478 ILLINOIS STATE ACADEMY OF SCIENCE 



analyzed and related to the various phases involved in 

 the definition of intelligence. The various phases of de- 

 velopment which may enter into the concept mental age 

 must be enumerated before that term can be definitely 

 understood. If this were done one of four problems, the 

 problem of the upper limit of development, could be de- 

 fined in an understandable way. At present, with one 

 person maintaining that it is at sixteen and another at 

 thirteen or some other point, one can only ask, "What do 

 they mean?" 



At the risk of seeming very much more certain about 

 these points than I am, I shall enumerate the various fac- 

 tors which may be involved in the use of the terms under 

 discussion. In the first place, there is that very 

 necessary machine, a nervous system. The com- 

 plexity of its development and interrelations of cel- 

 lular connections correlate with the complexities of 

 behavior. Intelligence in this sense indicates possibili- 

 ties of complexities of behavior or adjustment. In our 

 thinking and use of terms it must be contrasted sharply 

 with the forms of its expression which may relate them- 

 selves wholly or in part to the physical or cultural en- 

 vironment and to training. We should know, or strive 

 to know, whether our tests measure this innate quality, or 

 measure in addition to it something due to training or 

 which could not be expressed without a certain definite 

 training. For example, the civilized child who has 

 learned to read has expressed this innate quality in a cer- 

 tain form which is the result of formal training. The 

 savage who learns to read the meanings of the signs of 

 life in the forest about him has expressed it in another 

 form, which is, perhaps, as much the result of a 

 definite training. If these two factors are differ- 

 entiated clearly in our use of the term intelligence 

 we simplify and make definite the problem of the 

 upper limit of development, tho not necessarily at 

 once the answer to it. Donaldson, in his "Growth 

 of the Brain," expressed a wonder about this 

 problem with the same lack of clarity of definition. He 

 observed that about eighty percent of brain growth in all 

 of its aspects is completed at seven years of age and that 



