Herbst 247 
but an actual phyiogenesis which would repeat itself in 
each generation cx 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 
