8 



Problems, Concepts and Their History 



from essentially undifferentiated tissue. This 

 was to lead to his fundamental premise that 

 in animals as well as plants development 

 proceeds by gradual differentiation of orig- 

 inally homogeneous material. 



There is importance in the fact that he 

 considered the undifferentiated material to 

 be comparable in plants and animals. But 

 though his emphasis on this similarity of 

 construction of material in plant and animal 

 may, as Sachs and Huxley realized though 

 later generations have forgotten, have had 

 effect on the development of the doctrines 

 implying universality of cell and proto- 

 plasm, it was primarily his emphasis on its 

 early undifferentiatedness that was of more 

 immediate import. 



It has been said of Wolff, as of Harvey, 

 that he was without influence in his own 

 day, his writings neglected and without 

 effect until after their translation into Ger- 

 man by Meckel in 1812. This is inaccurate. 

 Haller knew his work, and certainly the 

 biological world was kept plentifully and 

 liberally informed of what Haller was think- 

 ing; Kant knew of the concept via Blumen- 

 bach (cf. Critique of Judgment, §81) and 

 from the beginning Kant had the attention of 

 the scientists. Diderot could affirm with confi- 

 dence, as early as 1769, when writing about 

 "germes preexistants," that 



Cela est contre I'experience et la raison: centre 

 I'experience qui chercherait inutilement ces germes 

 dans I'oeuf et dans la plupart des animaux avant un 

 certain age; contre la raison qui nous apprend que 

 la divisibilite de la matiere a un terme dans la 

 nature, quoiqu'elle n'en ait aucun dans I'entende- 

 ment (Entretien entre D'Alembert et Diderot; writ- 

 ten 1769, first published 1830; cited from 1875 edi- 

 tion, II, 110). 



Diderot was no technical embryologist; 

 while it is possible, it is hardly probable 

 that he had read the Theoria generationis. 

 It is far more likely that he was expressing 

 for political reasons an appropriate scien- 

 tific concept which was already sufficiently 

 widely disseminated to have reached his ad- 

 mittedly universal ear. 



May not this be the clue to Wolff's suc- 

 cess where Malpighi had failed? The new 

 century had brought new thinking with it, 

 a new thinking in terms of change. Social 

 and political change were soon to grow out 

 of it: revolution and evolution had a com- 

 mon philosophical background; it was even- 

 tually, with Hegel, to reach full fruition as 

 the central doctrine of a specific philosoph- 

 ical system. Wolff's work was an early ex- 

 pression of this tendency. It is no accidental 



coincidence that the Christian Wolff who 

 taught philosophy to Caspar Friedrich Wolff, 

 the originator of a biology of change, was 

 a popularizer of Leibniz who had invented 

 a calculus as a mathematics of change. 

 Without this background, it is as unlikely 

 that Wolff woiold have found a homogeneous 

 blastoderm under his microscope as it was 

 inevitable that Malpighi should have denied 

 one a century before. 



But Wolff's thinking typified too another 

 kind of thought that was soon to broaden 

 more generally. Wolff was prone to general- 

 ize from plant to animal (cf. "a bat is a 

 perfect leaf . . . for the mode of origin of 

 the two is the same"; from Theorie der Gen- 

 eration, 1764, §64, unavailable to me; cited 

 by Huxley, 1853b, p. 293). His first proof 

 of epigenesis in the chick came from his 

 demonstration that the blood vessels of the 

 chick blastoderm are not present from the 

 beginning; he was probably led to the in- 

 vestigation by his false and far-fetched 

 analogy between the vessels of the animal 

 and those of the plant whose development 

 he had already studied. Certainly if seman- 

 tics gives any indication, his preoccupation 

 with plants colored his later interpretations 

 of observation on animal development. When 

 he demonstrated that animal organs — the in- 

 testine and probably the central nervous sys- 

 tem — are formed by the folding of homoge- 

 neous layers into tubes he called them by 

 the name for leaf. 



This is a strong hint, as is his "tracing of 

 the body from principles and laws," of the 

 Naturphilosophie to come, and it is cm'ious, 

 from his own point of view, that the em- 

 phasis laid by history on his accomplishment 

 is centered so strongly on his microscopic 

 discovery of what was not there. In his time, 

 what influence he had probably spoke more 

 positively in the direction of transcendental- 

 ism. Upon his concept of epigenesis and 

 change and upon his intimations of layering 

 in the embryo — both concepts to which 

 Wolff was led by his tendencies towards 

 Naturphilosophie — embryology was to fol- 

 low with its whole momentous sequel, but 

 only after a serious delay during which con- 

 cepts were to arise which in many ways 

 negated the concept of change which Wolff 

 originated, concepts paradoxically enough 

 also derived on a basis of Naturphilosophie. 



EMBRYOLOGY AND NATURPHILOSOPHIE: 

 GOETHE AND VON BAER 



But why now Naturphilosophie, whose in- 

 fluence on embryology was to grow so strong 



