612 Causes and Course of Organic Evolution 



This again became compounded or linked with a thigmotactic 

 stimulus when the animal fastened on the body of its victim, 

 while finally the taste and absorption of the blood formed a 

 chemotactic stimulus that doubtless greatly excelled, though 

 it linked on with, those already compounded. The reaching 

 of the traveler's leg therefore is a compounded resultant re- 

 sponse, made up of at least two, possibly three, resultants each 

 built up of several distinct stimuli. 



The intrinsic capacity to link up these compounded resultants 

 is evidently the function of the ganglionic cells, as experiment 

 and observation will demonstrate from cases of a more and 

 more complicated kind cited below. But at this point it may 

 not be amiss to inquire as to the use of ganglion cells, and to 

 ascertain how they differ in action from root-tip, stem-tip, leaf- 

 stalk, and other plant cells, or the cells of some simple nucleate 

 animal like Amoeba. 



Jacques Loeb has well remarked as to the tropisms in rela- 

 tion to ganglia, "The mystery with which the ganglion cell has 

 been surrounded has led not only to no definite insight into 

 these processes, but has proved rather a hindrance in the at- 

 tempt to find the explanation of them." We have already seen 

 that in many plant responses each such is a resultant of several 

 stimuli. Each separate stimulus, whether geo-, hydro-, or 

 apoheliotactic for a root-tip, causes definite chemical and so 

 energizing changes in the perceptive root-tip region. These 

 are propagated backward to the responsive part. But here 

 the separate stimuli are combined into a resultant or proen- 

 vironal force, that causes the root to move in a resultant direc- 

 tion. This need not be in line with any one of the several 

 separate stimuli, though in nature such often happens, specially 

 if one stimulus be, in strength and direction, more powerful 

 than the others separately or combined. 



Now what occurs in the responsive part under such circum- 

 stances .^^ We can only consider that the chemical molecules, 

 severally affected by each separate stimulus, act and react on 

 each other there, so as to produce new and definitely com- 

 pounded molecular combinations whose energies distribute 



