CoGiTic Evolution of Man 611 



Tennent, in his ''Natural History of Ceylon" (201: 481), 

 gives the following account of an attack by land leeches on 

 himself and party. "In moving, the land leeches have the 

 power of planting one extremity on the earth and raising the 

 other perpendicularly to watch for their victim. Such is 

 their vigilance and instinct that, on the approach of a passer-by 

 to a spot which they infest, they may be seen amongst the grass 

 and fallen leaves on the edge of a native path, poised erect, and 

 preparing for their attack on man and horse. On descrying 

 their prey they advance rapidly by semicircular strides, fixing 

 one end firmly and arching the other forwards, till by successive 

 advances they can lay hold of the traveler's foot, when they 

 disengage themselves from the ground and ascend his dress 

 in search of an aperture to enter. In these encounters the 

 individuals in the rear of a party of travelers in the jungle 

 invariably fare worst, as the leeches, once warned of their 

 approach, congregate with singular celerity." 



Now here, from the sense-stimulation and sense-response 

 standpoints, we would analyze and then summarize the com- 

 plete action as follows. Chemotactic search for food, helio- 

 tactic light action, and geotactic stimuli that might correlate 

 their movements would all be combined into a resultant mo- 

 tion-response that caused them to frequent the native's path- 

 ways. But, on appearance of a man or other animal, the mov- 

 ing object started successively changing heliotactic stimuli and 

 heliotropic responses, simultaneously the sound of the moving 

 party started tonotactic stimulus,* while the poised erect at- 

 titude was an apogeotropic response due to the other stimuli 

 causing an overcoming of geotropic or diageotropic position. 

 These latter, combined into a resultant that linked up into the 

 ganglion cells of the ventral nerve-centers with the immedi- 

 ately previously formed resultant, together constituted a com- 

 y)Ounded cogitic resultant that manifested itself as an "advance 

 rapidly by semicircular strides." 



* As in the case of the Nemertinea we would regard the slightly depressed 

 sense-cells as a riuliinentary ear. 



