88 ROBERT M. YERKES. 



This table shows that the relative as well as the absolute vari- 

 ability is higher for the medusa than for the frog. In general it 

 is true that variability increases with increase in the time of reac- 

 tion. Stimuli or .intensities of stimulation which give extremely 

 short reaction-times may be expected to give low indices of vari- 

 ability ; similarly animals which are slow in reacting exhibit high 

 degrees of variability. The reflex reaction is absolutely and rela- 

 tively the least variable among the common types of action ; the 

 instinctive reaction is much more variable, and most variable of 

 all in time of execution as also in form, is the voluntary reaction 

 so-called 



RELATION OF REACTION-TIME TO REGION STIMULATED. 



As might be expected the reaction -time of Gonioneinits varies 

 with the region stimulated. When the electrodes are placed in 

 contact with the margin at the bases of the radial canals the re- 

 action is noticeably quicker than when the inter-radial regions or 

 other portions of the bell are stimulated. The average reaction- 

 time to a four-cell current applied to the inter-radial portions of 

 the margin is .605 second ; for the radial canal regions it is .507 

 second. It is not necessary, however, to make measurements to 

 thousandths or even hundredths of a second to exhibit this fact ; 

 stimulating different regions and simply watching the responses 

 will make clear the differences in reaction-time. Again the time 

 of reaction to light varies according as the light falls upon the 

 subumbrellar or the exumbrellar surfaces. It is much shorter 

 when the subumbrella is exposed to the light (3.4 seconds as 

 compared with 17.4 seconds for the other position). 1 



It is not at all likely that the differences in reaction-time here 

 noted for electric and photic stimuli are due to the same condi- 

 tions. The quicker reaction to stimulation of the radial canal 

 regions is doubtless due to the higher transmission rate of the 

 differentiated nerve tracts along the radial canals. Stimulation 

 of any other portion of the bell causes reaction less quickly sim- 

 ply because the tissues transmit impulses less rapidly, since they 

 possess less highly specialized nerve tracts. In case of the 

 quicker reaction to light when the medusa is resting with the 



1 Anier. Jour. Physiol,, Vol. 9, 1903, p. 301. 



