ber of repetitions, and the number of adequate presentations 

 from the accepted set of the consensus known as "native lan- 

 guage," and the total numbers of sets in the stored memories at 

 a given age. In addition, of course, is the adequate development 

 of the transmitting and of the receiving equipment needed for 

 speech and its ancillary behaviors. 



On the biological side, modern neurology says the number of 

 neurons in the human brain reaches maximum value before 

 birth at about 13 billions. After this point, the increase in weight 

 consists of increased numbers of fibers, increased connections, in- 

 creased size of elements, and increased efficiency and selectivity 

 of transmission. Thus the increase in weight of the human 

 brain from about 400 to 1400 grams seems to be devoted to im- 

 proving its internal (as well as external) communication, stor- 

 age, and computation networks. As I have stated elsewhere 

 {Man and Dolphin), it is my impression that there exist critical 

 threshold values in the brain's growth pattern at which certain 

 kinds of performance become possible. Complex speech acquisi- 

 tion seems related to brain weights of 800 to 1000 grams, but no 

 smaller. This assumes, of course, numbers of neurons (lo'") 

 and numbers of connections and opportunities for learning and 

 time to learn commonly found with humans. 



The critical psychological factors in speech acquisition are 

 slowly being dug out and described."^ ~ Among these the most 

 important seem to be a continuous background of presentations 

 to the child in rewarding circumstances of speech and its close 

 relations to objects, actions, satisfaction of needs, and persons. 

 Imitation of one's use of facial and vocal apparatus appears 

 spontaneously in the happy child. The virtuosity of the child 

 as a mimic is truly astonishing. 



I am also impressed by evidence for what I call the "transac- 

 tional drive." A bright child seems to seek and respond best to 

 those persons who respond in kind, back and forth in exchanges 

 of sounds and linked actions. For example, if one starts such a 



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