298 



SCIENTIFIC THOUGHT. 



13. 



Epigenesis 

 and evolu- 

 tion. 



14. 



C. F. Wolff. 



generalisation which he ventured to put forward, that 

 growth and development of the germ or embryo con- 

 sisted in the addition or formation of new parts and 

 structures through division or differentiation, was, how- 

 ever, obscured and cast into the shade by the opposite 

 doctrine, termed evolution, according to which every 

 form or particle of organisation was minutely pre-formed 

 in an invisible germ, and growth consisted merely in a 

 process of enlargement, as a particle of " dry gelatine 

 may be swelled up by the intussusception of water." 

 The supporters of this doctrine, to which the celebrated 

 names of Leibniz, Boerhaave, Haller, and Bonnet belonged, 

 seemed unable to conceive of any force in nature which 

 was capable of producing organisation, and were thus 

 compelled to accept in some form or other the doctrine 

 of the pre-existence of germs, a theory which has in 

 modern times been revived under an altered form. 



The real foundation of scientific embryology, of the 

 study of the genesis of vegetable and animal organisms, 

 is now pretty unanimously ^ traced to Caspar Friedrich 

 Wolff, whose ' Theoria generationis ' appeared in 1759. 

 His observations refer alike to plant and to animal life, 

 and his distinct object was to refute the theory of evolu- 



meut is carried still further, and 

 the origin of the molecular com- 

 ponents of the physically gross, 

 though sensibly minute, bodies 

 which we term germs is traced, 

 the theory of development will ap- 

 proach more nearly to metamor- 

 phosis than to epigenesis. . . . The 

 process, which in its superficial 

 aspect is epigenesis, appears in 

 essence to be evolution in the 

 modified sense adopted in Bonnet's 



later writings ; and development is 

 merely the expansion of a potential 

 organism or original pre-formation 

 according to fixed laws." 



^ See J. A. Thomson, loc. cit. , p. 

 121. Yves Delage, ' L'H^rddit^,' 

 p. 357, note ; and especiallly O. 

 Hertwig, ' The Biological Problem 

 of To-day,' transl. by P. C. Mitchell 

 (Heinemann's Scientific Handbooks, 

 1896), p. 4, &c. 



