no more rapid. In 1802 Lobstein was supporting the view (which 

 had been defended by Boerhaave) that the amniotic liquid nourished 

 the embryo per os, although Themel had shown forty years before 



