l62 T. H. MORGAN. 



success in explaining such actions. The vitalistic view goes to 

 the opposite extreme and postulates unknown or unknowable 

 processes. Both seem to have overlooked the possibility of ac- 

 counting for formative changes by the familiar processes of irrit- 

 ability and contractility, that appear to be fundamental attributes 

 of living materials. These cannot be reduced as yet to any known 

 chemical or physical phenomena, but neither does it follow that 

 they belong to a vitalistic category. 



To sum up : the formative principle as seen in development, 

 growth, and regeneration, appears to be an expression of the irrit- 

 ability of the living material. The problem of formative action is 

 therefore intrinsically one of response to external stimuli or to in- 

 ternal relations between the parts through the agency of irritability. 

 Contractility is one of the most usual methods of responding 

 to the condition of stimulation that exists in a given region, while 

 differentiation follows quickly in its train. The formative principle 

 is the outcome of a factor that is not one of the familiar chemical 

 or physical events. 1 This conclusion does not force us into vital- 

 ism ; for until we know something of the nature of irritability it 

 is premature to insist on referring it to any larger category. 

 As yet we are only at the threshold of a knowledge of formative 

 changes. 



1 To those who refer the varied manifestations of irritability to a psychical principle 

 the problem of development will appear to belong to that category ; but to those who 

 see in irritability only a "physiological process," the same problem will seem to 

 be physiological. The distinction may be verbal and conventional. 



