Herbst 247 



but an actual phylogenesis which would repeat itself in 

 each generation ex novo, because of the repetition always 

 in the same way of the same external formative influence. 



After having thus stored up a rich harvest of facts 

 upon the physiological actions exerted by the most differ- 

 ent stimuli upon organisms or upon given parts of 

 organisms, actions which are properly nothing else than 

 functional adaptations in a broad sense of the word, 

 Herbst believed he was able to build upon them his 

 epigenetic conception, by which he makes development 

 fundamentaly dependent upon a whole series of directive 

 stimuli. 



"Just as freely moving organisms are influenced in 

 the direction of their movements by external agents, so 

 do independent tissue cells react to definite directive 

 stimuli, and thereby make possible the production of a 

 large number of ontogenetic formative processes." 



"Just as the germinal vesicles of Cuscuta, for ex- 

 ample, develop their stinging barbs at the points of 

 contact with the plant upon which they lodge, just as in 

 the leaf stalks of Helleborus niger the traction of a weight 

 causes the formation of new mechanical elements which 

 otherwise do not appear, or just as roots may be made to 

 grow on a grass stalk because of the secretion of a gall 

 fly, so, in the interior of an animal embryo in process of 

 development a given organ can cause new formative pro- 

 cesses to come into existence in another organ by means 

 of contact, pressure, traction, by a specific product of 

 metabolism or in some other way." 



And so, "just as in plants and sessile animals 

 morphological formations of the most different kinds are 

 produced through formative stimuli which either arise 

 from the external world or are exerted by one organ of 



