298 SCIENTIFIC THOUGHT. 



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 

 is. structures through division or differentiation, was, how- 



Epigenesis 



andevoiu- 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, 

 14. is now pretty unanimously l traced to Caspar Friedrich 

 Wolff, whose ' Theoria genera tionis ' appeared in 1759. 

 His observations refer alike to plant and to animal life, 

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



ineiit is carried still further, and later writings ; and development is 



the origin of the molecular com- merely the expansion of a potential 



ponents of the physically gross, organism or original pre-formation 



though sensibly minute, bodies according to fixed laws." 



which we term germs is traced, l See J. A. Thomson, loc. cit., p. 



the theory of development will ap- 121. Yves Delage, ' L'HeYediteV 



proach more nearly to metamor- p. 357, note ; and especially O. 



phosis than to epigenesis. . . . The Hertwig, ' The Biological Problem 



process, which in its superficial of To-day,' transl. by P. C. Mitchell 



aspect is epigenesis, appears in (Heinemann's Scientific Handbooks, 



essence to be evolution in the 1896), p. 4, &c. 

 modified sense adopted in Bonnet's 



