102 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



potency is the real basis of the specific character of every 

 act in morphogenesis, and " means," including conditions, are 

 the sum of all external and internal general circumstances 

 which must be present in order that morphogenetic processes 

 may go on, without being responsible for their specificity 

 or localisation. 



It is implied in these definitions of cause and potency, 

 that the former almost always will be of that general 

 type which usually is called a stimulus or " Auslosung," 

 to use the untranslatable German word. There is no 

 quantitative correspondence between our " cause " and the 

 morphogenetic effect. 



Some Instances of Formative and Directive Stimuli 



Again it is to Herbst that we owe not only a very 

 thorough logical analysis of what he calls "formative and 

 directive stimuli ' but also some important discoveries 

 on this subject. We cannot do more here than barely 

 mention some of the most characteristic facts. 



Amongst plants it has long been known that the 

 direction of light or of gravity may determine where 

 roots or branches or other morphogenetic formations are to 

 arise ; in hydroids also we know that these factors of the 

 medium may be at work 2 as morphogenetic causes, though 



1 Herbst, " Ueber die Bedeutung die Reizphysiologie fur die kausale 

 Auffassung von V organ gen in der tierischen Ontogenese " (Biol. Centralblatt, 

 vols. xiv., 1894, and xv., 1895); Formative Reize in der tierischen Ontogcnesc, 

 Leipzig, 1901. These important papers must be studied by every one who 

 wishes to become familiar with the subject. The present state of science is 

 reviewed in my articles in the Ergebnisse der Anatomie und Entwickc- 

 lungsgeschichte, vols. xi. and xiv., 1902 and 1905. 



2 Compare the important papers by J. Loeb, Untersuchungen zur 

 physiologischen Morphologic der Tiere, Wiirzburg, 1891-2. 



